code review 7005055: runtime: faster mcentral alloc. (issue 7005055)

[sebastien...@gmail.com]

Reviewers: golang-dev_googlegroups.com,

Message:
Hello golan...@googlegroups.com,

I'd like you to review this change to
https://code.google.com/p/go/


Description:
runtime: faster mcentral alloc.

Reduce individual object handling by anticipating how much of them are
servable.

Not a chunked transfer cache, but decent enough to make sure the
bottleneck is not here.

Mac OSX, median of 10 runs:

benchmark old ns/op new ns/op delta
BenchmarkBinaryTree17 5358937333 4892813012 -8.70%
BenchmarkFannkuch11 3257752475 3315436116 +1.77%
BenchmarkGobDecode 23277349 23001114 -1.19%
BenchmarkGobEncode 14367327 14262925 -0.73%
BenchmarkGzip 441045541 440451719 -0.13%
BenchmarkGunzip 139117663 139622494 +0.36%
BenchmarkJSONEncode 45715854 45687802 -0.06%
BenchmarkJSONDecode 103949570 106530032 +2.48%
BenchmarkMandelbrot200 4542462 4548290 +0.13%
BenchmarkParse 7790558 7557540 -2.99%
BenchmarkRevcomp 831436684 832510381 +0.13%
BenchmarkTemplate 133789824 133007337 -0.58%

benchmark old MB/s new MB/s speedup
BenchmarkGobDecode 32.82 33.33 1.02x
BenchmarkGobEncode 53.42 53.86 1.01x
BenchmarkGzip 43.70 44.01 1.01x
BenchmarkGunzip 139.09 139.14 1.00x
BenchmarkJSONEncode 42.69 42.56 1.00x
BenchmarkJSONDecode 18.78 17.91 0.95x
BenchmarkParse 7.37 7.67 1.04x
BenchmarkRevcomp 306.83 305.70 1.00x
BenchmarkTemplate 14.57 14.56 1.00x

Please review this at https://codereview.appspot.com/7005055/

Affected files:
M src/pkg/runtime/mcentral.c


Index: src/pkg/runtime/mcentral.c
===================================================================
--- a/src/pkg/runtime/mcentral.c
+++ b/src/pkg/runtime/mcentral.c
@@ -34,12 +34,13 @@
// Allocate up to n objects from the central free list.
// Return the number of objects allocated.
// The objects are linked together by their first words.
-// On return, *pstart points at the first object and *pend at the last.
+// On return, *pstart points at the first object.
int32
runtime·MCentral_AllocList(MCentral *c, int32 n, MLink **pfirst)
{
- MLink *first, *last, *v;
- int32 i;
+ MSpan *s;
+ MLink *first, *last;
+ int32 cap, avail, i;

runtime·lock(c);
// Replenish central list if empty.
@@ -50,41 +51,34 @@
return 0;
}
}
+ s = c->nonempty.next;
+ cap = (s->npages << PageShift) / s->elemsize;
+ avail = cap - s->ref;
+ if(avail < n)
+ n = avail;

- // Copy from list, up to n.
// First one is guaranteed to work, because we just grew the list.
- first = MCentral_Alloc(c);
+ first = s->freelist;
last = first;
- for(i=1; i<n && (v = MCentral_Alloc(c)) != nil; i++) {
- last->next = v;
- last = v;
+ for(i=1; i<n; i++) {
+ last = last->next;
}
+ s->freelist = last->next;
last->next = nil;
- c->nfree -= i;
+ s->ref += n;
+ c->nfree -= n;
+
+ if(n == avail) {
+ if(s->freelist != nil || s->ref != cap) {
+ runtime·throw("invalid freelist");
+ }
+ runtime·MSpanList_Remove(s);
+ runtime·MSpanList_Insert(&c->empty, s);
+ }

runtime·unlock(c);
*pfirst = first;
- return i;
-}
-
-// Helper: allocate one object from the central free list.
-static void*
-MCentral_Alloc(MCentral *c)
-{
- MSpan *s;
- MLink *v;
-
- if(runtime·MSpanList_IsEmpty(&c->nonempty))
- return nil;
- s = c->nonempty.next;
- s->ref++;
- v = s->freelist;
- s->freelist = v->next;
- if(s->freelist == nil) {
- runtime·MSpanList_Remove(s);
- runtime·MSpanList_Insert(&c->empty, s);
- }
- return v;
+ return n;
}

// Free n objects back into the central free list.