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runtime: mark objects with non-atomic operations

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On the go.benchmarks/garbage benchmark with GOMAXPROCS=16:
                   old ns/op     new ns/op     delta
time               1392254       1353170       -2.81%
cputime            21995751      21373999      -2.83%
gc-pause-one       15044812      13050524      -13.26%
gc-pause-total     213636        185317        -13.26%

LGTM=rlh
R=golang-codereviews, rlh
CC=golang-codereviews, khr, rsc
https://golang.org/cl/123380043
This commit is contained in:
Dmitriy Vyukov 2014-08-14 21:38:24 +04:00
parent ba30c082c0
commit 0c32bd6262
1 changed files with 16 additions and 5 deletions
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21
src/pkg/runtime/mgc0.c
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@ -213,7 +213,7 @@ static struct {
static void static void
scanblock(byte *b, uintptr n, byte *ptrmask) scanblock(byte *b, uintptr n, byte *ptrmask)
{ {
byte *obj, *p, *arena_start, *arena_used, **wp, *scanbuf[8]; byte *obj, *p, *arena_start, *arena_used, **wp, *scanbuf[8], bits8;
uintptr i, nobj, size, idx, *bitp, bits, xbits, shift, x, off, cached, scanbufpos; uintptr i, nobj, size, idx, *bitp, bits, xbits, shift, x, off, cached, scanbufpos;
intptr ncached; intptr ncached;
Workbuf *wbuf; Workbuf *wbuf;
@ -440,16 +440,27 @@ scanblock(byte *b, uintptr n, byte *ptrmask)
// Only care about not marked objects. // Only care about not marked objects.
if((bits&bitMarked) != 0) if((bits&bitMarked) != 0)
continue; continue;
if(work.nproc == 1) // If obj size is greater than 8, then each byte of GC bitmap
*bitp |= bitMarked<<shift; // contains info for at most one object. In such case we use
// non-atomic byte store to mark the object. This can lead
// to double enqueue of the object for scanning, but scanning
// is an idempotent operation, so it is OK. This cannot lead
// to bitmap corruption because the single marked bit is the
// only thing that can change in the byte.
// For 8-byte objects we use non-atomic store, if the other
// quadruple is already marked. Otherwise we resort to CAS
// loop for marking.
bits8 = xbits>>(shift&~7);
if((bits8&(bitMask|(bitMask<<gcBits))) != (bitBoundary|(bitBoundary<<gcBits)))
((uint8*)bitp)[shift/8] = bits8 | (bitMarked<<(shift&7));
else { else {
for(;;) { for(;;) {
if(runtime·casp((void**)bitp, (void*)xbits, (void*)(xbits|(bitMarked<<shift))))
break;
xbits = *bitp; xbits = *bitp;
bits = (xbits>>shift) & bitMask; bits = (xbits>>shift) & bitMask;
if((bits&bitMarked) != 0) if((bits&bitMarked) != 0)
break; break;
if(runtime·casp((void**)bitp, (void*)xbits, (void*)(xbits|(bitMarked<<shift))))
break;
} }
if((bits&bitMarked) != 0) if((bits&bitMarked) != 0)
continue; continue;