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runtime: do not alloc never used tail bytes in fixalloc
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>
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parent
a35c5c98c0
commit
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@ -30,7 +30,8 @@ type fixalloc struct {
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arg unsafe.Pointer
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list *mlink
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chunk uintptr // use uintptr instead of unsafe.Pointer to avoid write barriers
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nchunk uint32
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nchunk uint32 // bytes remaining in current chunk
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nalloc uint32 // size of new chunks in bytes
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inuse uintptr // in-use bytes now
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stat *sysMemStat
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zero bool // zero allocations
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@ -63,6 +64,7 @@ func (f *fixalloc) init(size uintptr, first func(arg, p unsafe.Pointer), arg uns
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f.list = nil
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f.chunk = 0
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f.nchunk = 0
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f.nalloc = uint32(_FixAllocChunk / size * size) // Round _FixAllocChunk down to an exact multiple of size to eliminate tail waste
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f.inuse = 0
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f.stat = stat
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f.zero = true
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@ -84,8 +86,8 @@ func (f *fixalloc) alloc() unsafe.Pointer {
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return v
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}
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if uintptr(f.nchunk) < f.size {
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f.chunk = uintptr(persistentalloc(_FixAllocChunk, 0, f.stat))
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f.nchunk = _FixAllocChunk
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f.chunk = uintptr(persistentalloc(uintptr(f.nalloc), 0, f.stat))
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f.nchunk = f.nalloc
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}
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v := unsafe.Pointer(f.chunk)
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