runtime: transfer whole span from MCentral to MCache

Finer-grained transfers were relevant with per-M caches,
with per-P caches they are not relevant and harmful for performance.
For few small size classes where it makes difference,
it's fine to grab the whole span (4K).

benchmark          old ns/op    new ns/op    delta
BenchmarkMalloc           42           40   -4.45%

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/9374043

src/pkg/runtime/malloc.h

@@ -267,14 +267,10 @@ extern MStats mstats;
 // class_to_size[i] = largest size in class i
 // class_to_allocnpages[i] = number of pages to allocate when
 //	making new objects in class i
-// class_to_transfercount[i] = number of objects to move when
-//	taking a bunch of objects out of the central lists
-//	and putting them in the thread free list.
 
 int32	runtime·SizeToClass(int32);
 extern	int32	runtime·class_to_size[NumSizeClasses];
 extern	int32	runtime·class_to_allocnpages[NumSizeClasses];
-extern	int32	runtime·class_to_transfercount[NumSizeClasses];
 extern	int8	runtime·size_to_class8[1024/8 + 1];
 extern	int8	runtime·size_to_class128[(MaxSmallSize-1024)/128 + 1];
 extern	void	runtime·InitSizes(void);
@@ -399,7 +395,7 @@ struct MCentral
 };
 
 void	runtime·MCentral_Init(MCentral *c, int32 sizeclass);
-int32	runtime·MCentral_AllocList(MCentral *c, int32 n, MLink **first);
+int32	runtime·MCentral_AllocList(MCentral *c, MLink **first);
 void	runtime·MCentral_FreeList(MCentral *c, int32 n, MLink *first);
 void	runtime·MCentral_FreeSpan(MCentral *c, MSpan *s, int32 n, MLink *start, MLink *end);
 

src/pkg/runtime/mcache.c

@@ -21,8 +21,7 @@ runtime·MCache_Alloc(MCache *c, int32 sizeclass, uintptr size, int32 zeroed)
 	l = &c->list[sizeclass];
 	if(l->list == nil) {
 		// Replenish using central lists.
-		n = runtime·MCentral_AllocList(&runtime·mheap->central[sizeclass],
-			runtime·class_to_transfercount[sizeclass], &first);
+		n = runtime·MCentral_AllocList(&runtime·mheap->central[sizeclass], &first);
 		if(n == 0)
 			runtime·throw("out of memory");
 		l->list = first;
@@ -91,7 +90,7 @@ runtime·MCache_Free(MCache *c, void *v, int32 sizeclass, uintptr size)
 
 	if(l->nlist >= MaxMCacheListLen) {
 		// Release a chunk back.
-		ReleaseN(c, l, runtime·class_to_transfercount[sizeclass], sizeclass);
+		ReleaseN(c, l, l->nlist/2, sizeclass);
 	}
 
 	if(c->size >= MaxMCacheSize) {

src/pkg/runtime/mcentral.c

@@ -30,16 +30,15 @@ runtime·MCentral_Init(MCentral *c, int32 sizeclass)
 	runtime·MSpanList_Init(&c->empty);
 }
 
-// Allocate up to n objects from the central free list.
+// Allocate a list of 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.
 int32
-runtime·MCentral_AllocList(MCentral *c, int32 n, MLink **pfirst)
+runtime·MCentral_AllocList(MCentral *c, MLink **pfirst)
 {
 	MSpan *s;
-	MLink *first, *last;
-	int32 cap, avail, i;
+	int32 cap, n;
 
 	runtime·lock(c);
 	// Replenish central list if empty.
@@ -52,31 +51,14 @@ runtime·MCentral_AllocList(MCentral *c, int32 n, MLink **pfirst)
 	}
 	s = c->nonempty.next;
 	cap = (s->npages << PageShift) / s->elemsize;
-	avail = cap - s->ref;
-	if(avail < n)
-		n = avail;
-
-	// First one is guaranteed to work, because we just grew the list.
-	first = s->freelist;
-	last = first;
-	for(i=1; i<n; i++) {
-		last = last->next;
-	}
-	s->freelist = last->next;
-	last->next = nil;
+	n = cap - s->ref;
+	*pfirst = s->freelist;
+	s->freelist = nil;
 	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·MSpanList_Remove(s);
+	runtime·MSpanList_Insert(&c->empty, s);
 	runtime·unlock(c);
-	*pfirst = first;
 	return n;
 }
 

src/pkg/runtime/msize.c

@@ -31,7 +31,6 @@
 
 int32 runtime·class_to_size[NumSizeClasses];
 int32 runtime·class_to_allocnpages[NumSizeClasses];
-int32 runtime·class_to_transfercount[NumSizeClasses];
 
 // The SizeToClass lookup is implemented using two arrays,
 // one mapping sizes <= 1024 to their class and one mapping
@@ -137,16 +136,6 @@ runtime·InitSizes(void)
 	// Copy out for statistics table.
 	for(i=0; i<nelem(runtime·class_to_size); i++)
 		mstats.by_size[i].size = runtime·class_to_size[i];
-
-	// Initialize the runtime·class_to_transfercount table.
-	for(sizeclass = 1; sizeclass < NumSizeClasses; sizeclass++) {
-		n = 64*1024 / runtime·class_to_size[sizeclass];
-		if(n < 2)
-			n = 2;
-		if(n > 32)
-			n = 32;
-		runtime·class_to_transfercount[sizeclass] = n;
-	}
 	return;
 
 dump: