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cmd/compile: stack allocation phase in large functions slows down SSA compiler #14775

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dr2chase opened this issue Mar 11, 2016 · 5 comments
Closed

cmd/compile: stack allocation phase in large functions slows down SSA compiler #14775

dr2chase opened this issue Mar 11, 2016 · 5 comments
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FrozenDueToAge NeedsFix The path to resolution is known, but the work has not been done. ToolSpeed
Milestone
Go1.8Early

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@dr2chase
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Please answer these questions before submitting your issue. Thanks!

  1. What version of Go are you using (go version)?
    devel for 1.7
  2. What operating system and processor architecture are you using (go env)?
    amd64
  3. What did you do?
    Downloaded Dave Cheney's genpkg to explore performance problems.
mkdir x
go run gen.go -n 3000  -noinitfn > x/types.go
time go build  -gcflags='-d=ssa/check/off -memprofile=x.mprof' ./x
go tool pprof -alloc_space ${GOROOT}/pkg/tool/darwin_amd64/compile x/x.mprof

with results

Showing top 30 nodes out of 40 (cum >= 1810.24MB)
      flat  flat%   sum%        cum   cum%
 1819.24MB 20.43% 20.43%  1819.24MB 20.43%  cmd/compile/internal/ssa.(*Func).newValue
  947.75MB 10.64% 31.08%  2753.49MB 30.93%  cmd/compile/internal/gc.(*state).lookupVarOutgoing
  947.44MB 10.64% 41.72%   947.44MB 10.64%  cmd/compile/internal/ssa.(*stackAllocState).buildInterferenceGraph
  638.83MB  7.18% 48.89%   642.34MB  7.21%  cmd/compile/internal/ssa.(*stackAllocState).stackalloc
  625.34MB  7.02% 55.92%  1041.10MB 11.69%  cmd/compile/internal/ssa.(*regAllocState).init
  581.15MB  6.53% 62.45%   649.08MB  7.29%  cmd/compile/internal/gc.genssa
  525.89MB  5.91% 68.35%   526.62MB  5.91%  cmd/compile/internal/ssa.cse
  482.10MB  5.41% 73.77%   482.10MB  5.41%  cmd/compile/internal/ssa.schedule
  460.90MB  5.18% 78.94%  2667.34MB 29.96%  cmd/compile/internal/ssa.(*regAllocState).regalloc
  416.07MB  4.67% 83.62%  1476.37MB 16.58%  cmd/compile/internal/ssa.(*stackAllocState).init
  415.25MB  4.66% 88.28%   415.77MB  4.67%  cmd/compile/internal/ssa.(*regAllocState).computeLive
  312.07MB  3.51% 91.79%   312.07MB  3.51%  cmd/compile/internal/ssa.(*Func).newSparseSet
  169.05MB  1.90% 93.68%   169.05MB  1.90%  cmd/compile/internal/ssa.tighten
  111.31MB  1.25% 94.93%   112.86MB  1.27%  cmd/compile/internal/ssa.(*stackAllocState).computeLive
  1. What did you expect to see?
    Not that much slowdown and memory consumption.
  2. What did you see instead?
    Much slowdown and memory consumption.

Note that two similar bugs are filed, one against the ssa.go variable lookup code, the other against regalloc/stackalloc, since the fixes are independent.
@dr2chase dr2chase self-assigned this Mar 11, 2016
@ianlancetaylor ianlancetaylor changed the title Stack allocation phase in large functions slows down SSA compiler cmd/compile: stack allocation phase in large functions slows down SSA compiler Mar 11, 2016
@bradfitz bradfitz added this to the Go1.7 milestone Apr 9, 2016
@randall77 randall77 mentioned this issue May 10, 2016
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@dr2chase
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I'd like to push this off to 1.8; it's a hard problem, and we've made other progress for the pathological functions that trigger this:

and we have two more CLs queued up for 1.8 ( or possibly 1.7 if sufficiently horrible inputs appear) that will help some more:

@quentinmit quentinmit added the NeedsFix The path to resolution is known, but the work has not been done. label May 26, 2016
@quentinmit
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@randall77 @dr2chase Please change the milestone to Go1.8 if you're satisfied with what's already in 1.7.

@randall77 randall77 modified the milestones: Go1.8Early, Go1.7 May 26, 2016
@josharian josharian mentioned this issue Aug 24, 2016
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@bradfitz
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@dr2chase, what's the status of this? It's marked "Go1.8Early".

@dr2chase
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Very little work on the stack allocation code, and instead we beat on the more tractable problem of SSA phi-function placement -- one of the 1.8early changes went into 1.7late because of a horrifying compilation problem there: https://go-review.googlesource.com/c/23136/

I took at stab at this once and didn't get it right. I'd guess that it's a least a week's work if I'm lucky, probably two.

@randall77
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Tip is now faster than 1.6 (3.5 sec for 1.6, 3.2 sec for tip). Closing.

@golang golang locked and limited conversation to collaborators Oct 5, 2017
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