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: e08b4c66b82bc7be9d14fb7eb7580504d777481e
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>

diff --git a/src/runtime/mfixalloc.go b/src/runtime/mfixalloc.go
index a81139a389cbe37050b23422f2983e0c9a82bed9..b701a09b40a9471ab9b04c9742d7fddc11630aee 100644 (file)
--- a/src/runtime/mfixalloc.go
+++ b/src/runtime/mfixalloc.go
@@ -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)