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runtime: do not alloc never used tail bytes in fixalloc

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Currently, the '_FixAllocChunk % fixalloc.size' tail bytes
will never be used when allocing from persistentalloc.

Wasted bytes on darwin/amd64:
  _FixAllocChunk % mheap_.spanalloc.size             = 64
  _FixAllocChunk % mheap_.cachealloc.size            = 784
  _FixAllocChunk % mheap_.specialfinalizeralloc.size = 16
  _FixAllocChunk % mheap_.specialprofilealloc.size   = 16
  _FixAllocChunk % mheap_.specialReachableAlloc.size = 16
  _FixAllocChunk % mheap_.arenaHintAlloc.size        = 16

After this commit, fixalloc alloc '_FixAllocChunk / fixalloc.size'
objects exactly with zero waste. Sizeof(fixalloc{}) is unchanged.

Change-Id: Ifc551f5b7aa9d842fa559abbe532ffcfb4d3540c
GitHub-Last-Rev: e08b4c66b8
GitHub-Pull-Request: golang/go#47439
Reviewed-on: https://go-review.googlesource.com/c/go/+/338090
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Cherry Mui <cherryyz@google.com>
This commit is contained in:
Hans 2021-08-02 05:43:29 +00:00 committed by Austin Clements
parent a35c5c98c0
commit 4591f49938
1 changed files with 5 additions and 3 deletions
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8
src/runtime/mfixalloc.go
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@ -30,7 +30,8 @@ type fixalloc struct {
arg unsafe.Pointer
list *mlink
chunk uintptr // use uintptr instead of unsafe.Pointer to avoid write barriers
nchunk uint32
nchunk uint32 // bytes remaining in current chunk
nalloc uint32 // size of new chunks in bytes
inuse uintptr // in-use bytes now
stat *sysMemStat
zero bool // zero allocations
@ -63,6 +64,7 @@ func (f *fixalloc) init(size uintptr, first func(arg, p unsafe.Pointer), arg uns
f.list = nil
f.chunk = 0
f.nchunk = 0
f.nalloc = uint32(_FixAllocChunk / size * size) // Round _FixAllocChunk down to an exact multiple of size to eliminate tail waste
f.inuse = 0
f.stat = stat
f.zero = true
@ -84,8 +86,8 @@ func (f *fixalloc) alloc() unsafe.Pointer {
return v
}
if uintptr(f.nchunk) < f.size {
f.chunk = uintptr(persistentalloc(_FixAllocChunk, 0, f.stat))
f.nchunk = _FixAllocChunk
f.chunk = uintptr(persistentalloc(uintptr(f.nalloc), 0, f.stat))
f.nchunk = f.nalloc
}
v := unsafe.Pointer(f.chunk)