src/pkg/runtime/mgc0.c

// 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.

// Garbage collector -- step 0.
//
// Stop the world, mark and sweep garbage collector.
// NOT INTENDED FOR PRODUCTION USE.
//
// A mark and sweep collector provides a way to exercise
// and test the memory allocator and the stack walking machinery
// without also needing to get reference counting
// exactly right.

#include "runtime.h"
#include "malloc.h"

enum {
	Debug = 0
};

extern byte data[];
extern byte etext[];
extern byte end[];

static G *fing;
static Finalizer *finq;
static void sweepblock(byte*, int64, uint32*, int32);
static void runfinq(void);

enum {
	PtrSize = sizeof(void*)
};

static void
scanblock(int32 depth, byte *b, int64 n)
{
	int32 off;
	void *obj;
	uintptr size;
	uint32 *refp, ref;
	void **vp;
	int64 i;

	if(Debug > 1)
		printf("%d scanblock %p %D\n", depth, b, n);
	off = (uint32)(uintptr)b & (PtrSize-1);
	if(off) {
		b += PtrSize - off;
		n -= PtrSize - off;
	}

	vp = (void**)b;
	n /= PtrSize;
	for(i=0; i<n; i++) {
		obj = vp[i];
		if(obj == nil || (byte*)obj < mheap.min || (byte*)obj >= mheap.max)
			continue;
		if(mlookup(obj, &obj, &size, nil, &refp)) {
			ref = *refp;
			switch(ref & ~RefFlags) {
			case RefNone:
				if(Debug > 1)
					printf("%d found at %p: ", depth, &vp[i]);
				*refp = RefSome | (ref & RefFlags);
				if(!(ref & RefNoPointers))
					scanblock(depth+1, obj, size);
				break;
			}
		}
	}
}

static void
scanstack(G *gp)
{
	Stktop *stk;
	byte *sp;

	if(gp == g)
		sp = (byte*)&gp;
	else
		sp = gp->sched.sp;
	if(Debug > 1)
		printf("scanstack %d %p\n", gp->goid, sp);
	stk = (Stktop*)gp->stackbase;
	while(stk) {
		scanblock(0, sp, (byte*)stk - sp);
		sp = stk->gobuf.sp;
		stk = (Stktop*)stk->stackbase;
	}
}

static void
markfin(void *v)
{
	uintptr size;
	uint32 *refp;

	size = 0;
	refp = nil;
	if(!mlookup(v, &v, &size, nil, &refp) || !(*refp & RefHasFinalizer))
		throw("mark - finalizer inconsistency");
	
	// do not mark the finalizer block itself.  just mark the things it points at.
	scanblock(1, v, size);
}

static void
mark(void)
{
	G *gp;

	// mark data+bss.
	// skip mheap itself, which has no interesting pointers
	// and is mostly zeroed and would not otherwise be paged in.
	scanblock(0, data, (byte*)&mheap - data);
	scanblock(0, (byte*)(&mheap+1), end - (byte*)(&mheap+1));

	// mark stacks
	for(gp=allg; gp!=nil; gp=gp->alllink) {
		switch(gp->status){
		default:
			printf("unexpected G.status %d\n", gp->status);
			throw("mark - bad status");
		case Gdead:
			break;
		case Grunning:
			if(gp != g)
				throw("mark - world not stopped");
			scanstack(gp);
			break;
		case Grunnable:
		case Gsyscall:
		case Gwaiting:
			scanstack(gp);
			break;
		}
	}

	// mark things pointed at by objects with finalizers
	walkfintab(markfin);
}

// free RefNone, free & queue finalizers for RefNone|RefHasFinalizer, reset RefSome
static void
sweepspan(MSpan *s)
{
	int32 n, npages, size;
	byte *p;
	uint32 ref, *gcrefp, *gcrefep;
	MCache *c;
	Finalizer *f;

	p = (byte*)(s->start << PageShift);
	if(s->sizeclass == 0) {
		// Large block.
		ref = s->gcref0;
		switch(ref & ~(RefFlags^RefHasFinalizer)) {
		case RefNone:
			// Free large object.
			mstats.alloc -= s->npages<<PageShift;
			runtime_memclr(p, s->npages<<PageShift);
			if(ref & RefProfiled)
				MProf_Free(p, s->npages<<PageShift);
			s->gcref0 = RefFree;
			MHeap_Free(&mheap, s, 1);
			break;
		case RefNone|RefHasFinalizer:
			f = getfinalizer(p, 1);
			if(f == nil)
				throw("finalizer inconsistency");
			f->arg = p;
			f->next = finq;
			finq = f;
			ref &= ~RefHasFinalizer;
			// fall through
		case RefSome:
		case RefSome|RefHasFinalizer:
			s->gcref0 = RefNone | (ref&RefFlags);
			break;
		}
		return;
	}

	// Chunk full of small blocks.
	MGetSizeClassInfo(s->sizeclass, &size, &npages, &n);
	gcrefp = s->gcref;
	gcrefep = s->gcref + n;
	for(; gcrefp < gcrefep; gcrefp++, p += size) {
		ref = *gcrefp;
		if(ref < RefNone)	// RefFree or RefStack
			continue;
		switch(ref & ~(RefFlags^RefHasFinalizer)) {
		case RefNone:
			// Free small object.
			if(ref & RefProfiled)
				MProf_Free(p, size);
			*gcrefp = RefFree;
			c = m->mcache;
			if(size > sizeof(uintptr))
				((uintptr*)p)[1] = 1;	// mark as "needs to be zeroed"
			mstats.alloc -= size;
			mstats.by_size[s->sizeclass].nfree++;
			MCache_Free(c, p, s->sizeclass, size);
			break;
		case RefNone|RefHasFinalizer:
			f = getfinalizer(p, 1);
			if(f == nil)
				throw("finalizer inconsistency");
			f->arg = p;
			f->next = finq;
			finq = f;
			ref &= ~RefHasFinalizer;
			// fall through
		case RefSome:
		case RefSome|RefHasFinalizer:
			*gcrefp = RefNone | (ref&RefFlags);
			break;
		}
	}
}

static void
sweep(void)
{
	MSpan *s;

	for(s = mheap.allspans; s != nil; s = s->allnext)
		if(s->state == MSpanInUse)
			sweepspan(s);
}

// Semaphore, not Lock, so that the goroutine
// reschedules when there is contention rather
// than spinning.
static uint32 gcsema = 1;

// Initialized from $GOGC.  GOGC=off means no gc.
//
// Next gc is after we've allocated an extra amount of
// memory proportional to the amount already in use.
// If gcpercent=100 and we're using 4M, we'll gc again
// when we get to 8M.  This keeps the gc cost in linear
// proportion to the allocation cost.  Adjusting gcpercent
// just changes the linear constant (and also the amount of
// extra memory used).
static int32 gcpercent = -2;

static void
stealcache(void)
{
	M *m;
	
	for(m=allm; m; m=m->alllink)
		MCache_ReleaseAll(m->mcache);
}

void
gc(int32 force)
{
	int64 t0, t1;
	byte *p;
	Finalizer *fp;

	// The gc is turned off (via enablegc) until
	// the bootstrap has completed.
	// Also, malloc gets called in the guts
	// of a number of libraries that might be
	// holding locks.  To avoid priority inversion
	// problems, don't bother trying to run gc
	// while holding a lock.  The next mallocgc
	// without a lock will do the gc instead.
	if(!mstats.enablegc || m->locks > 0 || panicking)
		return;

	if(gcpercent == -2) {	// first time through
		p = getenv("GOGC");
		if(p == nil || p[0] == '\0')
			gcpercent = 100;
		else if(strcmp(p, (byte*)"off") == 0)
			gcpercent = -1;
		else
			gcpercent = atoi(p);
	}
	if(gcpercent < 0)
		return;

	semacquire(&gcsema);
	t0 = nanotime();
	m->gcing = 1;
	stoptheworld();
	if(mheap.Lock.key != 0)
		throw("mheap locked during gc");
	if(force || mstats.heap_alloc >= mstats.next_gc) {
		mark();
		sweep();
		stealcache();
		mstats.next_gc = mstats.heap_alloc+mstats.heap_alloc*gcpercent/100;
	}
	m->gcing = 0;

	m->locks++;	// disable gc during the mallocs in newproc
	fp = finq;
	if(fp != nil) {
		// kick off or wake up goroutine to run queued finalizers
		if(fing == nil)
			fing = newproc1((byte*)runfinq, nil, 0, 0);
		else if(fing->status == Gwaiting)
			ready(fing);
	}
	m->locks--;

	t1 = nanotime();
	mstats.numgc++;
	mstats.pause_ns += t1 - t0;
	if(mstats.debuggc)
		printf("pause %D\n", t1-t0);
	semrelease(&gcsema);
	starttheworld();
	
	// give the queued finalizers, if any, a chance to run
	if(fp != nil)
		gosched();
}

static void
runfinq(void)
{
	Finalizer *f, *next;
	byte *frame;

	for(;;) {
		// There's no need for a lock in this section
		// because it only conflicts with the garbage
		// collector, and the garbage collector only
		// runs when everyone else is stopped, and
		// runfinq only stops at the gosched() or
		// during the calls in the for loop.
		f = finq;
		finq = nil;
		if(f == nil) {
			g->status = Gwaiting;
			gosched();
			continue;
		}
		for(; f; f=next) {
			next = f->next;
			frame = mal(sizeof(uintptr) + f->nret);
			*(void**)frame = f->arg;
			reflect·call((byte*)f->fn, frame, sizeof(uintptr) + f->nret);
			free(frame);
			f->fn = nil;
			f->arg = nil;
			f->next = nil;
		}
		gc(1);	// trigger another gc to clean up the finalized objects, if possible
	}
}