1 // Copyright 2009 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 // Page heap. 6 // 7 // See malloc.go for overview. 8 9 package runtime 10 11 import ( 12 "internal/cpu" 13 "runtime/internal/atomic" 14 "runtime/internal/sys" 15 "unsafe" 16 ) 17 18 // minPhysPageSize is a lower-bound on the physical page size. The 19 // true physical page size may be larger than this. In contrast, 20 // sys.PhysPageSize is an upper-bound on the physical page size. 21 const minPhysPageSize = 4096 22 23 // Main malloc heap. 24 // The heap itself is the "free" and "scav" treaps, 25 // but all the other global data is here too. 26 // 27 // mheap must not be heap-allocated because it contains mSpanLists, 28 // which must not be heap-allocated. 29 // 30 //go:notinheap 31 type mheap struct { 32 lock mutex 33 free mTreap // free and non-scavenged spans 34 scav mTreap // free and scavenged spans 35 sweepgen uint32 // sweep generation, see comment in mspan 36 sweepdone uint32 // all spans are swept 37 sweepers uint32 // number of active sweepone calls 38 39 // allspans is a slice of all mspans ever created. Each mspan 40 // appears exactly once. 41 // 42 // The memory for allspans is manually managed and can be 43 // reallocated and move as the heap grows. 44 // 45 // In general, allspans is protected by mheap_.lock, which 46 // prevents concurrent access as well as freeing the backing 47 // store. Accesses during STW might not hold the lock, but 48 // must ensure that allocation cannot happen around the 49 // access (since that may free the backing store). 50 allspans []*mspan // all spans out there 51 52 // sweepSpans contains two mspan stacks: one of swept in-use 53 // spans, and one of unswept in-use spans. These two trade 54 // roles on each GC cycle. Since the sweepgen increases by 2 55 // on each cycle, this means the swept spans are in 56 // sweepSpans[sweepgen/2%2] and the unswept spans are in 57 // sweepSpans[1-sweepgen/2%2]. Sweeping pops spans from the 58 // unswept stack and pushes spans that are still in-use on the 59 // swept stack. Likewise, allocating an in-use span pushes it 60 // on the swept stack. 61 sweepSpans [2]gcSweepBuf 62 63 _ uint32 // align uint64 fields on 32-bit for atomics 64 65 // Proportional sweep 66 // 67 // These parameters represent a linear function from heap_live 68 // to page sweep count. The proportional sweep system works to 69 // stay in the black by keeping the current page sweep count 70 // above this line at the current heap_live. 71 // 72 // The line has slope sweepPagesPerByte and passes through a 73 // basis point at (sweepHeapLiveBasis, pagesSweptBasis). At 74 // any given time, the system is at (memstats.heap_live, 75 // pagesSwept) in this space. 76 // 77 // It's important that the line pass through a point we 78 // control rather than simply starting at a (0,0) origin 79 // because that lets us adjust sweep pacing at any time while 80 // accounting for current progress. If we could only adjust 81 // the slope, it would create a discontinuity in debt if any 82 // progress has already been made. 83 pagesInUse uint64 // pages of spans in stats mSpanInUse; R/W with mheap.lock 84 pagesSwept uint64 // pages swept this cycle; updated atomically 85 pagesSweptBasis uint64 // pagesSwept to use as the origin of the sweep ratio; updated atomically 86 sweepHeapLiveBasis uint64 // value of heap_live to use as the origin of sweep ratio; written with lock, read without 87 sweepPagesPerByte float64 // proportional sweep ratio; written with lock, read without 88 // TODO(austin): pagesInUse should be a uintptr, but the 386 89 // compiler can't 8-byte align fields. 90 91 // Page reclaimer state 92 93 // reclaimIndex is the page index in allArenas of next page to 94 // reclaim. Specifically, it refers to page (i % 95 // pagesPerArena) of arena allArenas[i / pagesPerArena]. 96 // 97 // If this is >= 1<<63, the page reclaimer is done scanning 98 // the page marks. 99 // 100 // This is accessed atomically. 101 reclaimIndex uint64 102 // reclaimCredit is spare credit for extra pages swept. Since 103 // the page reclaimer works in large chunks, it may reclaim 104 // more than requested. Any spare pages released go to this 105 // credit pool. 106 // 107 // This is accessed atomically. 108 reclaimCredit uintptr 109 110 // scavengeCredit is spare credit for extra bytes scavenged. 111 // Since the scavenging mechanisms operate on spans, it may 112 // scavenge more than requested. Any spare pages released 113 // go to this credit pool. 114 // 115 // This is protected by the mheap lock. 116 scavengeCredit uintptr 117 118 // Malloc stats. 119 largealloc uint64 // bytes allocated for large objects 120 nlargealloc uint64 // number of large object allocations 121 largefree uint64 // bytes freed for large objects (>maxsmallsize) 122 nlargefree uint64 // number of frees for large objects (>maxsmallsize) 123 nsmallfree [_NumSizeClasses]uint64 // number of frees for small objects (<=maxsmallsize) 124 125 // arenas is the heap arena map. It points to the metadata for 126 // the heap for every arena frame of the entire usable virtual 127 // address space. 128 // 129 // Use arenaIndex to compute indexes into this array. 130 // 131 // For regions of the address space that are not backed by the 132 // Go heap, the arena map contains nil. 133 // 134 // Modifications are protected by mheap_.lock. Reads can be 135 // performed without locking; however, a given entry can 136 // transition from nil to non-nil at any time when the lock 137 // isn't held. (Entries never transitions back to nil.) 138 // 139 // In general, this is a two-level mapping consisting of an L1 140 // map and possibly many L2 maps. This saves space when there 141 // are a huge number of arena frames. However, on many 142 // platforms (even 64-bit), arenaL1Bits is 0, making this 143 // effectively a single-level map. In this case, arenas[0] 144 // will never be nil. 145 arenas [1 << arenaL1Bits]*[1 << arenaL2Bits]*heapArena 146 147 // heapArenaAlloc is pre-reserved space for allocating heapArena 148 // objects. This is only used on 32-bit, where we pre-reserve 149 // this space to avoid interleaving it with the heap itself. 150 heapArenaAlloc linearAlloc 151 152 // arenaHints is a list of addresses at which to attempt to 153 // add more heap arenas. This is initially populated with a 154 // set of general hint addresses, and grown with the bounds of 155 // actual heap arena ranges. 156 arenaHints *arenaHint 157 158 // arena is a pre-reserved space for allocating heap arenas 159 // (the actual arenas). This is only used on 32-bit. 160 arena linearAlloc 161 162 // allArenas is the arenaIndex of every mapped arena. This can 163 // be used to iterate through the address space. 164 // 165 // Access is protected by mheap_.lock. However, since this is 166 // append-only and old backing arrays are never freed, it is 167 // safe to acquire mheap_.lock, copy the slice header, and 168 // then release mheap_.lock. 169 allArenas []arenaIdx 170 171 // sweepArenas is a snapshot of allArenas taken at the 172 // beginning of the sweep cycle. This can be read safely by 173 // simply blocking GC (by disabling preemption). 174 sweepArenas []arenaIdx 175 176 // _ uint32 // ensure 64-bit alignment of central 177 178 // central free lists for small size classes. 179 // the padding makes sure that the mcentrals are 180 // spaced CacheLinePadSize bytes apart, so that each mcentral.lock 181 // gets its own cache line. 182 // central is indexed by spanClass. 183 central [numSpanClasses]struct { 184 mcentral mcentral 185 pad [cpu.CacheLinePadSize - unsafe.Sizeof(mcentral{})%cpu.CacheLinePadSize]byte 186 } 187 188 spanalloc fixalloc // allocator for span* 189 cachealloc fixalloc // allocator for mcache* 190 treapalloc fixalloc // allocator for treapNodes* 191 specialfinalizeralloc fixalloc // allocator for specialfinalizer* 192 specialprofilealloc fixalloc // allocator for specialprofile* 193 speciallock mutex // lock for special record allocators. 194 arenaHintAlloc fixalloc // allocator for arenaHints 195 196 unused *specialfinalizer // never set, just here to force the specialfinalizer type into DWARF 197 } 198 199 var mheap_ mheap 200 201 // A heapArena stores metadata for a heap arena. heapArenas are stored 202 // outside of the Go heap and accessed via the mheap_.arenas index. 203 // 204 // This gets allocated directly from the OS, so ideally it should be a 205 // multiple of the system page size. For example, avoid adding small 206 // fields. 207 // 208 //go:notinheap 209 type heapArena struct { 210 // bitmap stores the pointer/scalar bitmap for the words in 211 // this arena. See mbitmap.go for a description. Use the 212 // heapBits type to access this. 213 bitmap [heapArenaBitmapBytes]byte 214 215 // spans maps from virtual address page ID within this arena to *mspan. 216 // For allocated spans, their pages map to the span itself. 217 // For free spans, only the lowest and highest pages map to the span itself. 218 // Internal pages map to an arbitrary span. 219 // For pages that have never been allocated, spans entries are nil. 220 // 221 // Modifications are protected by mheap.lock. Reads can be 222 // performed without locking, but ONLY from indexes that are 223 // known to contain in-use or stack spans. This means there 224 // must not be a safe-point between establishing that an 225 // address is live and looking it up in the spans array. 226 spans [pagesPerArena]*mspan 227 228 // pageInUse is a bitmap that indicates which spans are in 229 // state mSpanInUse. This bitmap is indexed by page number, 230 // but only the bit corresponding to the first page in each 231 // span is used. 232 // 233 // Writes are protected by mheap_.lock. 234 pageInUse [pagesPerArena / 8]uint8 235 236 // pageMarks is a bitmap that indicates which spans have any 237 // marked objects on them. Like pageInUse, only the bit 238 // corresponding to the first page in each span is used. 239 // 240 // Writes are done atomically during marking. Reads are 241 // non-atomic and lock-free since they only occur during 242 // sweeping (and hence never race with writes). 243 // 244 // This is used to quickly find whole spans that can be freed. 245 // 246 // TODO(austin): It would be nice if this was uint64 for 247 // faster scanning, but we don't have 64-bit atomic bit 248 // operations. 249 pageMarks [pagesPerArena / 8]uint8 250 } 251 252 // arenaHint is a hint for where to grow the heap arenas. See 253 // mheap_.arenaHints. 254 // 255 //go:notinheap 256 type arenaHint struct { 257 addr uintptr 258 down bool 259 next *arenaHint 260 } 261 262 // An mspan is a run of pages. 263 // 264 // When a mspan is in the heap free treap, state == mSpanFree 265 // and heapmap(s->start) == span, heapmap(s->start+s->npages-1) == span. 266 // If the mspan is in the heap scav treap, then in addition to the 267 // above scavenged == true. scavenged == false in all other cases. 268 // 269 // When a mspan is allocated, state == mSpanInUse or mSpanManual 270 // and heapmap(i) == span for all s->start <= i < s->start+s->npages. 271 272 // Every mspan is in one doubly-linked list, either in the mheap's 273 // busy list or one of the mcentral's span lists. 274 275 // An mspan representing actual memory has state mSpanInUse, 276 // mSpanManual, or mSpanFree. Transitions between these states are 277 // constrained as follows: 278 // 279 // * A span may transition from free to in-use or manual during any GC 280 // phase. 281 // 282 // * During sweeping (gcphase == _GCoff), a span may transition from 283 // in-use to free (as a result of sweeping) or manual to free (as a 284 // result of stacks being freed). 285 // 286 // * During GC (gcphase != _GCoff), a span *must not* transition from 287 // manual or in-use to free. Because concurrent GC may read a pointer 288 // and then look up its span, the span state must be monotonic. 289 type mSpanState uint8 290 291 const ( 292 mSpanDead mSpanState = iota 293 mSpanInUse // allocated for garbage collected heap 294 mSpanManual // allocated for manual management (e.g., stack allocator) 295 mSpanFree 296 ) 297 298 // mSpanStateNames are the names of the span states, indexed by 299 // mSpanState. 300 var mSpanStateNames = []string{ 301 "mSpanDead", 302 "mSpanInUse", 303 "mSpanManual", 304 "mSpanFree", 305 } 306 307 // mSpanList heads a linked list of spans. 308 // 309 //go:notinheap 310 type mSpanList struct { 311 first *mspan // first span in list, or nil if none 312 last *mspan // last span in list, or nil if none 313 } 314 315 //go:notinheap 316 type mspan struct { 317 next *mspan // next span in list, or nil if none 318 prev *mspan // previous span in list, or nil if none 319 list *mSpanList // For debugging. TODO: Remove. 320 321 startAddr uintptr // address of first byte of span aka s.base() 322 npages uintptr // number of pages in span 323 324 manualFreeList gclinkptr // list of free objects in mSpanManual spans 325 326 // freeindex is the slot index between 0 and nelems at which to begin scanning 327 // for the next free object in this span. 328 // Each allocation scans allocBits starting at freeindex until it encounters a 0 329 // indicating a free object. freeindex is then adjusted so that subsequent scans begin 330 // just past the newly discovered free object. 331 // 332 // If freeindex == nelem, this span has no free objects. 333 // 334 // allocBits is a bitmap of objects in this span. 335 // If n >= freeindex and allocBits[n/8] & (1<<(n%8)) is 0 336 // then object n is free; 337 // otherwise, object n is allocated. Bits starting at nelem are 338 // undefined and should never be referenced. 339 // 340 // Object n starts at address n*elemsize + (start << pageShift). 341 freeindex uintptr 342 // TODO: Look up nelems from sizeclass and remove this field if it 343 // helps performance. 344 nelems uintptr // number of object in the span. 345 346 // Cache of the allocBits at freeindex. allocCache is shifted 347 // such that the lowest bit corresponds to the bit freeindex. 348 // allocCache holds the complement of allocBits, thus allowing 349 // ctz (count trailing zero) to use it directly. 350 // allocCache may contain bits beyond s.nelems; the caller must ignore 351 // these. 352 allocCache uint64 353 354 // allocBits and gcmarkBits hold pointers to a span's mark and 355 // allocation bits. The pointers are 8 byte aligned. 356 // There are three arenas where this data is held. 357 // free: Dirty arenas that are no longer accessed 358 // and can be reused. 359 // next: Holds information to be used in the next GC cycle. 360 // current: Information being used during this GC cycle. 361 // previous: Information being used during the last GC cycle. 362 // A new GC cycle starts with the call to finishsweep_m. 363 // finishsweep_m moves the previous arena to the free arena, 364 // the current arena to the previous arena, and 365 // the next arena to the current arena. 366 // The next arena is populated as the spans request 367 // memory to hold gcmarkBits for the next GC cycle as well 368 // as allocBits for newly allocated spans. 369 // 370 // The pointer arithmetic is done "by hand" instead of using 371 // arrays to avoid bounds checks along critical performance 372 // paths. 373 // The sweep will free the old allocBits and set allocBits to the 374 // gcmarkBits. The gcmarkBits are replaced with a fresh zeroed 375 // out memory. 376 allocBits *gcBits 377 gcmarkBits *gcBits 378 379 // sweep generation: 380 // if sweepgen == h->sweepgen - 2, the span needs sweeping 381 // if sweepgen == h->sweepgen - 1, the span is currently being swept 382 // if sweepgen == h->sweepgen, the span is swept and ready to use 383 // if sweepgen == h->sweepgen + 1, the span was cached before sweep began and is still cached, and needs sweeping 384 // if sweepgen == h->sweepgen + 3, the span was swept and then cached and is still cached 385 // h->sweepgen is incremented by 2 after every GC 386 387 sweepgen uint32 388 divMul uint16 // for divide by elemsize - divMagic.mul 389 baseMask uint16 // if non-0, elemsize is a power of 2, & this will get object allocation base 390 allocCount uint16 // number of allocated objects 391 spanclass spanClass // size class and noscan (uint8) 392 state mSpanState // mspaninuse etc 393 needzero uint8 // needs to be zeroed before allocation 394 divShift uint8 // for divide by elemsize - divMagic.shift 395 divShift2 uint8 // for divide by elemsize - divMagic.shift2 396 scavenged bool // whether this span has had its pages released to the OS 397 elemsize uintptr // computed from sizeclass or from npages 398 unusedsince int64 // first time spotted by gc in mspanfree state 399 limit uintptr // end of data in span 400 speciallock mutex // guards specials list 401 specials *special // linked list of special records sorted by offset. 402 } 403 404 func (s *mspan) base() uintptr { 405 return s.startAddr 406 } 407 408 func (s *mspan) layout() (size, n, total uintptr) { 409 total = s.npages << _PageShift 410 size = s.elemsize 411 if size > 0 { 412 n = total / size 413 } 414 return 415 } 416 417 // physPageBounds returns the start and end of the span 418 // rounded in to the physical page size. 419 func (s *mspan) physPageBounds() (uintptr, uintptr) { 420 start := s.base() 421 end := start + s.npages<<_PageShift 422 if physPageSize > _PageSize { 423 // Round start and end in. 424 start = (start + physPageSize - 1) &^ (physPageSize - 1) 425 end &^= physPageSize - 1 426 } 427 return start, end 428 } 429 430 func (h *mheap) coalesce(s *mspan) { 431 // We scavenge s at the end after coalescing if s or anything 432 // it merged with is marked scavenged. 433 needsScavenge := false 434 prescavenged := s.released() // number of bytes already scavenged. 435 436 // merge is a helper which merges other into s, deletes references to other 437 // in heap metadata, and then discards it. other must be adjacent to s. 438 merge := func(other *mspan) { 439 // Adjust s via base and npages and also in heap metadata. 440 s.npages += other.npages 441 s.needzero |= other.needzero 442 if other.startAddr < s.startAddr { 443 s.startAddr = other.startAddr 444 h.setSpan(s.base(), s) 445 } else { 446 h.setSpan(s.base()+s.npages*pageSize-1, s) 447 } 448 449 // If before or s are scavenged, then we need to scavenge the final coalesced span. 450 needsScavenge = needsScavenge || other.scavenged || s.scavenged 451 prescavenged += other.released() 452 453 // The size is potentially changing so the treap needs to delete adjacent nodes and 454 // insert back as a combined node. 455 if other.scavenged { 456 h.scav.removeSpan(other) 457 } else { 458 h.free.removeSpan(other) 459 } 460 other.state = mSpanDead 461 h.spanalloc.free(unsafe.Pointer(other)) 462 } 463 464 // realign is a helper which shrinks other and grows s such that their 465 // boundary is on a physical page boundary. 466 realign := func(a, b, other *mspan) { 467 // Caller must ensure a.startAddr < b.startAddr and that either a or 468 // b is s. a and b must be adjacent. other is whichever of the two is 469 // not s. 470 471 // If pageSize <= physPageSize then spans are always aligned 472 // to physical page boundaries, so just exit. 473 if pageSize <= physPageSize { 474 return 475 } 476 // Since we're resizing other, we must remove it from the treap. 477 if other.scavenged { 478 h.scav.removeSpan(other) 479 } else { 480 h.free.removeSpan(other) 481 } 482 // Round boundary to the nearest physical page size, toward the 483 // scavenged span. 484 boundary := b.startAddr 485 if a.scavenged { 486 boundary &^= (physPageSize - 1) 487 } else { 488 boundary = (boundary + physPageSize - 1) &^ (physPageSize - 1) 489 } 490 a.npages = (boundary - a.startAddr) / pageSize 491 b.npages = (b.startAddr + b.npages*pageSize - boundary) / pageSize 492 b.startAddr = boundary 493 494 h.setSpan(boundary-1, a) 495 h.setSpan(boundary, b) 496 497 // Re-insert other now that it has a new size. 498 if other.scavenged { 499 h.scav.insert(other) 500 } else { 501 h.free.insert(other) 502 } 503 } 504 505 // Coalesce with earlier, later spans. 506 if before := spanOf(s.base() - 1); before != nil && before.state == mSpanFree { 507 if s.scavenged == before.scavenged { 508 merge(before) 509 } else { 510 realign(before, s, before) 511 } 512 } 513 514 // Now check to see if next (greater addresses) span is free and can be coalesced. 515 if after := spanOf(s.base() + s.npages*pageSize); after != nil && after.state == mSpanFree { 516 if s.scavenged == after.scavenged { 517 merge(after) 518 } else { 519 realign(s, after, after) 520 } 521 } 522 523 if needsScavenge { 524 // When coalescing spans, some physical pages which 525 // were not returned to the OS previously because 526 // they were only partially covered by the span suddenly 527 // become available for scavenging. We want to make sure 528 // those holes are filled in, and the span is properly 529 // scavenged. Rather than trying to detect those holes 530 // directly, we collect how many bytes were already 531 // scavenged above and subtract that from heap_released 532 // before re-scavenging the entire newly-coalesced span, 533 // which will implicitly bump up heap_released. 534 memstats.heap_released -= uint64(prescavenged) 535 s.scavenge() 536 } 537 } 538 539 func (s *mspan) scavenge() uintptr { 540 // start and end must be rounded in, otherwise madvise 541 // will round them *out* and release more memory 542 // than we want. 543 start, end := s.physPageBounds() 544 if end <= start { 545 // start and end don't span a whole physical page. 546 return 0 547 } 548 released := end - start 549 memstats.heap_released += uint64(released) 550 s.scavenged = true 551 sysUnused(unsafe.Pointer(start), released) 552 return released 553 } 554 555 // released returns the number of bytes in this span 556 // which were returned back to the OS. 557 func (s *mspan) released() uintptr { 558 if !s.scavenged { 559 return 0 560 } 561 start, end := s.physPageBounds() 562 return end - start 563 } 564 565 // recordspan adds a newly allocated span to h.allspans. 566 // 567 // This only happens the first time a span is allocated from 568 // mheap.spanalloc (it is not called when a span is reused). 569 // 570 // Write barriers are disallowed here because it can be called from 571 // gcWork when allocating new workbufs. However, because it's an 572 // indirect call from the fixalloc initializer, the compiler can't see 573 // this. 574 // 575 //go:nowritebarrierrec 576 func recordspan(vh unsafe.Pointer, p unsafe.Pointer) { 577 h := (*mheap)(vh) 578 s := (*mspan)(p) 579 if len(h.allspans) >= cap(h.allspans) { 580 n := 64 * 1024 / sys.PtrSize 581 if n < cap(h.allspans)*3/2 { 582 n = cap(h.allspans) * 3 / 2 583 } 584 var new []*mspan 585 sp := (*slice)(unsafe.Pointer(&new)) 586 sp.array = sysAlloc(uintptr(n)*sys.PtrSize, &memstats.other_sys) 587 if sp.array == nil { 588 throw("runtime: cannot allocate memory") 589 } 590 sp.len = len(h.allspans) 591 sp.cap = n 592 if len(h.allspans) > 0 { 593 copy(new, h.allspans) 594 } 595 oldAllspans := h.allspans 596 *(*notInHeapSlice)(unsafe.Pointer(&h.allspans)) = *(*notInHeapSlice)(unsafe.Pointer(&new)) 597 if len(oldAllspans) != 0 { 598 sysFree(unsafe.Pointer(&oldAllspans[0]), uintptr(cap(oldAllspans))*unsafe.Sizeof(oldAllspans[0]), &memstats.other_sys) 599 } 600 } 601 h.allspans = h.allspans[:len(h.allspans)+1] 602 h.allspans[len(h.allspans)-1] = s 603 } 604 605 // A spanClass represents the size class and noscan-ness of a span. 606 // 607 // Each size class has a noscan spanClass and a scan spanClass. The 608 // noscan spanClass contains only noscan objects, which do not contain 609 // pointers and thus do not need to be scanned by the garbage 610 // collector. 611 type spanClass uint8 612 613 const ( 614 numSpanClasses = _NumSizeClasses << 1 615 tinySpanClass = spanClass(tinySizeClass<<1 | 1) 616 ) 617 618 func makeSpanClass(sizeclass uint8, noscan bool) spanClass { 619 return spanClass(sizeclass<<1) | spanClass(bool2int(noscan)) 620 } 621 622 func (sc spanClass) sizeclass() int8 { 623 return int8(sc >> 1) 624 } 625 626 func (sc spanClass) noscan() bool { 627 return sc&1 != 0 628 } 629 630 // arenaIndex returns the index into mheap_.arenas of the arena 631 // containing metadata for p. This index combines of an index into the 632 // L1 map and an index into the L2 map and should be used as 633 // mheap_.arenas[ai.l1()][ai.l2()]. 634 // 635 // If p is outside the range of valid heap addresses, either l1() or 636 // l2() will be out of bounds. 637 // 638 // It is nosplit because it's called by spanOf and several other 639 // nosplit functions. 640 // 641 //go:nosplit 642 func arenaIndex(p uintptr) arenaIdx { 643 return arenaIdx((p + arenaBaseOffset) / heapArenaBytes) 644 } 645 646 // arenaBase returns the low address of the region covered by heap 647 // arena i. 648 func arenaBase(i arenaIdx) uintptr { 649 return uintptr(i)*heapArenaBytes - arenaBaseOffset 650 } 651 652 type arenaIdx uint 653 654 func (i arenaIdx) l1() uint { 655 if arenaL1Bits == 0 { 656 // Let the compiler optimize this away if there's no 657 // L1 map. 658 return 0 659 } else { 660 return uint(i) >> arenaL1Shift 661 } 662 } 663 664 func (i arenaIdx) l2() uint { 665 if arenaL1Bits == 0 { 666 return uint(i) 667 } else { 668 return uint(i) & (1<<arenaL2Bits - 1) 669 } 670 } 671 672 // inheap reports whether b is a pointer into a (potentially dead) heap object. 673 // It returns false for pointers into mSpanManual spans. 674 // Non-preemptible because it is used by write barriers. 675 //go:nowritebarrier 676 //go:nosplit 677 func inheap(b uintptr) bool { 678 return spanOfHeap(b) != nil 679 } 680 681 // inHeapOrStack is a variant of inheap that returns true for pointers 682 // into any allocated heap span. 683 // 684 //go:nowritebarrier 685 //go:nosplit 686 func inHeapOrStack(b uintptr) bool { 687 s := spanOf(b) 688 if s == nil || b < s.base() { 689 return false 690 } 691 switch s.state { 692 case mSpanInUse, mSpanManual: 693 return b < s.limit 694 default: 695 return false 696 } 697 } 698 699 // spanOf returns the span of p. If p does not point into the heap 700 // arena or no span has ever contained p, spanOf returns nil. 701 // 702 // If p does not point to allocated memory, this may return a non-nil 703 // span that does *not* contain p. If this is a possibility, the 704 // caller should either call spanOfHeap or check the span bounds 705 // explicitly. 706 // 707 // Must be nosplit because it has callers that are nosplit. 708 // 709 //go:nosplit 710 func spanOf(p uintptr) *mspan { 711 // This function looks big, but we use a lot of constant 712 // folding around arenaL1Bits to get it under the inlining 713 // budget. Also, many of the checks here are safety checks 714 // that Go needs to do anyway, so the generated code is quite 715 // short. 716 ri := arenaIndex(p) 717 if arenaL1Bits == 0 { 718 // If there's no L1, then ri.l1() can't be out of bounds but ri.l2() can. 719 if ri.l2() >= uint(len(mheap_.arenas[0])) { 720 return nil 721 } 722 } else { 723 // If there's an L1, then ri.l1() can be out of bounds but ri.l2() can't. 724 if ri.l1() >= uint(len(mheap_.arenas)) { 725 return nil 726 } 727 } 728 l2 := mheap_.arenas[ri.l1()] 729 if arenaL1Bits != 0 && l2 == nil { // Should never happen if there's no L1. 730 return nil 731 } 732 ha := l2[ri.l2()] 733 if ha == nil { 734 return nil 735 } 736 return ha.spans[(p/pageSize)%pagesPerArena] 737 } 738 739 // spanOfUnchecked is equivalent to spanOf, but the caller must ensure 740 // that p points into an allocated heap arena. 741 // 742 // Must be nosplit because it has callers that are nosplit. 743 // 744 //go:nosplit 745 func spanOfUnchecked(p uintptr) *mspan { 746 ai := arenaIndex(p) 747 return mheap_.arenas[ai.l1()][ai.l2()].spans[(p/pageSize)%pagesPerArena] 748 } 749 750 // spanOfHeap is like spanOf, but returns nil if p does not point to a 751 // heap object. 752 // 753 // Must be nosplit because it has callers that are nosplit. 754 // 755 //go:nosplit 756 func spanOfHeap(p uintptr) *mspan { 757 s := spanOf(p) 758 // If p is not allocated, it may point to a stale span, so we 759 // have to check the span's bounds and state. 760 if s == nil || p < s.base() || p >= s.limit || s.state != mSpanInUse { 761 return nil 762 } 763 return s 764 } 765 766 // pageIndexOf returns the arena, page index, and page mask for pointer p. 767 // The caller must ensure p is in the heap. 768 func pageIndexOf(p uintptr) (arena *heapArena, pageIdx uintptr, pageMask uint8) { 769 ai := arenaIndex(p) 770 arena = mheap_.arenas[ai.l1()][ai.l2()] 771 pageIdx = ((p / pageSize) / 8) % uintptr(len(arena.pageInUse)) 772 pageMask = byte(1 << ((p / pageSize) % 8)) 773 return 774 } 775 776 // Initialize the heap. 777 func (h *mheap) init() { 778 h.treapalloc.init(unsafe.Sizeof(treapNode{}), nil, nil, &memstats.other_sys) 779 h.spanalloc.init(unsafe.Sizeof(mspan{}), recordspan, unsafe.Pointer(h), &memstats.mspan_sys) 780 h.cachealloc.init(unsafe.Sizeof(mcache{}), nil, nil, &memstats.mcache_sys) 781 h.specialfinalizeralloc.init(unsafe.Sizeof(specialfinalizer{}), nil, nil, &memstats.other_sys) 782 h.specialprofilealloc.init(unsafe.Sizeof(specialprofile{}), nil, nil, &memstats.other_sys) 783 h.arenaHintAlloc.init(unsafe.Sizeof(arenaHint{}), nil, nil, &memstats.other_sys) 784 785 // Don't zero mspan allocations. Background sweeping can 786 // inspect a span concurrently with allocating it, so it's 787 // important that the span's sweepgen survive across freeing 788 // and re-allocating a span to prevent background sweeping 789 // from improperly cas'ing it from 0. 790 // 791 // This is safe because mspan contains no heap pointers. 792 h.spanalloc.zero = false 793 794 // h->mapcache needs no init 795 796 for i := range h.central { 797 h.central[i].mcentral.init(spanClass(i)) 798 } 799 } 800 801 // reclaim sweeps and reclaims at least npage pages into the heap. 802 // It is called before allocating npage pages to keep growth in check. 803 // 804 // reclaim implements the page-reclaimer half of the sweeper. 805 // 806 // h must NOT be locked. 807 func (h *mheap) reclaim(npage uintptr) { 808 // This scans pagesPerChunk at a time. Higher values reduce 809 // contention on h.reclaimPos, but increase the minimum 810 // latency of performing a reclaim. 811 // 812 // Must be a multiple of the pageInUse bitmap element size. 813 // 814 // The time required by this can vary a lot depending on how 815 // many spans are actually freed. Experimentally, it can scan 816 // for pages at ~300 GB/ms on a 2.6GHz Core i7, but can only 817 // free spans at ~32 MB/ms. Using 512 pages bounds this at 818 // roughly 100µs. 819 // 820 // TODO(austin): Half of the time spent freeing spans is in 821 // locking/unlocking the heap (even with low contention). We 822 // could make the slow path here several times faster by 823 // batching heap frees. 824 const pagesPerChunk = 512 825 826 // Bail early if there's no more reclaim work. 827 if atomic.Load64(&h.reclaimIndex) >= 1<<63 { 828 return 829 } 830 831 // Disable preemption so the GC can't start while we're 832 // sweeping, so we can read h.sweepArenas, and so 833 // traceGCSweepStart/Done pair on the P. 834 mp := acquirem() 835 836 if trace.enabled { 837 traceGCSweepStart() 838 } 839 840 arenas := h.sweepArenas 841 locked := false 842 for npage > 0 { 843 // Pull from accumulated credit first. 844 if credit := atomic.Loaduintptr(&h.reclaimCredit); credit > 0 { 845 take := credit 846 if take > npage { 847 // Take only what we need. 848 take = npage 849 } 850 if atomic.Casuintptr(&h.reclaimCredit, credit, credit-take) { 851 npage -= take 852 } 853 continue 854 } 855 856 // Claim a chunk of work. 857 idx := uintptr(atomic.Xadd64(&h.reclaimIndex, pagesPerChunk) - pagesPerChunk) 858 if idx/pagesPerArena >= uintptr(len(arenas)) { 859 // Page reclaiming is done. 860 atomic.Store64(&h.reclaimIndex, 1<<63) 861 break 862 } 863 864 if !locked { 865 // Lock the heap for reclaimChunk. 866 lock(&h.lock) 867 locked = true 868 } 869 870 // Scan this chunk. 871 nfound := h.reclaimChunk(arenas, idx, pagesPerChunk) 872 if nfound <= npage { 873 npage -= nfound 874 } else { 875 // Put spare pages toward global credit. 876 atomic.Xadduintptr(&h.reclaimCredit, nfound-npage) 877 npage = 0 878 } 879 } 880 if locked { 881 unlock(&h.lock) 882 } 883 884 if trace.enabled { 885 traceGCSweepDone() 886 } 887 releasem(mp) 888 } 889 890 // reclaimChunk sweeps unmarked spans that start at page indexes [pageIdx, pageIdx+n). 891 // It returns the number of pages returned to the heap. 892 // 893 // h.lock must be held and the caller must be non-preemptible. 894 func (h *mheap) reclaimChunk(arenas []arenaIdx, pageIdx, n uintptr) uintptr { 895 // The heap lock must be held because this accesses the 896 // heapArena.spans arrays using potentially non-live pointers. 897 // In particular, if a span were freed and merged concurrently 898 // with this probing heapArena.spans, it would be possible to 899 // observe arbitrary, stale span pointers. 900 n0 := n 901 var nFreed uintptr 902 sg := h.sweepgen 903 for n > 0 { 904 ai := arenas[pageIdx/pagesPerArena] 905 ha := h.arenas[ai.l1()][ai.l2()] 906 907 // Get a chunk of the bitmap to work on. 908 arenaPage := uint(pageIdx % pagesPerArena) 909 inUse := ha.pageInUse[arenaPage/8:] 910 marked := ha.pageMarks[arenaPage/8:] 911 if uintptr(len(inUse)) > n/8 { 912 inUse = inUse[:n/8] 913 marked = marked[:n/8] 914 } 915 916 // Scan this bitmap chunk for spans that are in-use 917 // but have no marked objects on them. 918 for i := range inUse { 919 inUseUnmarked := inUse[i] &^ marked[i] 920 if inUseUnmarked == 0 { 921 continue 922 } 923 924 for j := uint(0); j < 8; j++ { 925 if inUseUnmarked&(1<<j) != 0 { 926 s := ha.spans[arenaPage+uint(i)*8+j] 927 if atomic.Load(&s.sweepgen) == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) { 928 npages := s.npages 929 unlock(&h.lock) 930 if s.sweep(false) { 931 nFreed += npages 932 } 933 lock(&h.lock) 934 // Reload inUse. It's possible nearby 935 // spans were freed when we dropped the 936 // lock and we don't want to get stale 937 // pointers from the spans array. 938 inUseUnmarked = inUse[i] &^ marked[i] 939 } 940 } 941 } 942 } 943 944 // Advance. 945 pageIdx += uintptr(len(inUse) * 8) 946 n -= uintptr(len(inUse) * 8) 947 } 948 if trace.enabled { 949 // Account for pages scanned but not reclaimed. 950 traceGCSweepSpan((n0 - nFreed) * pageSize) 951 } 952 return nFreed 953 } 954 955 // alloc_m is the internal implementation of mheap.alloc. 956 // 957 // alloc_m must run on the system stack because it locks the heap, so 958 // any stack growth during alloc_m would self-deadlock. 959 // 960 //go:systemstack 961 func (h *mheap) alloc_m(npage uintptr, spanclass spanClass, large bool) *mspan { 962 _g_ := getg() 963 964 // To prevent excessive heap growth, before allocating n pages 965 // we need to sweep and reclaim at least n pages. 966 if h.sweepdone == 0 { 967 h.reclaim(npage) 968 } 969 970 lock(&h.lock) 971 // transfer stats from cache to global 972 memstats.heap_scan += uint64(_g_.m.mcache.local_scan) 973 _g_.m.mcache.local_scan = 0 974 memstats.tinyallocs += uint64(_g_.m.mcache.local_tinyallocs) 975 _g_.m.mcache.local_tinyallocs = 0 976 977 s := h.allocSpanLocked(npage, &memstats.heap_inuse) 978 if s != nil { 979 // Record span info, because gc needs to be 980 // able to map interior pointer to containing span. 981 atomic.Store(&s.sweepgen, h.sweepgen) 982 h.sweepSpans[h.sweepgen/2%2].push(s) // Add to swept in-use list. 983 s.state = mSpanInUse 984 s.allocCount = 0 985 s.spanclass = spanclass 986 if sizeclass := spanclass.sizeclass(); sizeclass == 0 { 987 s.elemsize = s.npages << _PageShift 988 s.divShift = 0 989 s.divMul = 0 990 s.divShift2 = 0 991 s.baseMask = 0 992 } else { 993 s.elemsize = uintptr(class_to_size[sizeclass]) 994 m := &class_to_divmagic[sizeclass] 995 s.divShift = m.shift 996 s.divMul = m.mul 997 s.divShift2 = m.shift2 998 s.baseMask = m.baseMask 999 } 1000 1001 // Mark in-use span in arena page bitmap. 1002 arena, pageIdx, pageMask := pageIndexOf(s.base()) 1003 arena.pageInUse[pageIdx] |= pageMask 1004 1005 // update stats, sweep lists 1006 h.pagesInUse += uint64(npage) 1007 if large { 1008 memstats.heap_objects++ 1009 mheap_.largealloc += uint64(s.elemsize) 1010 mheap_.nlargealloc++ 1011 atomic.Xadd64(&memstats.heap_live, int64(npage<<_PageShift)) 1012 } 1013 } 1014 // heap_scan and heap_live were updated. 1015 if gcBlackenEnabled != 0 { 1016 gcController.revise() 1017 } 1018 1019 if trace.enabled { 1020 traceHeapAlloc() 1021 } 1022 1023 // h.spans is accessed concurrently without synchronization 1024 // from other threads. Hence, there must be a store/store 1025 // barrier here to ensure the writes to h.spans above happen 1026 // before the caller can publish a pointer p to an object 1027 // allocated from s. As soon as this happens, the garbage 1028 // collector running on another processor could read p and 1029 // look up s in h.spans. The unlock acts as the barrier to 1030 // order these writes. On the read side, the data dependency 1031 // between p and the index in h.spans orders the reads. 1032 unlock(&h.lock) 1033 return s 1034 } 1035 1036 // alloc allocates a new span of npage pages from the GC'd heap. 1037 // 1038 // Either large must be true or spanclass must indicates the span's 1039 // size class and scannability. 1040 // 1041 // If needzero is true, the memory for the returned span will be zeroed. 1042 func (h *mheap) alloc(npage uintptr, spanclass spanClass, large bool, needzero bool) *mspan { 1043 // Don't do any operations that lock the heap on the G stack. 1044 // It might trigger stack growth, and the stack growth code needs 1045 // to be able to allocate heap. 1046 var s *mspan 1047 systemstack(func() { 1048 s = h.alloc_m(npage, spanclass, large) 1049 }) 1050 1051 if s != nil { 1052 if needzero && s.needzero != 0 { 1053 memclrNoHeapPointers(unsafe.Pointer(s.base()), s.npages<<_PageShift) 1054 } 1055 s.needzero = 0 1056 } 1057 return s 1058 } 1059 1060 // allocManual allocates a manually-managed span of npage pages. 1061 // allocManual returns nil if allocation fails. 1062 // 1063 // allocManual adds the bytes used to *stat, which should be a 1064 // memstats in-use field. Unlike allocations in the GC'd heap, the 1065 // allocation does *not* count toward heap_inuse or heap_sys. 1066 // 1067 // The memory backing the returned span may not be zeroed if 1068 // span.needzero is set. 1069 // 1070 // allocManual must be called on the system stack to prevent stack 1071 // growth. Since this is used by the stack allocator, stack growth 1072 // during allocManual would self-deadlock. 1073 // 1074 //go:systemstack 1075 func (h *mheap) allocManual(npage uintptr, stat *uint64) *mspan { 1076 lock(&h.lock) 1077 s := h.allocSpanLocked(npage, stat) 1078 if s != nil { 1079 s.state = mSpanManual 1080 s.manualFreeList = 0 1081 s.allocCount = 0 1082 s.spanclass = 0 1083 s.nelems = 0 1084 s.elemsize = 0 1085 s.limit = s.base() + s.npages<<_PageShift 1086 // Manually managed memory doesn't count toward heap_sys. 1087 memstats.heap_sys -= uint64(s.npages << _PageShift) 1088 } 1089 1090 // This unlock acts as a release barrier. See mheap.alloc_m. 1091 unlock(&h.lock) 1092 1093 return s 1094 } 1095 1096 // setSpan modifies the span map so spanOf(base) is s. 1097 func (h *mheap) setSpan(base uintptr, s *mspan) { 1098 ai := arenaIndex(base) 1099 h.arenas[ai.l1()][ai.l2()].spans[(base/pageSize)%pagesPerArena] = s 1100 } 1101 1102 // setSpans modifies the span map so [spanOf(base), spanOf(base+npage*pageSize)) 1103 // is s. 1104 func (h *mheap) setSpans(base, npage uintptr, s *mspan) { 1105 p := base / pageSize 1106 ai := arenaIndex(base) 1107 ha := h.arenas[ai.l1()][ai.l2()] 1108 for n := uintptr(0); n < npage; n++ { 1109 i := (p + n) % pagesPerArena 1110 if i == 0 { 1111 ai = arenaIndex(base + n*pageSize) 1112 ha = h.arenas[ai.l1()][ai.l2()] 1113 } 1114 ha.spans[i] = s 1115 } 1116 } 1117 1118 // pickFreeSpan acquires a free span from internal free list 1119 // structures if one is available. Otherwise returns nil. 1120 // h must be locked. 1121 func (h *mheap) pickFreeSpan(npage uintptr) *mspan { 1122 tf := h.free.find(npage) 1123 ts := h.scav.find(npage) 1124 1125 // Check for whichever treap gave us the smaller, non-nil result. 1126 // Note that we want the _smaller_ free span, i.e. the free span 1127 // closer in size to the amount we requested (npage). 1128 var s *mspan 1129 if tf != nil && (ts == nil || tf.spanKey.npages <= ts.spanKey.npages) { 1130 s = tf.spanKey 1131 h.free.removeNode(tf) 1132 } else if ts != nil && (tf == nil || tf.spanKey.npages > ts.spanKey.npages) { 1133 s = ts.spanKey 1134 h.scav.removeNode(ts) 1135 } 1136 return s 1137 } 1138 1139 // Allocates a span of the given size. h must be locked. 1140 // The returned span has been removed from the 1141 // free structures, but its state is still mSpanFree. 1142 func (h *mheap) allocSpanLocked(npage uintptr, stat *uint64) *mspan { 1143 var s *mspan 1144 1145 s = h.pickFreeSpan(npage) 1146 if s != nil { 1147 goto HaveSpan 1148 } 1149 // On failure, grow the heap and try again. 1150 if !h.grow(npage) { 1151 return nil 1152 } 1153 s = h.pickFreeSpan(npage) 1154 if s != nil { 1155 goto HaveSpan 1156 } 1157 throw("grew heap, but no adequate free span found") 1158 1159 HaveSpan: 1160 // Mark span in use. 1161 if s.state != mSpanFree { 1162 throw("candidate mspan for allocation is not free") 1163 } 1164 if s.npages < npage { 1165 throw("candidate mspan for allocation is too small") 1166 } 1167 1168 // First, subtract any memory that was released back to 1169 // the OS from s. We will re-scavenge the trimmed section 1170 // if necessary. 1171 memstats.heap_released -= uint64(s.released()) 1172 1173 if s.npages > npage { 1174 // Trim extra and put it back in the heap. 1175 t := (*mspan)(h.spanalloc.alloc()) 1176 t.init(s.base()+npage<<_PageShift, s.npages-npage) 1177 s.npages = npage 1178 h.setSpan(t.base()-1, s) 1179 h.setSpan(t.base(), t) 1180 h.setSpan(t.base()+t.npages*pageSize-1, t) 1181 t.needzero = s.needzero 1182 // If s was scavenged, then t may be scavenged. 1183 start, end := t.physPageBounds() 1184 if s.scavenged && start < end { 1185 memstats.heap_released += uint64(end - start) 1186 t.scavenged = true 1187 } 1188 s.state = mSpanManual // prevent coalescing with s 1189 t.state = mSpanManual 1190 h.freeSpanLocked(t, false, false, s.unusedsince) 1191 s.state = mSpanFree 1192 } 1193 // "Unscavenge" s only AFTER splitting so that 1194 // we only sysUsed whatever we actually need. 1195 if s.scavenged { 1196 // sysUsed all the pages that are actually available 1197 // in the span. Note that we don't need to decrement 1198 // heap_released since we already did so earlier. 1199 sysUsed(unsafe.Pointer(s.base()), s.npages<<_PageShift) 1200 s.scavenged = false 1201 } 1202 s.unusedsince = 0 1203 1204 h.setSpans(s.base(), npage, s) 1205 1206 *stat += uint64(npage << _PageShift) 1207 memstats.heap_idle -= uint64(npage << _PageShift) 1208 1209 //println("spanalloc", hex(s.start<<_PageShift)) 1210 if s.inList() { 1211 throw("still in list") 1212 } 1213 return s 1214 } 1215 1216 // Try to add at least npage pages of memory to the heap, 1217 // returning whether it worked. 1218 // 1219 // h must be locked. 1220 func (h *mheap) grow(npage uintptr) bool { 1221 ask := npage << _PageShift 1222 v, size := h.sysAlloc(ask) 1223 if v == nil { 1224 print("runtime: out of memory: cannot allocate ", ask, "-byte block (", memstats.heap_sys, " in use)\n") 1225 return false 1226 } 1227 1228 // Scavenge some pages out of the free treap to make up for 1229 // the virtual memory space we just allocated. We prefer to 1230 // scavenge the largest spans first since the cost of scavenging 1231 // is proportional to the number of sysUnused() calls rather than 1232 // the number of pages released, so we make fewer of those calls 1233 // with larger spans. 1234 h.scavengeLargest(size) 1235 1236 // Create a fake "in use" span and free it, so that the 1237 // right coalescing happens. 1238 s := (*mspan)(h.spanalloc.alloc()) 1239 s.init(uintptr(v), size/pageSize) 1240 h.setSpans(s.base(), s.npages, s) 1241 atomic.Store(&s.sweepgen, h.sweepgen) 1242 s.state = mSpanInUse 1243 h.pagesInUse += uint64(s.npages) 1244 h.freeSpanLocked(s, false, true, 0) 1245 return true 1246 } 1247 1248 // Free the span back into the heap. 1249 // 1250 // large must match the value of large passed to mheap.alloc. This is 1251 // used for accounting. 1252 func (h *mheap) freeSpan(s *mspan, large bool) { 1253 systemstack(func() { 1254 mp := getg().m 1255 lock(&h.lock) 1256 memstats.heap_scan += uint64(mp.mcache.local_scan) 1257 mp.mcache.local_scan = 0 1258 memstats.tinyallocs += uint64(mp.mcache.local_tinyallocs) 1259 mp.mcache.local_tinyallocs = 0 1260 if msanenabled { 1261 // Tell msan that this entire span is no longer in use. 1262 base := unsafe.Pointer(s.base()) 1263 bytes := s.npages << _PageShift 1264 msanfree(base, bytes) 1265 } 1266 if large { 1267 // Match accounting done in mheap.alloc. 1268 memstats.heap_objects-- 1269 } 1270 if gcBlackenEnabled != 0 { 1271 // heap_scan changed. 1272 gcController.revise() 1273 } 1274 h.freeSpanLocked(s, true, true, 0) 1275 unlock(&h.lock) 1276 }) 1277 } 1278 1279 // freeManual frees a manually-managed span returned by allocManual. 1280 // stat must be the same as the stat passed to the allocManual that 1281 // allocated s. 1282 // 1283 // This must only be called when gcphase == _GCoff. See mSpanState for 1284 // an explanation. 1285 // 1286 // freeManual must be called on the system stack to prevent stack 1287 // growth, just like allocManual. 1288 // 1289 //go:systemstack 1290 func (h *mheap) freeManual(s *mspan, stat *uint64) { 1291 s.needzero = 1 1292 lock(&h.lock) 1293 *stat -= uint64(s.npages << _PageShift) 1294 memstats.heap_sys += uint64(s.npages << _PageShift) 1295 h.freeSpanLocked(s, false, true, 0) 1296 unlock(&h.lock) 1297 } 1298 1299 // s must be on the busy list or unlinked. 1300 func (h *mheap) freeSpanLocked(s *mspan, acctinuse, acctidle bool, unusedsince int64) { 1301 switch s.state { 1302 case mSpanManual: 1303 if s.allocCount != 0 { 1304 throw("mheap.freeSpanLocked - invalid stack free") 1305 } 1306 case mSpanInUse: 1307 if s.allocCount != 0 || s.sweepgen != h.sweepgen { 1308 print("mheap.freeSpanLocked - span ", s, " ptr ", hex(s.base()), " allocCount ", s.allocCount, " sweepgen ", s.sweepgen, "/", h.sweepgen, "\n") 1309 throw("mheap.freeSpanLocked - invalid free") 1310 } 1311 h.pagesInUse -= uint64(s.npages) 1312 1313 // Clear in-use bit in arena page bitmap. 1314 arena, pageIdx, pageMask := pageIndexOf(s.base()) 1315 arena.pageInUse[pageIdx] &^= pageMask 1316 default: 1317 throw("mheap.freeSpanLocked - invalid span state") 1318 } 1319 1320 if acctinuse { 1321 memstats.heap_inuse -= uint64(s.npages << _PageShift) 1322 } 1323 if acctidle { 1324 memstats.heap_idle += uint64(s.npages << _PageShift) 1325 } 1326 s.state = mSpanFree 1327 1328 // Stamp newly unused spans. The scavenger will use that 1329 // info to potentially give back some pages to the OS. 1330 s.unusedsince = unusedsince 1331 if unusedsince == 0 { 1332 s.unusedsince = nanotime() 1333 } 1334 1335 // Coalesce span with neighbors. 1336 h.coalesce(s) 1337 1338 // Insert s into the appropriate treap. 1339 if s.scavenged { 1340 h.scav.insert(s) 1341 } else { 1342 h.free.insert(s) 1343 } 1344 } 1345 1346 // scavengeLargest scavenges nbytes worth of spans in unscav 1347 // starting from the largest span and working down. It then takes those spans 1348 // and places them in scav. h must be locked. 1349 func (h *mheap) scavengeLargest(nbytes uintptr) { 1350 // Use up scavenge credit if there's any available. 1351 if nbytes > h.scavengeCredit { 1352 nbytes -= h.scavengeCredit 1353 h.scavengeCredit = 0 1354 } else { 1355 h.scavengeCredit -= nbytes 1356 return 1357 } 1358 // Iterate over the treap backwards (from largest to smallest) scavenging spans 1359 // until we've reached our quota of nbytes. 1360 released := uintptr(0) 1361 for t := h.free.end(); released < nbytes && t.valid(); { 1362 s := t.span() 1363 r := s.scavenge() 1364 if r == 0 { 1365 // Since we're going in order of largest-to-smallest span, this 1366 // means all other spans are no bigger than s. There's a high 1367 // chance that the other spans don't even cover a full page, 1368 // (though they could) but iterating further just for a handful 1369 // of pages probably isn't worth it, so just stop here. 1370 // 1371 // This check also preserves the invariant that spans that have 1372 // `scavenged` set are only ever in the `scav` treap, and 1373 // those which have it unset are only in the `free` treap. 1374 return 1375 } 1376 n := t.prev() 1377 h.free.erase(t) 1378 // Now that s is scavenged, we must eagerly coalesce it 1379 // with its neighbors to prevent having two spans with 1380 // the same scavenged state adjacent to each other. 1381 h.coalesce(s) 1382 t = n 1383 h.scav.insert(s) 1384 released += r 1385 } 1386 // If we over-scavenged, turn that extra amount into credit. 1387 if released > nbytes { 1388 h.scavengeCredit += released - nbytes 1389 } 1390 } 1391 1392 // scavengeAll visits each node in the unscav treap and scavenges the 1393 // treapNode's span. It then removes the scavenged span from 1394 // unscav and adds it into scav before continuing. h must be locked. 1395 func (h *mheap) scavengeAll(now, limit uint64) uintptr { 1396 // Iterate over the treap scavenging spans if unused for at least limit time. 1397 released := uintptr(0) 1398 for t := h.free.start(); t.valid(); { 1399 s := t.span() 1400 n := t.next() 1401 if (now - uint64(s.unusedsince)) > limit { 1402 r := s.scavenge() 1403 if r != 0 { 1404 h.free.erase(t) 1405 // Now that s is scavenged, we must eagerly coalesce it 1406 // with its neighbors to prevent having two spans with 1407 // the same scavenged state adjacent to each other. 1408 h.coalesce(s) 1409 h.scav.insert(s) 1410 released += r 1411 } 1412 } 1413 t = n 1414 } 1415 return released 1416 } 1417 1418 func (h *mheap) scavenge(k int32, now, limit uint64) { 1419 // Disallow malloc or panic while holding the heap lock. We do 1420 // this here because this is an non-mallocgc entry-point to 1421 // the mheap API. 1422 gp := getg() 1423 gp.m.mallocing++ 1424 lock(&h.lock) 1425 released := h.scavengeAll(now, limit) 1426 unlock(&h.lock) 1427 gp.m.mallocing-- 1428 1429 if debug.gctrace > 0 { 1430 if released > 0 { 1431 print("scvg", k, ": ", released>>20, " MB released\n") 1432 } 1433 print("scvg", k, ": inuse: ", memstats.heap_inuse>>20, ", idle: ", memstats.heap_idle>>20, ", sys: ", memstats.heap_sys>>20, ", released: ", memstats.heap_released>>20, ", consumed: ", (memstats.heap_sys-memstats.heap_released)>>20, " (MB)\n") 1434 } 1435 } 1436 1437 //go:linkname runtime_debug_freeOSMemory runtime/debug.freeOSMemory 1438 func runtime_debug_freeOSMemory() { 1439 GC() 1440 systemstack(func() { mheap_.scavenge(-1, ^uint64(0), 0) }) 1441 } 1442 1443 // Initialize a new span with the given start and npages. 1444 func (span *mspan) init(base uintptr, npages uintptr) { 1445 // span is *not* zeroed. 1446 span.next = nil 1447 span.prev = nil 1448 span.list = nil 1449 span.startAddr = base 1450 span.npages = npages 1451 span.allocCount = 0 1452 span.spanclass = 0 1453 span.elemsize = 0 1454 span.state = mSpanDead 1455 span.unusedsince = 0 1456 span.scavenged = false 1457 span.speciallock.key = 0 1458 span.specials = nil 1459 span.needzero = 0 1460 span.freeindex = 0 1461 span.allocBits = nil 1462 span.gcmarkBits = nil 1463 } 1464 1465 func (span *mspan) inList() bool { 1466 return span.list != nil 1467 } 1468 1469 // Initialize an empty doubly-linked list. 1470 func (list *mSpanList) init() { 1471 list.first = nil 1472 list.last = nil 1473 } 1474 1475 func (list *mSpanList) remove(span *mspan) { 1476 if span.list != list { 1477 print("runtime: failed mSpanList.remove span.npages=", span.npages, 1478 " span=", span, " prev=", span.prev, " span.list=", span.list, " list=", list, "\n") 1479 throw("mSpanList.remove") 1480 } 1481 if list.first == span { 1482 list.first = span.next 1483 } else { 1484 span.prev.next = span.next 1485 } 1486 if list.last == span { 1487 list.last = span.prev 1488 } else { 1489 span.next.prev = span.prev 1490 } 1491 span.next = nil 1492 span.prev = nil 1493 span.list = nil 1494 } 1495 1496 func (list *mSpanList) isEmpty() bool { 1497 return list.first == nil 1498 } 1499 1500 func (list *mSpanList) insert(span *mspan) { 1501 if span.next != nil || span.prev != nil || span.list != nil { 1502 println("runtime: failed mSpanList.insert", span, span.next, span.prev, span.list) 1503 throw("mSpanList.insert") 1504 } 1505 span.next = list.first 1506 if list.first != nil { 1507 // The list contains at least one span; link it in. 1508 // The last span in the list doesn't change. 1509 list.first.prev = span 1510 } else { 1511 // The list contains no spans, so this is also the last span. 1512 list.last = span 1513 } 1514 list.first = span 1515 span.list = list 1516 } 1517 1518 func (list *mSpanList) insertBack(span *mspan) { 1519 if span.next != nil || span.prev != nil || span.list != nil { 1520 println("runtime: failed mSpanList.insertBack", span, span.next, span.prev, span.list) 1521 throw("mSpanList.insertBack") 1522 } 1523 span.prev = list.last 1524 if list.last != nil { 1525 // The list contains at least one span. 1526 list.last.next = span 1527 } else { 1528 // The list contains no spans, so this is also the first span. 1529 list.first = span 1530 } 1531 list.last = span 1532 span.list = list 1533 } 1534 1535 // takeAll removes all spans from other and inserts them at the front 1536 // of list. 1537 func (list *mSpanList) takeAll(other *mSpanList) { 1538 if other.isEmpty() { 1539 return 1540 } 1541 1542 // Reparent everything in other to list. 1543 for s := other.first; s != nil; s = s.next { 1544 s.list = list 1545 } 1546 1547 // Concatenate the lists. 1548 if list.isEmpty() { 1549 *list = *other 1550 } else { 1551 // Neither list is empty. Put other before list. 1552 other.last.next = list.first 1553 list.first.prev = other.last 1554 list.first = other.first 1555 } 1556 1557 other.first, other.last = nil, nil 1558 } 1559 1560 const ( 1561 _KindSpecialFinalizer = 1 1562 _KindSpecialProfile = 2 1563 // Note: The finalizer special must be first because if we're freeing 1564 // an object, a finalizer special will cause the freeing operation 1565 // to abort, and we want to keep the other special records around 1566 // if that happens. 1567 ) 1568 1569 //go:notinheap 1570 type special struct { 1571 next *special // linked list in span 1572 offset uint16 // span offset of object 1573 kind byte // kind of special 1574 } 1575 1576 // Adds the special record s to the list of special records for 1577 // the object p. All fields of s should be filled in except for 1578 // offset & next, which this routine will fill in. 1579 // Returns true if the special was successfully added, false otherwise. 1580 // (The add will fail only if a record with the same p and s->kind 1581 // already exists.) 1582 func addspecial(p unsafe.Pointer, s *special) bool { 1583 span := spanOfHeap(uintptr(p)) 1584 if span == nil { 1585 throw("addspecial on invalid pointer") 1586 } 1587 1588 // Ensure that the span is swept. 1589 // Sweeping accesses the specials list w/o locks, so we have 1590 // to synchronize with it. And it's just much safer. 1591 mp := acquirem() 1592 span.ensureSwept() 1593 1594 offset := uintptr(p) - span.base() 1595 kind := s.kind 1596 1597 lock(&span.speciallock) 1598 1599 // Find splice point, check for existing record. 1600 t := &span.specials 1601 for { 1602 x := *t 1603 if x == nil { 1604 break 1605 } 1606 if offset == uintptr(x.offset) && kind == x.kind { 1607 unlock(&span.speciallock) 1608 releasem(mp) 1609 return false // already exists 1610 } 1611 if offset < uintptr(x.offset) || (offset == uintptr(x.offset) && kind < x.kind) { 1612 break 1613 } 1614 t = &x.next 1615 } 1616 1617 // Splice in record, fill in offset. 1618 s.offset = uint16(offset) 1619 s.next = *t 1620 *t = s 1621 unlock(&span.speciallock) 1622 releasem(mp) 1623 1624 return true 1625 } 1626 1627 // Removes the Special record of the given kind for the object p. 1628 // Returns the record if the record existed, nil otherwise. 1629 // The caller must FixAlloc_Free the result. 1630 func removespecial(p unsafe.Pointer, kind uint8) *special { 1631 span := spanOfHeap(uintptr(p)) 1632 if span == nil { 1633 throw("removespecial on invalid pointer") 1634 } 1635 1636 // Ensure that the span is swept. 1637 // Sweeping accesses the specials list w/o locks, so we have 1638 // to synchronize with it. And it's just much safer. 1639 mp := acquirem() 1640 span.ensureSwept() 1641 1642 offset := uintptr(p) - span.base() 1643 1644 lock(&span.speciallock) 1645 t := &span.specials 1646 for { 1647 s := *t 1648 if s == nil { 1649 break 1650 } 1651 // This function is used for finalizers only, so we don't check for 1652 // "interior" specials (p must be exactly equal to s->offset). 1653 if offset == uintptr(s.offset) && kind == s.kind { 1654 *t = s.next 1655 unlock(&span.speciallock) 1656 releasem(mp) 1657 return s 1658 } 1659 t = &s.next 1660 } 1661 unlock(&span.speciallock) 1662 releasem(mp) 1663 return nil 1664 } 1665 1666 // The described object has a finalizer set for it. 1667 // 1668 // specialfinalizer is allocated from non-GC'd memory, so any heap 1669 // pointers must be specially handled. 1670 // 1671 //go:notinheap 1672 type specialfinalizer struct { 1673 special special 1674 fn *funcval // May be a heap pointer. 1675 nret uintptr 1676 fint *_type // May be a heap pointer, but always live. 1677 ot *ptrtype // May be a heap pointer, but always live. 1678 } 1679 1680 // Adds a finalizer to the object p. Returns true if it succeeded. 1681 func addfinalizer(p unsafe.Pointer, f *funcval, nret uintptr, fint *_type, ot *ptrtype) bool { 1682 lock(&mheap_.speciallock) 1683 s := (*specialfinalizer)(mheap_.specialfinalizeralloc.alloc()) 1684 unlock(&mheap_.speciallock) 1685 s.special.kind = _KindSpecialFinalizer 1686 s.fn = f 1687 s.nret = nret 1688 s.fint = fint 1689 s.ot = ot 1690 if addspecial(p, &s.special) { 1691 // This is responsible for maintaining the same 1692 // GC-related invariants as markrootSpans in any 1693 // situation where it's possible that markrootSpans 1694 // has already run but mark termination hasn't yet. 1695 if gcphase != _GCoff { 1696 base, _, _ := findObject(uintptr(p), 0, 0) 1697 mp := acquirem() 1698 gcw := &mp.p.ptr().gcw 1699 // Mark everything reachable from the object 1700 // so it's retained for the finalizer. 1701 scanobject(base, gcw) 1702 // Mark the finalizer itself, since the 1703 // special isn't part of the GC'd heap. 1704 scanblock(uintptr(unsafe.Pointer(&s.fn)), sys.PtrSize, &oneptrmask[0], gcw, nil) 1705 releasem(mp) 1706 } 1707 return true 1708 } 1709 1710 // There was an old finalizer 1711 lock(&mheap_.speciallock) 1712 mheap_.specialfinalizeralloc.free(unsafe.Pointer(s)) 1713 unlock(&mheap_.speciallock) 1714 return false 1715 } 1716 1717 // Removes the finalizer (if any) from the object p. 1718 func removefinalizer(p unsafe.Pointer) { 1719 s := (*specialfinalizer)(unsafe.Pointer(removespecial(p, _KindSpecialFinalizer))) 1720 if s == nil { 1721 return // there wasn't a finalizer to remove 1722 } 1723 lock(&mheap_.speciallock) 1724 mheap_.specialfinalizeralloc.free(unsafe.Pointer(s)) 1725 unlock(&mheap_.speciallock) 1726 } 1727 1728 // The described object is being heap profiled. 1729 // 1730 //go:notinheap 1731 type specialprofile struct { 1732 special special 1733 b *bucket 1734 } 1735 1736 // Set the heap profile bucket associated with addr to b. 1737 func setprofilebucket(p unsafe.Pointer, b *bucket) { 1738 lock(&mheap_.speciallock) 1739 s := (*specialprofile)(mheap_.specialprofilealloc.alloc()) 1740 unlock(&mheap_.speciallock) 1741 s.special.kind = _KindSpecialProfile 1742 s.b = b 1743 if !addspecial(p, &s.special) { 1744 throw("setprofilebucket: profile already set") 1745 } 1746 } 1747 1748 // Do whatever cleanup needs to be done to deallocate s. It has 1749 // already been unlinked from the mspan specials list. 1750 func freespecial(s *special, p unsafe.Pointer, size uintptr) { 1751 switch s.kind { 1752 case _KindSpecialFinalizer: 1753 sf := (*specialfinalizer)(unsafe.Pointer(s)) 1754 queuefinalizer(p, sf.fn, sf.nret, sf.fint, sf.ot) 1755 lock(&mheap_.speciallock) 1756 mheap_.specialfinalizeralloc.free(unsafe.Pointer(sf)) 1757 unlock(&mheap_.speciallock) 1758 case _KindSpecialProfile: 1759 sp := (*specialprofile)(unsafe.Pointer(s)) 1760 mProf_Free(sp.b, size) 1761 lock(&mheap_.speciallock) 1762 mheap_.specialprofilealloc.free(unsafe.Pointer(sp)) 1763 unlock(&mheap_.speciallock) 1764 default: 1765 throw("bad special kind") 1766 panic("not reached") 1767 } 1768 } 1769 1770 // gcBits is an alloc/mark bitmap. This is always used as *gcBits. 1771 // 1772 //go:notinheap 1773 type gcBits uint8 1774 1775 // bytep returns a pointer to the n'th byte of b. 1776 func (b *gcBits) bytep(n uintptr) *uint8 { 1777 return addb((*uint8)(b), n) 1778 } 1779 1780 // bitp returns a pointer to the byte containing bit n and a mask for 1781 // selecting that bit from *bytep. 1782 func (b *gcBits) bitp(n uintptr) (bytep *uint8, mask uint8) { 1783 return b.bytep(n / 8), 1 << (n % 8) 1784 } 1785 1786 const gcBitsChunkBytes = uintptr(64 << 10) 1787 const gcBitsHeaderBytes = unsafe.Sizeof(gcBitsHeader{}) 1788 1789 type gcBitsHeader struct { 1790 free uintptr // free is the index into bits of the next free byte. 1791 next uintptr // *gcBits triggers recursive type bug. (issue 14620) 1792 } 1793 1794 //go:notinheap 1795 type gcBitsArena struct { 1796 // gcBitsHeader // side step recursive type bug (issue 14620) by including fields by hand. 1797 free uintptr // free is the index into bits of the next free byte; read/write atomically 1798 next *gcBitsArena 1799 bits [gcBitsChunkBytes - gcBitsHeaderBytes]gcBits 1800 } 1801 1802 var gcBitsArenas struct { 1803 lock mutex 1804 free *gcBitsArena 1805 next *gcBitsArena // Read atomically. Write atomically under lock. 1806 current *gcBitsArena 1807 previous *gcBitsArena 1808 } 1809 1810 // tryAlloc allocates from b or returns nil if b does not have enough room. 1811 // This is safe to call concurrently. 1812 func (b *gcBitsArena) tryAlloc(bytes uintptr) *gcBits { 1813 if b == nil || atomic.Loaduintptr(&b.free)+bytes > uintptr(len(b.bits)) { 1814 return nil 1815 } 1816 // Try to allocate from this block. 1817 end := atomic.Xadduintptr(&b.free, bytes) 1818 if end > uintptr(len(b.bits)) { 1819 return nil 1820 } 1821 // There was enough room. 1822 start := end - bytes 1823 return &b.bits[start] 1824 } 1825 1826 // newMarkBits returns a pointer to 8 byte aligned bytes 1827 // to be used for a span's mark bits. 1828 func newMarkBits(nelems uintptr) *gcBits { 1829 blocksNeeded := uintptr((nelems + 63) / 64) 1830 bytesNeeded := blocksNeeded * 8 1831 1832 // Try directly allocating from the current head arena. 1833 head := (*gcBitsArena)(atomic.Loadp(unsafe.Pointer(&gcBitsArenas.next))) 1834 if p := head.tryAlloc(bytesNeeded); p != nil { 1835 return p 1836 } 1837 1838 // There's not enough room in the head arena. We may need to 1839 // allocate a new arena. 1840 lock(&gcBitsArenas.lock) 1841 // Try the head arena again, since it may have changed. Now 1842 // that we hold the lock, the list head can't change, but its 1843 // free position still can. 1844 if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil { 1845 unlock(&gcBitsArenas.lock) 1846 return p 1847 } 1848 1849 // Allocate a new arena. This may temporarily drop the lock. 1850 fresh := newArenaMayUnlock() 1851 // If newArenaMayUnlock dropped the lock, another thread may 1852 // have put a fresh arena on the "next" list. Try allocating 1853 // from next again. 1854 if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil { 1855 // Put fresh back on the free list. 1856 // TODO: Mark it "already zeroed" 1857 fresh.next = gcBitsArenas.free 1858 gcBitsArenas.free = fresh 1859 unlock(&gcBitsArenas.lock) 1860 return p 1861 } 1862 1863 // Allocate from the fresh arena. We haven't linked it in yet, so 1864 // this cannot race and is guaranteed to succeed. 1865 p := fresh.tryAlloc(bytesNeeded) 1866 if p == nil { 1867 throw("markBits overflow") 1868 } 1869 1870 // Add the fresh arena to the "next" list. 1871 fresh.next = gcBitsArenas.next 1872 atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), unsafe.Pointer(fresh)) 1873 1874 unlock(&gcBitsArenas.lock) 1875 return p 1876 } 1877 1878 // newAllocBits returns a pointer to 8 byte aligned bytes 1879 // to be used for this span's alloc bits. 1880 // newAllocBits is used to provide newly initialized spans 1881 // allocation bits. For spans not being initialized the 1882 // mark bits are repurposed as allocation bits when 1883 // the span is swept. 1884 func newAllocBits(nelems uintptr) *gcBits { 1885 return newMarkBits(nelems) 1886 } 1887 1888 // nextMarkBitArenaEpoch establishes a new epoch for the arenas 1889 // holding the mark bits. The arenas are named relative to the 1890 // current GC cycle which is demarcated by the call to finishweep_m. 1891 // 1892 // All current spans have been swept. 1893 // During that sweep each span allocated room for its gcmarkBits in 1894 // gcBitsArenas.next block. gcBitsArenas.next becomes the gcBitsArenas.current 1895 // where the GC will mark objects and after each span is swept these bits 1896 // will be used to allocate objects. 1897 // gcBitsArenas.current becomes gcBitsArenas.previous where the span's 1898 // gcAllocBits live until all the spans have been swept during this GC cycle. 1899 // The span's sweep extinguishes all the references to gcBitsArenas.previous 1900 // by pointing gcAllocBits into the gcBitsArenas.current. 1901 // The gcBitsArenas.previous is released to the gcBitsArenas.free list. 1902 func nextMarkBitArenaEpoch() { 1903 lock(&gcBitsArenas.lock) 1904 if gcBitsArenas.previous != nil { 1905 if gcBitsArenas.free == nil { 1906 gcBitsArenas.free = gcBitsArenas.previous 1907 } else { 1908 // Find end of previous arenas. 1909 last := gcBitsArenas.previous 1910 for last = gcBitsArenas.previous; last.next != nil; last = last.next { 1911 } 1912 last.next = gcBitsArenas.free 1913 gcBitsArenas.free = gcBitsArenas.previous 1914 } 1915 } 1916 gcBitsArenas.previous = gcBitsArenas.current 1917 gcBitsArenas.current = gcBitsArenas.next 1918 atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), nil) // newMarkBits calls newArena when needed 1919 unlock(&gcBitsArenas.lock) 1920 } 1921 1922 // newArenaMayUnlock allocates and zeroes a gcBits arena. 1923 // The caller must hold gcBitsArena.lock. This may temporarily release it. 1924 func newArenaMayUnlock() *gcBitsArena { 1925 var result *gcBitsArena 1926 if gcBitsArenas.free == nil { 1927 unlock(&gcBitsArenas.lock) 1928 result = (*gcBitsArena)(sysAlloc(gcBitsChunkBytes, &memstats.gc_sys)) 1929 if result == nil { 1930 throw("runtime: cannot allocate memory") 1931 } 1932 lock(&gcBitsArenas.lock) 1933 } else { 1934 result = gcBitsArenas.free 1935 gcBitsArenas.free = gcBitsArenas.free.next 1936 memclrNoHeapPointers(unsafe.Pointer(result), gcBitsChunkBytes) 1937 } 1938 result.next = nil 1939 // If result.bits is not 8 byte aligned adjust index so 1940 // that &result.bits[result.free] is 8 byte aligned. 1941 if uintptr(unsafe.Offsetof(gcBitsArena{}.bits))&7 == 0 { 1942 result.free = 0 1943 } else { 1944 result.free = 8 - (uintptr(unsafe.Pointer(&result.bits[0])) & 7) 1945 } 1946 return result 1947 } 1948
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