mcentral.go

Documentation: runtime

     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  // Central free lists.
     6  //
     7  // See malloc.go for an overview.
     8  //
     9  // The mcentral doesn't actually contain the list of free objects; the mspan does.
    10  // Each mcentral is two lists of mspans: those with free objects (c->nonempty)
    11  // and those that are completely allocated (c->empty).
    12  
    13  package runtime
    14  
    15  import "runtime/internal/atomic"
    16  
    17  // Central list of free objects of a given size.
    18  //
    19  //go:notinheap
    20  type mcentral struct {
    21  	lock      mutex
    22  	spanclass spanClass
    23  	nonempty  mSpanList // list of spans with a free object, ie a nonempty free list
    24  	empty     mSpanList // list of spans with no free objects (or cached in an mcache)
    25  
    26  	// nmalloc is the cumulative count of objects allocated from
    27  	// this mcentral, assuming all spans in mcaches are
    28  	// fully-allocated. Written atomically, read under STW.
    29  	nmalloc uint64
    30  }
    31  
    32  // Initialize a single central free list.
    33  func (c *mcentral) init(spc spanClass) {
    34  	c.spanclass = spc
    35  	c.nonempty.init()
    36  	c.empty.init()
    37  }
    38  
    39  // Allocate a span to use in an mcache.
    40  func (c *mcentral) cacheSpan() *mspan {
    41  	// Deduct credit for this span allocation and sweep if necessary.
    42  	spanBytes := uintptr(class_to_allocnpages[c.spanclass.sizeclass()]) * _PageSize
    43  	deductSweepCredit(spanBytes, 0)
    44  
    45  	lock(&c.lock)
    46  	traceDone := false
    47  	if trace.enabled {
    48  		traceGCSweepStart()
    49  	}
    50  	sg := mheap_.sweepgen
    51  retry:
    52  	var s *mspan
    53  	for s = c.nonempty.first; s != nil; s = s.next {
    54  		if s.sweepgen == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
    55  			c.nonempty.remove(s)
    56  			c.empty.insertBack(s)
    57  			unlock(&c.lock)
    58  			s.sweep(true)
    59  			goto havespan
    60  		}
    61  		if s.sweepgen == sg-1 {
    62  			// the span is being swept by background sweeper, skip
    63  			continue
    64  		}
    65  		// we have a nonempty span that does not require sweeping, allocate from it
    66  		c.nonempty.remove(s)
    67  		c.empty.insertBack(s)
    68  		unlock(&c.lock)
    69  		goto havespan
    70  	}
    71  
    72  	for s = c.empty.first; s != nil; s = s.next {
    73  		if s.sweepgen == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
    74  			// we have an empty span that requires sweeping,
    75  			// sweep it and see if we can free some space in it
    76  			c.empty.remove(s)
    77  			// swept spans are at the end of the list
    78  			c.empty.insertBack(s)
    79  			unlock(&c.lock)
    80  			s.sweep(true)
    81  			freeIndex := s.nextFreeIndex()
    82  			if freeIndex != s.nelems {
    83  				s.freeindex = freeIndex
    84  				goto havespan
    85  			}
    86  			lock(&c.lock)
    87  			// the span is still empty after sweep
    88  			// it is already in the empty list, so just retry
    89  			goto retry
    90  		}
    91  		if s.sweepgen == sg-1 {
    92  			// the span is being swept by background sweeper, skip
    93  			continue
    94  		}
    95  		// already swept empty span,
    96  		// all subsequent ones must also be either swept or in process of sweeping
    97  		break
    98  	}
    99  	if trace.enabled {
   100  		traceGCSweepDone()
   101  		traceDone = true
   102  	}
   103  	unlock(&c.lock)
   104  
   105  	// Replenish central list if empty.
   106  	s = c.grow()
   107  	if s == nil {
   108  		return nil
   109  	}
   110  	lock(&c.lock)
   111  	c.empty.insertBack(s)
   112  	unlock(&c.lock)
   113  
   114  	// At this point s is a non-empty span, queued at the end of the empty list,
   115  	// c is unlocked.
   116  havespan:
   117  	if trace.enabled && !traceDone {
   118  		traceGCSweepDone()
   119  	}
   120  	n := int(s.nelems) - int(s.allocCount)
   121  	if n == 0 || s.freeindex == s.nelems || uintptr(s.allocCount) == s.nelems {
   122  		throw("span has no free objects")
   123  	}
   124  	// Assume all objects from this span will be allocated in the
   125  	// mcache. If it gets uncached, we'll adjust this.
   126  	atomic.Xadd64(&c.nmalloc, int64(n))
   127  	usedBytes := uintptr(s.allocCount) * s.elemsize
   128  	atomic.Xadd64(&memstats.heap_live, int64(spanBytes)-int64(usedBytes))
   129  	if trace.enabled {
   130  		// heap_live changed.
   131  		traceHeapAlloc()
   132  	}
   133  	if gcBlackenEnabled != 0 {
   134  		// heap_live changed.
   135  		gcController.revise()
   136  	}
   137  	freeByteBase := s.freeindex &^ (64 - 1)
   138  	whichByte := freeByteBase / 8
   139  	// Init alloc bits cache.
   140  	s.refillAllocCache(whichByte)
   141  
   142  	// Adjust the allocCache so that s.freeindex corresponds to the low bit in
   143  	// s.allocCache.
   144  	s.allocCache >>= s.freeindex % 64
   145  
   146  	return s
   147  }
   148  
   149  // Return span from an mcache.
   150  func (c *mcentral) uncacheSpan(s *mspan) {
   151  	if s.allocCount == 0 {
   152  		throw("uncaching span but s.allocCount == 0")
   153  	}
   154  
   155  	sg := mheap_.sweepgen
   156  	stale := s.sweepgen == sg+1
   157  	if stale {
   158  		// Span was cached before sweep began. It's our
   159  		// responsibility to sweep it.
   160  		//
   161  		// Set sweepgen to indicate it's not cached but needs
   162  		// sweeping and can't be allocated from. sweep will
   163  		// set s.sweepgen to indicate s is swept.
   164  		atomic.Store(&s.sweepgen, sg-1)
   165  	} else {
   166  		// Indicate that s is no longer cached.
   167  		atomic.Store(&s.sweepgen, sg)
   168  	}
   169  
   170  	n := int(s.nelems) - int(s.allocCount)
   171  	if n > 0 {
   172  		// cacheSpan updated alloc assuming all objects on s
   173  		// were going to be allocated. Adjust for any that
   174  		// weren't. We must do this before potentially
   175  		// sweeping the span.
   176  		atomic.Xadd64(&c.nmalloc, -int64(n))
   177  
   178  		lock(&c.lock)
   179  		c.empty.remove(s)
   180  		c.nonempty.insert(s)
   181  		if !stale {
   182  			// mCentral_CacheSpan conservatively counted
   183  			// unallocated slots in heap_live. Undo this.
   184  			//
   185  			// If this span was cached before sweep, then
   186  			// heap_live was totally recomputed since
   187  			// caching this span, so we don't do this for
   188  			// stale spans.
   189  			atomic.Xadd64(&memstats.heap_live, -int64(n)*int64(s.elemsize))
   190  		}
   191  		unlock(&c.lock)
   192  	}
   193  
   194  	if stale {
   195  		// Now that s is in the right mcentral list, we can
   196  		// sweep it.
   197  		s.sweep(false)
   198  	}
   199  }
   200  
   201  // freeSpan updates c and s after sweeping s.
   202  // It sets s's sweepgen to the latest generation,
   203  // and, based on the number of free objects in s,
   204  // moves s to the appropriate list of c or returns it
   205  // to the heap.
   206  // freeSpan reports whether s was returned to the heap.
   207  // If preserve=true, it does not move s (the caller
   208  // must take care of it).
   209  func (c *mcentral) freeSpan(s *mspan, preserve bool, wasempty bool) bool {
   210  	if sg := mheap_.sweepgen; s.sweepgen == sg+1 || s.sweepgen == sg+3 {
   211  		throw("freeSpan given cached span")
   212  	}
   213  	s.needzero = 1
   214  
   215  	if preserve {
   216  		// preserve is set only when called from (un)cacheSpan above,
   217  		// the span must be in the empty list.
   218  		if !s.inList() {
   219  			throw("can't preserve unlinked span")
   220  		}
   221  		atomic.Store(&s.sweepgen, mheap_.sweepgen)
   222  		return false
   223  	}
   224  
   225  	lock(&c.lock)
   226  
   227  	// Move to nonempty if necessary.
   228  	if wasempty {
   229  		c.empty.remove(s)
   230  		c.nonempty.insert(s)
   231  	}
   232  
   233  	// delay updating sweepgen until here. This is the signal that
   234  	// the span may be used in an mcache, so it must come after the
   235  	// linked list operations above (actually, just after the
   236  	// lock of c above.)
   237  	atomic.Store(&s.sweepgen, mheap_.sweepgen)
   238  
   239  	if s.allocCount != 0 {
   240  		unlock(&c.lock)
   241  		return false
   242  	}
   243  
   244  	c.nonempty.remove(s)
   245  	unlock(&c.lock)
   246  	mheap_.freeSpan(s, false)
   247  	return true
   248  }
   249  
   250  // grow allocates a new empty span from the heap and initializes it for c's size class.
   251  func (c *mcentral) grow() *mspan {
   252  	npages := uintptr(class_to_allocnpages[c.spanclass.sizeclass()])
   253  	size := uintptr(class_to_size[c.spanclass.sizeclass()])
   254  	n := (npages << _PageShift) / size
   255  
   256  	s := mheap_.alloc(npages, c.spanclass, false, true)
   257  	if s == nil {
   258  		return nil
   259  	}
   260  
   261  	p := s.base()
   262  	s.limit = p + size*n
   263  
   264  	heapBitsForAddr(s.base()).initSpan(s)
   265  	return s
   266  }
   267
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