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runtime: fix malloc sampling bug 

The malloc sample trigger was not being set in a
new m, so the first allocation in each new m - the
goroutine structure - was being sampled with
probability 1 instead of probability sizeof(G)/rate,
an oversampling of about 5000x for the default
rate of 1 MB.  This bug made pprof graphs show
far more G allocations than there actually were.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5224041
This commit is contained in:
Russ Cox 2011-10-06 11:30:48 -04:00
parent 5695915833
commit ad35cea762
1 changed files with 9 additions and 0 deletions
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9
src/pkg/runtime/malloc.goc
View File

@ -80,6 +80,7 @@ runtime·mallocgc(uintptr size, uint32 flag, int32 dogc, int32 zeroed)
m->mcache->next_sample -= size;
else {
// pick next profile time
// If you change this, also change allocmcache.
if(rate > 0x3fffffff) // make 2*rate not overflow
rate = 0x3fffffff;
m->mcache->next_sample = runtime·fastrand1() % (2*rate);
@ -205,6 +206,7 @@ runtime·mlookup(void *v, byte **base, uintptr *size, MSpan **sp)
MCache*
runtime·allocmcache(void)
{
int32 rate;
MCache *c;
runtime·lock(&runtime·mheap);
@ -212,6 +214,13 @@ runtime·allocmcache(void)
mstats.mcache_inuse = runtime·mheap.cachealloc.inuse;
mstats.mcache_sys = runtime·mheap.cachealloc.sys;
runtime·unlock(&runtime·mheap);
// Set first allocation sample size.
rate = runtime·MemProfileRate;
if(rate > 0x3fffffff) // make 2*rate not overflow
rate = 0x3fffffff;
c->next_sample = runtime·fastrand1() % (2*rate);
return c;
}