// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Central free lists.
//
// See malloc.h for an overview.
//
// The MCentral doesn't actually contain the list of free objects; the MSpan does.
// Each MCentral is two lists of MSpans: those with free objects (c->nonempty)
// and those that are completely allocated (c->empty).
//
// TODO(rsc): tcmalloc uses a "transfer cache" to split the list
// into sections of class_to_transfercount[sizeclass] objects
// so that it is faster to move those lists between MCaches and MCentrals.
#include "runtime.h"
#include "arch_GOARCH.h"
#include "malloc.h"
static bool MCentral_Grow(MCentral *c);
static void MCentral_Free(MCentral *c, MLink *v);
static void MCentral_ReturnToHeap(MCentral *c, MSpan *s);
// Initialize a single central free list.
void
runtime·MCentral_Init(MCentral *c, int32 sizeclass)
{
c->sizeclass = sizeclass;
runtime·MSpanList_Init(&c->nonempty);
runtime·MSpanList_Init(&c->empty);
}
// Allocate a span to use in an MCache.
MSpan*
runtime·MCentral_CacheSpan(MCentral *c)
{
MSpan *s;
int32 cap, n;
uint32 sg;
runtime·lock(c);
sg = runtime·mheap.sweepgen;
retry:
for(s = c->nonempty.next; s != &c->nonempty; s = s->next) {
if(s->sweepgen == sg-2 && runtime·cas(&s->sweepgen, sg-2, sg-1)) {
runtime·unlock(c);
runtime·MSpan_Sweep(s);
runtime·lock(c);
// the span could have been moved to heap, retry
goto retry;
}
if(s->sweepgen == sg-1) {
// the span is being swept by background sweeper, skip
continue;
}
// we have a nonempty span that does not require sweeping, allocate from it
goto havespan;
}
for(s = c->empty.next; s != &c->empty; s = s->next) {
if(s->sweepgen == sg-2 && runtime·cas(&s->sweepgen, sg-2, sg-1)) {
// we have an empty span that requires sweeping,
// sweep it and see if we can free some space in it
runtime·MSpanList_Remove(s);
// swept spans are at the end of the list
runtime·MSpanList_InsertBack(&c->empty, s);
runtime·unlock(c);
runtime·MSpan_Sweep(s);
runtime·lock(c);
// the span could be moved to nonempty or heap, retry
goto retry;
}
if(s->sweepgen == sg-1) {
// the span is being swept by background sweeper, skip
continue;
}
// already swept empty span,
// all subsequent ones must also be either swept or in process of sweeping
break;
}
// Replenish central list if empty.
if(!MCentral_Grow(c)) {
runtime·unlock(c);
return nil;
}
goto retry;
havespan:
cap = (s->npages << PageShift) / s->elemsize;
n = cap - s->ref;
if(n == 0)
runtime·throw("empty span");
if(s->freelist == nil)
runtime·throw("freelist empty");
c->nfree -= n;
runtime·MSpanList_Remove(s);
runtime·MSpanList_InsertBack(&c->empty, s);
s->incache = true;
runtime·unlock(c);
return s;
}
// Return span from an MCache.
void
runtime·MCentral_UncacheSpan(MCentral *c, MSpan *s)
{
MLink *v;
int32 cap, n;
runtime·lock(c);
s->incache = false;
// Move any explicitly freed items from the freebuf to the freelist.
while((v = s->freebuf) != nil) {
s->freebuf = v->next;
runtime·markfreed(v);
v->next = s->freelist;
s->freelist = v;
s->ref--;
}
if(s->ref == 0) {
// Free back to heap. Unlikely, but possible.
MCentral_ReturnToHeap(c, s); // unlocks c
return;
}
cap = (s->npages << PageShift) / s->elemsize;
n = cap - s->ref;
if(n > 0) {
c->nfree += n;
runtime·MSpanList_Remove(s);
runtime·MSpanList_Insert(&c->nonempty, s);
}
runtime·unlock(c);
}
// Free the list of objects back into the central free list c.
// Called from runtime·free.
void
runtime·MCentral_FreeList(MCentral *c, MLink *start)
{
MLink *next;
runtime·lock(c);
for(; start != nil; start = next) {
next = start->next;
MCentral_Free(c, start);
}
runtime·unlock(c);
}
// Helper: free one object back into the central free list.
// Caller must hold lock on c on entry. Holds lock on exit.
static void
MCentral_Free(MCentral *c, MLink *v)
{
MSpan *s;
// Find span for v.
s = runtime·MHeap_Lookup(&runtime·mheap, v);
if(s == nil || s->ref == 0)
runtime·throw("invalid free");
if(s->sweepgen != runtime·mheap.sweepgen)
runtime·throw("free into unswept span");
// If the span is currently being used unsynchronized by an MCache,
// we can't modify the freelist. Add to the freebuf instead. The
// items will get moved to the freelist when the span is returned
// by the MCache.
if(s->incache) {
v->next = s->freebuf;
s->freebuf = v;
return;
}
// Move span to nonempty if necessary.
if(s->freelist == nil) {
runtime·MSpanList_Remove(s);
runtime·MSpanList_Insert(&c->nonempty, s);
}
// Add the object to span's free list.
runtime·markfreed(v);
v->next = s->freelist;
s->freelist = v;
s->ref--;
c->nfree++;
// If s is completely freed, return it to the heap.
if(s->ref == 0) {
MCentral_ReturnToHeap(c, s); // unlocks c
runtime·lock(c);
}
}
// Free n objects from a span s back into the central free list c.
// Called during sweep.
// Returns true if the span was returned to heap. Sets sweepgen to
// the latest generation.
bool
runtime·MCentral_FreeSpan(MCentral *c, MSpan *s, int32 n, MLink *start, MLink *end)
{
if(s->incache)
runtime·throw("freespan into cached span");
runtime·lock(c);
// Move to nonempty if necessary.
if(s->freelist == nil) {
runtime·MSpanList_Remove(s);
runtime·MSpanList_Insert(&c->nonempty, s);
}
// Add the objects back to s's free list.
end->next = s->freelist;
s->freelist = start;
s->ref -= n;
c->nfree += n;
// delay updating sweepgen until here. This is the signal that
// the span may be used in an MCache, so it must come after the
// linked list operations above (actually, just after the
// lock of c above.)
runtime·atomicstore(&s->sweepgen, runtime·mheap.sweepgen);
if(s->ref != 0) {
runtime·unlock(c);
return false;
}
// s is completely freed, return it to the heap.
MCentral_ReturnToHeap(c, s); // unlocks c
return true;
}
void
runtime·MGetSizeClassInfo(int32 sizeclass, uintptr *sizep, int32 *npagesp, int32 *nobj)
{
int32 size;
int32 npages;
npages = runtime·class_to_allocnpages[sizeclass];
size = runtime·class_to_size[sizeclass];
*npagesp = npages;
*sizep = size;
*nobj = (npages << PageShift) / size;
}
// Fetch a new span from the heap and
// carve into objects for the free list.
static bool
MCentral_Grow(MCentral *c)
{
int32 i, n, npages;
uintptr size;
MLink **tailp, *v;
byte *p;
MSpan *s;
runtime·unlock(c);
runtime·MGetSizeClassInfo(c->sizeclass, &size, &npages, &n);
s = runtime·MHeap_Alloc(&runtime·mheap, npages, c->sizeclass, 0, 1);
if(s == nil) {
// TODO(rsc): Log out of memory
runtime·lock(c);
return false;
}
// Carve span into sequence of blocks.
tailp = &s->freelist;
p = (byte*)(s->start << PageShift);
s->limit = p + size*n;
for(i=0; i<n; i++) {
v = (MLink*)p;
*tailp = v;
tailp = &v->next;
p += size;
}
*tailp = nil;
runtime·markspan((byte*)(s->start<<PageShift), size, n, size*n < (s->npages<<PageShift));
runtime·lock(c);
c->nfree += n;
runtime·MSpanList_Insert(&c->nonempty, s);
return true;
}
// Return s to the heap. s must be unused (s->ref == 0). Unlocks c.
static void
MCentral_ReturnToHeap(MCentral *c, MSpan *s)
{
int32 size;
size = runtime·class_to_size[c->sizeclass];
runtime·MSpanList_Remove(s);
s->needzero = 1;
s->freelist = nil;
if(s->ref != 0)
runtime·throw("ref wrong");
c->nfree -= (s->npages << PageShift) / size;
runtime·unlock(c);
runtime·unmarkspan((byte*)(s->start<<PageShift), s->npages<<PageShift);
runtime·MHeap_Free(&runtime·mheap, s, 0);
}