// Garbage collector (GC).
//
-// GC is:
-// - mark&sweep
-// - mostly precise (with the exception of some C-allocated objects, assembly frames/arguments, etc)
-// - parallel (up to MaxGcproc threads)
-// - partially concurrent (mark is stop-the-world, while sweep is concurrent)
-// - non-moving/non-compacting
-// - full (non-partial)
+// The GC runs concurrently with mutator threads, is type accurate (aka precise), allows multiple GC
+// thread to run in parallel. It is a concurrent mark and sweep that uses a write barrier. It is
+// non-generational and non-compacting. Allocation is done using size segregated per P allocation
+// areas to minimize fragmentation while eliminating locks in the common case.
//
-// GC rate.
-// Next GC is after we've allocated an extra amount of memory proportional to
-// the amount already in use. The proportion is controlled by GOGC environment variable
-// (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M
-// (this mark is tracked in next_gc variable). This keeps the GC cost in linear
-// proportion to the allocation cost. Adjusting GOGC just changes the linear constant
-// (and also the amount of extra memory used).
+// The algorithm decomposes into several steps.
+// This is a high level description of the algorithm being used. For an overview of GC a good
+// place to start is Richard Jones' gchandbook.org.
+//
+// The algorithm's intellectual heritage includes Dijkstra's on-the-fly algorithm, see
+// Edsger W. Dijkstra, Leslie Lamport, A. J. Martin, C. S. Scholten, and E. F. M. Steffens. 1978.
+// On-the-fly garbage collection: an exercise in cooperation. Commun. ACM 21, 11 (November 1978), 966-975.
+// For journal quality proofs that these steps are complete, correct, and terminate see
+// Hudson, R., and Moss, J.E.B. Copying Garbage Collection without stopping the world.
+// Concurrency and Computation: Practice and Experience 15(3-5), 2003.
//
+// 0. Set phase = GCscan from GCoff.
+// 1. Wait for all P's to acknowledge phase change.
+// At this point all goroutines have passed through a GC safepoint and
+// know we are in the GCscan phase.
+// 2. GC scans all goroutine stacks, mark and enqueues all encountered pointers
+// (marking avoids most duplicate enqueuing but races may produce duplication which is benign).
+// Preempted goroutines are scanned before P schedules next goroutine.
+// 3. Set phase = GCmark.
+// 4. Wait for all P's to acknowledge phase change.
+// 5. Now write barrier marks and enqueues black or grey to white pointers. If a pointer is
+// stored into a white slot, such pointer is not marked.
+// Malloc still allocates white (non-marked) objects.
+// 6. Meanwhile GC transitively walks the heap marking reachable objects.
+// 7. When GC finishes marking heap, it preempts P's one-by-one and
+// retakes partial wbufs (filled by write barrier or during a stack scan of the goroutine
+// currently scheduled on the P).
+// 8. Once the GC has exhausted all available marking work it sets phase = marktermination.
+// 9. Wait for all P's to acknowledge phase change.
+// 10. Malloc now allocates black objects, so number of unmarked reachable objects
+// monotonically decreases.
+// 11. GC preempts P's one-by-one taking partial wbufs and marks all unmarked yet reachable objects.
+// 12. When GC completes a full cycle over P's and discovers no new grey
+// objects, (which means all reachable objects are marked) set phase = GCsweep.
+// 13. Wait for all P's to acknowledge phase change.
+// 14. Now malloc allocates white (but sweeps spans before use).
+// Write barrier becomes nop.
+// 15. GC does background sweeping, see description below.
+// 16. When sweeping is complete set phase to GCoff.
+// 17. When sufficient allocation has taken place replay the sequence starting at 0 above,
+// see discussion of GC rate below.
+
+// Changing phases.
+// Phases are changed by setting the gcphase to the next phase and possibly calling ackgcphase.
+// All phase action must be benign in the presence of a change.
+// Starting with GCoff
+// GCoff to GCscan
+// GSscan scans stacks and globals greying them and never marks an object black.
+// Once all the P's are aware of the new phase they will scan gs on preemption.
+// This means that the scanning of preempted gs can't start until all the Ps
+// have acknowledged.
+// GCscan to GCmark
+// GCMark turns on the write barrier which also only greys objects. No scanning
+// of objects (making them black) can happen until all the Ps have acknowledged
+// the phase change.
+// GCmark to GCmarktermination
+// The only change here is that we start allocating black so the Ps must acknowledge
+// the change before we begin the termination algorithm
+// GCmarktermination to GSsweep
+// Object currently on the freelist must be marked black for this to work.
+// Are things on the free lists black or white? How does the sweep phase work?
+
// Concurrent sweep.
// The sweep phase proceeds concurrently with normal program execution.
// The heap is swept span-by-span both lazily (when a goroutine needs another span)
// The finalizer goroutine is kicked off only when all spans are swept.
// When the next GC starts, it sweeps all not-yet-swept spans (if any).
+// GC rate.
+// Next GC is after we've allocated an extra amount of memory proportional to
+// the amount already in use. The proportion is controlled by GOGC environment variable
+// (100 by default). If GOGC=100 and we're using 4M, we'll GC again when we get to 8M
+// (this mark is tracked in next_gc variable). This keeps the GC cost in linear
+// proportion to the allocation cost. Adjusting GOGC just changes the linear constant
+// (and also the amount of extra memory used).
+
#include "runtime.h"
#include "arch_GOARCH.h"
#include "malloc.h"
enum {
Debug = 0,
- DebugPtrs = 0, // if 1, print trace of every pointer load during GC
- ConcurrentSweep = 0,
+ ConcurrentSweep = 1,
- WorkbufSize = 4*1024,
FinBlockSize = 4*1024,
RootData = 0,
RootBss = 1,
// ptrmask for an allocation containing a single pointer.
static byte oneptr[] = {BitsPointer};
-// Initialized from $GOGC. GOGC=off means no gc.
+// Initialized from $GOGC. GOGC=off means no GC.
extern int32 runtime·gcpercent;
// Holding worldsema grants an M the right to try to stop the world.
//
uint32 runtime·worldsema = 1;
-typedef struct Workbuf Workbuf;
-struct Workbuf
-{
- LFNode node; // must be first
- uintptr nobj;
- byte* obj[(WorkbufSize-sizeof(LFNode)-sizeof(uintptr))/PtrSize];
+typedef struct Markbits Markbits;
+struct Markbits {
+ byte *bitp; // pointer to the byte holding xbits
+ byte shift; // bits xbits needs to be shifted to get bits
+ byte xbits; // byte holding all the bits from *bitp
+ byte bits; // bits relevant to corresponding slot.
};
extern byte runtime·data[];
Mutex runtime·gclock;
-static uintptr badblock[1024];
-static int32 nbadblock;
-
+static Workbuf* getpartialorempty(void);
+static void putpartial(Workbuf*);
static Workbuf* getempty(Workbuf*);
static Workbuf* getfull(Workbuf*);
static void putempty(Workbuf*);
+static void putfull(Workbuf*);
static Workbuf* handoff(Workbuf*);
static void gchelperstart(void);
static void flushallmcaches(void);
-static bool scanframe(Stkframe *frame, void *unused);
-static void scanstack(G *gp);
-static BitVector unrollglobgcprog(byte *prog, uintptr size);
+static bool scanframe(Stkframe*, void*);
+static void scanstack(G*);
+static BitVector unrollglobgcprog(byte*, uintptr);
+static void scanblock(byte*, uintptr, byte*);
+static byte* objectstart(byte*, Markbits*);
+static Workbuf* greyobject(byte*, Markbits*, Workbuf*);
+static bool inheap(byte*);
+static bool shaded(byte*);
+static void shade(byte*);
+static void slottombits(byte*, Markbits*);
void runtime·bgsweep(void);
+void runtime·finishsweep_m(void);
static FuncVal bgsweepv = {runtime·bgsweep};
typedef struct WorkData WorkData;
struct WorkData {
- uint64 full; // lock-free list of full blocks
- uint64 empty; // lock-free list of empty blocks
+ uint64 full; // lock-free list of full blocks
+ uint64 empty; // lock-free list of empty blocks
+ uint64 partial; // lock-free list of partially filled blocks
byte pad0[CacheLineSize]; // prevents false-sharing between full/empty and nproc/nwait
uint32 nproc;
int64 tstart;
};
WorkData runtime·work;
-// Is _cgo_allocate linked into the binary?
+// Is address b in the known heap. If it doesn't have a valid gcmap
+// returns false. For example pointers into stacks will return false.
static bool
-have_cgo_allocate(void)
+inheap(byte *b)
{
- extern byte go·weak·runtime·_cgo_allocate_internal[1];
- return go·weak·runtime·_cgo_allocate_internal != nil;
+ MSpan *s;
+ pageID k;
+ uintptr x;
+
+ if(b == nil || b < runtime·mheap.arena_start || b >= runtime·mheap.arena_used)
+ return false;
+ // Not a beginning of a block, consult span table to find the block beginning.
+ k = (uintptr)b>>PageShift;
+ x = k;
+ x -= (uintptr)runtime·mheap.arena_start>>PageShift;
+ s = runtime·mheap.spans[x];
+ if(s == nil || k < s->start || b >= s->limit || s->state != MSpanInUse)
+ return false;
+ return true;
}
-// scanblock scans a block of n bytes starting at pointer b for references
-// to other objects, scanning any it finds recursively until there are no
-// unscanned objects left. Instead of using an explicit recursion, it keeps
-// a work list in the Workbuf* structures and loops in the main function
-// body. Keeping an explicit work list is easier on the stack allocator and
-// more efficient.
+// Given an address in the heap return the relevant byte from the gcmap. This routine
+// can be used on addresses to the start of an object or to the interior of the an object.
static void
-scanblock(byte *b, uintptr n, byte *ptrmask)
+slottombits(byte *obj, Markbits *mbits)
{
- byte *obj, *obj0, *p, *arena_start, *arena_used, **wp, *scanbuf[8], *ptrbitp, *bitp;
- uintptr i, j, nobj, size, idx, x, off, scanbufpos, bits, xbits, shift;
- Workbuf *wbuf;
- Iface *iface;
- Eface *eface;
- Type *typ;
+ uintptr off;
+
+ off = (uintptr*)((uintptr)obj&~(PtrSize-1)) - (uintptr*)runtime·mheap.arena_start;
+ mbits->bitp = runtime·mheap.arena_start - off/wordsPerBitmapByte - 1;
+ mbits->shift = (off % wordsPerBitmapByte) * gcBits;
+ mbits->xbits = *mbits->bitp;
+ mbits->bits = (mbits->xbits >> mbits->shift) & bitMask;
+}
+
+// b is a pointer into the heap.
+// Find the start of the object refered to by b.
+// Set mbits to the associated bits from the bit map.
+static byte*
+objectstart(byte *b, Markbits *mbits)
+{
+ byte *obj, *p;
MSpan *s;
pageID k;
- bool keepworking;
+ uintptr x, size, idx;
- // Cache memory arena parameters in local vars.
- arena_start = runtime·mheap.arena_start;
- arena_used = runtime·mheap.arena_used;
+ obj = (byte*)((uintptr)b&~(PtrSize-1));
+ for(;;) {
+ slottombits(obj, mbits);
+ if(mbits->bits&bitBoundary == bitBoundary)
+ break;
+
+ // Not a beginning of a block, consult span table to find the block beginning.
+ k = (uintptr)obj>>PageShift;
+ x = k;
+ x -= (uintptr)runtime·mheap.arena_start>>PageShift;
+ s = runtime·mheap.spans[x];
+ if(s == nil || k < s->start || obj >= s->limit || s->state != MSpanInUse){
+ if(s->state == MSpanStack)
+ break; // This is legit.
+
+ // The following is catching some bugs left over from
+ // us not being rigerous about what data structures are
+ // hold valid pointers and different parts of the system
+ // considering different structures as roots. For example
+ // if there is a pointer into a stack that is left in
+ // a global data structure but that part of the runtime knows that
+ // those structures will be reinitialized before they are
+ // reused. Unfortunately the GC believes these roots are valid.
+ // Typically a stack gets moved and only the structures that part of
+ // the system knows are alive are updated. The span is freed
+ // after the stack copy and the pointer is still alive. This
+ // check is catching that bug but for now we will not throw,
+ // instead we will simply break out of this routine and depend
+ // on the caller to recognize that this pointer is not a valid
+ // heap pointer. I leave the code that catches the bug so that once
+ // resolved we can turn this check back on and throw.
+
+ //runtime·printf("Runtime: Span weird: obj=%p, k=%p", obj, k);
+ //if (s == nil)
+ // runtime·printf(" s=nil\n");
+ //else
+ // runtime·printf(" s->start=%p s->limit=%p, s->state=%d\n", s->start*PageSize, s->limit, s->state);
+ //runtime·throw("Blowup on weird span");
+ break; // We are not in a real block throw??
+ }
+ p = (byte*)((uintptr)s->start<<PageShift);
+ if(s->sizeclass != 0) {
+ size = s->elemsize;
+ idx = ((byte*)obj - p)/size;
+ p = p+idx*size;
+ }
+ if(p == obj) {
+ runtime·printf("runtime: failed to find block beginning for %p s=%p s->limit=%p\n",
+ p, s->start*PageSize, s->limit);
+ runtime·throw("failed to find block beginning");
+ }
+ obj = p;
+ }
+ // if size(obj.firstfield) < PtrSize, the &obj.secondfield could map to the boundary bit
+ // Clear any low bits to get to the start of the object.
+ // greyobject depends on this.
+ return obj;
+}
- wbuf = getempty(nil);
- nobj = wbuf->nobj;
- wp = &wbuf->obj[nobj];
- keepworking = b == nil;
- scanbufpos = 0;
- for(i = 0; i < nelem(scanbuf); i++)
- scanbuf[i] = nil;
+// obj is the start of an object with mark mbits.
+// If it isn't already marked, mark it and enqueue into workbuf.
+// Return possibly new workbuf to use.
+static Workbuf*
+greyobject(byte *obj, Markbits *mbits, Workbuf *wbuf)
+{
+ // obj should be start of allocation, and so must be at least pointer-aligned.
+ if(((uintptr)obj & (PtrSize-1)) != 0)
+ runtime·throw("greyobject: obj not pointer-aligned");
+
+ // If marked we have nothing to do.
+ if((mbits->bits&bitMarked) != 0)
+ return wbuf;
+
+ // Each byte of GC bitmap holds info for two words.
+ // If the current object is larger than two words, or if the object is one word
+ // but the object it shares the byte with is already marked,
+ // then all the possible concurrent updates are trying to set the same bit,
+ // so we can use a non-atomic update.
+ if((mbits->xbits&(bitMask|(bitMask<<gcBits))) != (bitBoundary|(bitBoundary<<gcBits)) || runtime·work.nproc == 1)
+ *mbits->bitp = mbits->xbits | (bitMarked<<mbits->shift);
+ else
+ runtime·atomicor8(mbits->bitp, bitMarked<<mbits->shift);
+
+ if(((mbits->xbits>>(mbits->shift+2))&BitsMask) == BitsDead)
+ return wbuf; // noscan object
+
+ // Queue the obj for scanning. The PREFETCH(obj) logic has been removed but
+ // seems like a nice optimization that can be added back in.
+ // There needs to be time between the PREFETCH and the use.
+ // Previously we put the obj in an 8 element buffer that is drained at a rate
+ // to give the PREFETCH time to do its work.
+ // Use of PREFETCHNTA might be more appropriate than PREFETCH
+
+ // If workbuf is full, obtain an empty one.
+ if(wbuf->nobj >= nelem(wbuf->obj)) {
+ wbuf = getempty(wbuf);
+ }
+
+ wbuf->obj[wbuf->nobj] = obj;
+ wbuf->nobj++;
+ return wbuf;
+}
+
+// Scan the object b of size n, adding pointers to wbuf.
+// Return possibly new wbuf to use.
+// If ptrmask != nil, it specifies where pointers are in b.
+// If ptrmask == nil, the GC bitmap should be consulted.
+// In this case, n may be an overestimate of the size; the GC bitmap
+// must also be used to make sure the scan stops at the end of b.
+static Workbuf*
+scanobject(byte *b, uintptr n, byte *ptrmask, Workbuf *wbuf)
+{
+ byte *obj, *arena_start, *arena_used, *ptrbitp;
+ uintptr i, j;
+ int32 bits;
+ Markbits mbits;
+ arena_start = (byte*)runtime·mheap.arena_start;
+ arena_used = runtime·mheap.arena_used;
ptrbitp = nil;
+ // Find bits of the beginning of the object.
+ if(ptrmask == nil) {
+ b = objectstart(b, &mbits);
+ ptrbitp = mbits.bitp; //arena_start - off/wordsPerBitmapByte - 1;
+ }
+ for(i = 0; i < n; i += PtrSize) {
+ // Find bits for this word.
+ if(ptrmask != nil) {
+ // dense mask (stack or data)
+ bits = (ptrmask[(i/PtrSize)/4]>>(((i/PtrSize)%4)*BitsPerPointer))&BitsMask;
+ } else {
+ // Check if we have reached end of span.
+ if((((uintptr)b+i)%PageSize) == 0 &&
+ runtime·mheap.spans[(b-arena_start)>>PageShift] != runtime·mheap.spans[(b+i-arena_start)>>PageShift])
+ break;
+ // Consult GC bitmap.
+ bits = *ptrbitp;
+ if(wordsPerBitmapByte != 2)
+ runtime·throw("alg doesn't work for wordsPerBitmapByte != 2");
+ j = ((uintptr)b+i)/PtrSize & 1;
+ bits >>= gcBits*j;
+ if(i == 0)
+ bits &= ~bitBoundary;
+ ptrbitp -= j;
+
+ if((bits&bitBoundary) != 0 && i != 0)
+ break; // reached beginning of the next object
+ bits = (bits>>2)&BitsMask;
+ if(bits == BitsDead)
+ break; // reached no-scan part of the object
+ }
+
+ if(bits <= BitsScalar) // Bits Scalar || BitsDead
+ continue;
+ if(bits != BitsPointer) {
+ runtime·printf("gc bits=%x\n", bits);
+ runtime·throw("unexpected garbage collection bits");
+ }
+
+ obj = *(byte**)(b+i);
+ // At this point we have extracted the next potential pointer.
+ // Check if it points into heap.
+ if(obj == nil || obj < arena_start || obj >= arena_used)
+ continue;
+ // Mark the object. return some important bits.
+ // We we combine the following two rotines we don't have to pass mbits or obj around.
+ obj = objectstart(obj, &mbits);
+ wbuf = greyobject(obj, &mbits, wbuf);
+ }
+ return wbuf;
+}
+
+// scanblock starts by scanning b as scanobject would.
+// If the gcphase is GCscan, that's all scanblock does.
+// Otherwise it traverses some fraction of the pointers it found in b, recursively.
+// As a special case, scanblock(nil, 0, nil) means to scan previously queued work,
+// stopping only when no work is left in the system.
+static void
+scanblock(byte *b, uintptr n, byte *ptrmask)
+{
+ Workbuf *wbuf;
+ bool keepworking;
+
+ wbuf = getpartialorempty();
+ if(b != nil) {
+ wbuf = scanobject(b, n, ptrmask, wbuf);
+ if(runtime·gcphase == GCscan) {
+ if(inheap(b) && !ptrmask)
+ // b is in heap, we are in GCscan so there should be a ptrmask.
+ runtime·throw("scanblock: In GCscan phase and inheap is true.");
+ // GCscan only goes one level deep since mark wb not turned on.
+ putpartial(wbuf);
+ return;
+ }
+ }
+ if(runtime·gcphase == GCscan) {
+ runtime·throw("scanblock: In GCscan phase but no b passed in.");
+ }
+
+ keepworking = b == nil;
+
// ptrmask can have 2 possible values:
// 1. nil - obtain pointer mask from GC bitmap.
// 2. pointer to a compact mask (for stacks and data).
- if(b != nil)
- goto scanobj;
for(;;) {
- if(nobj == 0) {
- // Out of work in workbuf.
- // First, see is there is any work in scanbuf.
- for(i = 0; i < nelem(scanbuf); i++) {
- b = scanbuf[scanbufpos];
- scanbuf[scanbufpos++] = nil;
- scanbufpos %= nelem(scanbuf);
- if(b != nil) {
- n = arena_used - b; // scan until bitBoundary or BitsDead
- ptrmask = nil; // use GC bitmap for pointer info
- goto scanobj;
- }
- }
+ if(wbuf->nobj == 0) {
if(!keepworking) {
putempty(wbuf);
return;
}
// Refill workbuf from global queue.
wbuf = getfull(wbuf);
- if(wbuf == nil)
+ if(wbuf == nil) // nil means out of work barrier reached
return;
- nobj = wbuf->nobj;
- wp = &wbuf->obj[nobj];
+
+ if(wbuf->nobj<=0) {
+ runtime·throw("runtime:scanblock getfull returns empty buffer");
+ }
+
}
// If another proc wants a pointer, give it some.
- if(runtime·work.nwait > 0 && nobj > 4 && runtime·work.full == 0) {
- wbuf->nobj = nobj;
+ if(runtime·work.nwait > 0 && wbuf->nobj > 4 && runtime·work.full == 0) {
wbuf = handoff(wbuf);
- nobj = wbuf->nobj;
- wp = &wbuf->obj[nobj];
- }
-
- wp--;
- nobj--;
- b = *wp;
- n = arena_used - b; // scan until next bitBoundary or BitsDead
- ptrmask = nil; // use GC bitmap for pointer info
-
- scanobj:
- if(DebugPtrs)
- runtime·printf("scanblock %p +%p %p\n", b, n, ptrmask);
- // Find bits of the beginning of the object.
- if(ptrmask == nil) {
- off = (uintptr*)b - (uintptr*)arena_start;
- ptrbitp = arena_start - off/wordsPerBitmapByte - 1;
}
- for(i = 0; i < n; i += PtrSize) {
- obj = nil;
- // Find bits for this word.
- if(ptrmask == nil) {
- // Check is we have reached end of span.
- if((((uintptr)b+i)%PageSize) == 0 &&
- runtime·mheap.spans[(b-arena_start)>>PageShift] != runtime·mheap.spans[(b+i-arena_start)>>PageShift])
- break;
- // Consult GC bitmap.
- bits = *ptrbitp;
-
- if(wordsPerBitmapByte != 2)
- runtime·throw("alg doesn't work for wordsPerBitmapByte != 2");
- j = ((uintptr)b+i)/PtrSize & 1;
- ptrbitp -= j;
- bits >>= gcBits*j;
-
- if((bits&bitBoundary) != 0 && i != 0)
- break; // reached beginning of the next object
- bits = (bits>>2)&BitsMask;
- if(bits == BitsDead)
- break; // reached no-scan part of the object
- } else // dense mask (stack or data)
- bits = (ptrmask[(i/PtrSize)/4]>>(((i/PtrSize)%4)*BitsPerPointer))&BitsMask;
-
- if(bits <= BitsScalar) // BitsScalar || BitsDead
- continue;
- if(bits == BitsPointer) {
- obj = *(byte**)(b+i);
- obj0 = obj;
- goto markobj;
- }
- // With those three out of the way, must be multi-word.
- if(Debug && bits != BitsMultiWord)
- runtime·throw("unexpected garbage collection bits");
- // Find the next pair of bits.
- if(ptrmask == nil) {
- bits = *ptrbitp;
- j = ((uintptr)b+i+PtrSize)/PtrSize & 1;
- ptrbitp -= j;
- bits >>= gcBits*j;
- bits = (bits>>2)&BitsMask;
- } else
- bits = (ptrmask[((i+PtrSize)/PtrSize)/4]>>((((i+PtrSize)/PtrSize)%4)*BitsPerPointer))&BitsMask;
-
- if(Debug && bits != BitsIface && bits != BitsEface)
- runtime·throw("unexpected garbage collection bits");
-
- if(bits == BitsIface) {
- iface = (Iface*)(b+i);
- if(iface->tab != nil) {
- typ = iface->tab->type;
- if(!(typ->kind&KindDirectIface) || !(typ->kind&KindNoPointers))
- obj = iface->data;
- }
- } else {
- eface = (Eface*)(b+i);
- typ = eface->type;
- if(typ != nil) {
- if(!(typ->kind&KindDirectIface) || !(typ->kind&KindNoPointers))
- obj = eface->data;
- }
- }
-
- i += PtrSize;
-
- obj0 = obj;
- markobj:
- // At this point we have extracted the next potential pointer.
- // Check if it points into heap.
- if(obj == nil)
- continue;
- if(obj < arena_start || obj >= arena_used) {
- if((uintptr)obj < PhysPageSize && runtime·invalidptr) {
- s = nil;
- goto badobj;
- }
- continue;
- }
- // Mark the object.
- obj = (byte*)((uintptr)obj & ~(PtrSize-1));
- off = (uintptr*)obj - (uintptr*)arena_start;
- bitp = arena_start - off/wordsPerBitmapByte - 1;
- shift = (off % wordsPerBitmapByte) * gcBits;
- xbits = *bitp;
- bits = (xbits >> shift) & bitMask;
- if((bits&bitBoundary) == 0) {
- // Not a beginning of a block, consult span table to find the block beginning.
- k = (uintptr)obj>>PageShift;
- x = k;
- x -= (uintptr)arena_start>>PageShift;
- s = runtime·mheap.spans[x];
- if(s == nil || k < s->start || obj >= s->limit || s->state != MSpanInUse) {
- // Stack pointers lie within the arena bounds but are not part of the GC heap.
- // Ignore them.
- if(s != nil && s->state == MSpanStack)
- continue;
-
- badobj:
- // If cgo_allocate is linked into the binary, it can allocate
- // memory as []unsafe.Pointer that may not contain actual
- // pointers and must be scanned conservatively.
- // In this case alone, allow the bad pointer.
- if(have_cgo_allocate() && ptrmask == nil)
- continue;
-
- // Anything else indicates a bug somewhere.
- // If we're in the middle of chasing down a different bad pointer,
- // don't confuse the trace by printing about this one.
- if(nbadblock > 0)
- continue;
-
- runtime·printf("runtime: garbage collector found invalid heap pointer *(%p+%p)=%p", b, i, obj);
- if(s == nil)
- runtime·printf(" s=nil\n");
- else
- runtime·printf(" span=%p-%p-%p state=%d\n", (uintptr)s->start<<PageShift, s->limit, (uintptr)(s->start+s->npages)<<PageShift, s->state);
- if(ptrmask != nil)
- runtime·throw("invalid heap pointer");
- // Add to badblock list, which will cause the garbage collection
- // to keep repeating until it has traced the chain of pointers
- // leading to obj all the way back to a root.
- if(nbadblock == 0)
- badblock[nbadblock++] = (uintptr)b;
- continue;
- }
- p = (byte*)((uintptr)s->start<<PageShift);
- if(s->sizeclass != 0) {
- size = s->elemsize;
- idx = ((byte*)obj - p)/size;
- p = p+idx*size;
- }
- if(p == obj) {
- runtime·printf("runtime: failed to find block beginning for %p s=%p s->limit=%p\n",
- p, s->start*PageSize, s->limit);
- runtime·throw("failed to find block beginning");
- }
- obj = p;
- goto markobj;
- }
- if(DebugPtrs)
- runtime·printf("scan *%p = %p => base %p\n", b+i, obj0, obj);
-
- if(nbadblock > 0 && (uintptr)obj == badblock[nbadblock-1]) {
- // Running garbage collection again because
- // we want to find the path from a root to a bad pointer.
- // Found possible next step; extend or finish path.
- for(j=0; j<nbadblock; j++)
- if(badblock[j] == (uintptr)b)
- goto AlreadyBad;
- runtime·printf("runtime: found *(%p+%p) = %p+%p\n", b, i, obj0, (uintptr)(obj-obj0));
- if(ptrmask != nil)
- runtime·throw("bad pointer");
- if(nbadblock >= nelem(badblock))
- runtime·throw("badblock trace too long");
- badblock[nbadblock++] = (uintptr)b;
- AlreadyBad:;
- }
-
- // Now we have bits, bitp, and shift correct for
- // obj pointing at the base of the object.
- // Only care about not marked objects.
- if((bits&bitMarked) != 0)
- continue;
- // If obj size is greater than 8, then each byte of GC bitmap
- // contains info for at most one object. In such case we use
- // non-atomic byte store to mark the object. This can lead
- // to double enqueue of the object for scanning, but scanning
- // is an idempotent operation, so it is OK. This cannot lead
- // to bitmap corruption because the single marked bit is the
- // only thing that can change in the byte.
- // For 8-byte objects we use non-atomic store, if the other
- // quadruple is already marked. Otherwise we resort to CAS
- // loop for marking.
- if((xbits&(bitMask|(bitMask<<gcBits))) != (bitBoundary|(bitBoundary<<gcBits)) ||
- runtime·work.nproc == 1)
- *bitp = xbits | (bitMarked<<shift);
- else
- runtime·atomicor8(bitp, bitMarked<<shift);
-
- if(((xbits>>(shift+2))&BitsMask) == BitsDead)
- continue; // noscan object
-
- // Queue the obj for scanning.
- PREFETCH(obj);
- p = scanbuf[scanbufpos];
- scanbuf[scanbufpos++] = obj;
- scanbufpos %= nelem(scanbuf);
- if(p == nil)
- continue;
-
- // If workbuf is full, obtain an empty one.
- if(nobj >= nelem(wbuf->obj)) {
- wbuf->nobj = nobj;
- wbuf = getempty(wbuf);
- nobj = wbuf->nobj;
- wp = &wbuf->obj[nobj];
- }
- *wp = p;
- wp++;
- nobj++;
- }
- if(DebugPtrs)
- runtime·printf("end scanblock %p +%p %p\n", b, n, ptrmask);
-
- if(Debug && ptrmask == nil) {
- // For heap objects ensure that we did not overscan.
- n = 0;
- p = nil;
- if(!runtime·mlookup(b, &p, &n, nil) || b != p || i > n) {
- runtime·printf("runtime: scanned (%p,%p), heap object (%p,%p)\n", b, i, p, n);
- runtime·throw("scanblock: scanned invalid object");
- }
- }
+ // This might be a good place to add prefetch code...
+ // if(wbuf->nobj > 4) {
+ // PREFETCH(wbuf->obj[wbuf->nobj - 3];
+ // }
+ --wbuf->nobj;
+ b = wbuf->obj[wbuf->nobj];
+ wbuf = scanobject(b, runtime·mheap.arena_used - b, nil, wbuf);
}
}
void *p;
uint32 status;
bool restart;
-
+
USED(&desc);
// Note: if you add a case here, please also update heapdump.c:dumproots.
switch(i) {
spf = (SpecialFinalizer*)sp;
// A finalizer can be set for an inner byte of an object, find object beginning.
p = (void*)((s->start << PageShift) + spf->special.offset/s->elemsize*s->elemsize);
- scanblock(p, s->elemsize, nil);
+ if(runtime·gcphase != GCscan)
+ scanblock(p, s->elemsize, nil); // Scanned during mark phase
scanblock((void*)&spf->fn, PtrSize, oneptr);
}
}
break;
case RootFlushCaches:
- flushallmcaches();
+ if (runtime·gcphase != GCscan) // Do not flush mcaches during GCscan phase.
+ flushallmcaches();
break;
default:
gp = runtime·allg[i - RootCount];
// remember when we've first observed the G blocked
// needed only to output in traceback
- status = runtime·readgstatus(gp);
+ status = runtime·readgstatus(gp); // We are not in a scan state
if((status == Gwaiting || status == Gsyscall) && gp->waitsince == 0)
gp->waitsince = runtime·work.tstart;
- // Shrink a stack if not much of it is being used.
- runtime·shrinkstack(gp);
- if(runtime·readgstatus(gp) == Gdead)
+ // Shrink a stack if not much of it is being used but not in the scan phase.
+ if (runtime·gcphase != GCscan) // Do not shrink during GCscan phase.
+ runtime·shrinkstack(gp);
+ if(runtime·readgstatus(gp) == Gdead)
gp->gcworkdone = true;
else
gp->gcworkdone = false;
restart = runtime·stopg(gp);
- scanstack(gp);
+
+ // goroutine will scan its own stack when it stops running.
+ // Wait until it has.
+ while(runtime·readgstatus(gp) == Grunning && !gp->gcworkdone) {
+ }
+
+ // scanstack(gp) is done as part of gcphasework
+ // But to make sure we finished we need to make sure that
+ // the stack traps have all responded so drop into
+ // this while loop until they respond.
+ while(!gp->gcworkdone){
+ status = runtime·readgstatus(gp);
+ if(status == Gdead) {
+ gp->gcworkdone = true; // scan is a noop
+ break;
+ //do nothing, scan not needed.
+ }
+ if(status == Gwaiting || status == Grunnable)
+ restart = runtime·stopg(gp);
+ }
if(restart)
runtime·restartg(gp);
break;
}
}
+// wblock is used for creating new empty work buffer blocks.
+static Mutex wblock;
+
// Get an empty work buffer off the work.empty list,
// allocating new buffers as needed.
static Workbuf*
getempty(Workbuf *b)
{
- MCache *c;
-
- if(b != nil)
- runtime·lfstackpush(&runtime·work.full, &b->node);
- b = nil;
- c = g->m->mcache;
- if(c->gcworkbuf != nil) {
- b = c->gcworkbuf;
- c->gcworkbuf = nil;
+ if(b != nil) {
+ putfull(b);
+ b = nil;
}
- if(b == nil)
+ if(runtime·work.empty)
b = (Workbuf*)runtime·lfstackpop(&runtime·work.empty);
- if(b == nil)
+
+ if(b && b->nobj != 0) {
+ runtime·printf("m%d: getempty: popped b=%p with non-zero b->nobj=%d\n", g->m->id, b, (uint32)b->nobj);
+ runtime·throw("getempty: workbuffer not empty, b->nobj not 0");
+ }
+ if(b == nil) {
+ runtime·lock(&wblock);
b = runtime·persistentalloc(sizeof(*b), CacheLineSize, &mstats.gc_sys);
- b->nobj = 0;
+ b->nobj = 0;
+ runtime·unlock(&wblock);
+ }
return b;
}
static void
putempty(Workbuf *b)
{
- MCache *c;
-
- c = g->m->mcache;
- if(c->gcworkbuf == nil) {
- c->gcworkbuf = b;
- return;
+ if(b->nobj != 0) {
+ runtime·throw("putempty: b->nobj not 0\n");
}
runtime·lfstackpush(&runtime·work.empty, &b->node);
}
+// Put a full or partially full workbuf on the full list.
+static void
+putfull(Workbuf *b)
+{
+ if(b->nobj <= 0) {
+ runtime·throw("putfull: b->nobj <= 0\n");
+ }
+ runtime·lfstackpush(&runtime·work.full, &b->node);
+}
+
+// Get an partially empty work buffer
+// if none are available get an empty one.
+static Workbuf*
+getpartialorempty(void)
+{
+ Workbuf *b;
+
+ b = (Workbuf*)runtime·lfstackpop(&runtime·work.partial);
+ if(b == nil)
+ b = getempty(nil);
+ return b;
+}
+
+static void
+putpartial(Workbuf *b)
+{
+
+ if(b->nobj == 0)
+ runtime·lfstackpush(&runtime·work.empty, &b->node);
+ else if (b->nobj < nelem(b->obj))
+ runtime·lfstackpush(&runtime·work.partial, &b->node);
+ else if (b->nobj == nelem(b->obj))
+ runtime·lfstackpush(&runtime·work.full, &b->node);
+ else {
+ runtime·printf("b=%p, b->nobj=%d, nelem(b->obj)=%d\n", b, (uint32)b->nobj, (uint32)nelem(b->obj));
+ runtime·throw("putpartial: bad Workbuf b->nobj");
+ }
+}
+
void
-runtime·gcworkbuffree(void *b)
+runtime·gcworkbuffree(Workbuf *b)
{
- if(b != nil)
+ if(b == nil)
+ return;
+ if(b->nobj == 0)
putempty(b);
+ else
+ putfull(b);
}
-// Get a full work buffer off the work.full list, or return nil.
+// Get a full work buffer off the work.full or a partially
+// filled one off the work.partial list. If nothing is available
+// wait until all the other gc helpers have finished and then
+// return nil.
+// getfull acts as a barrier for work.nproc helpers. As long as one
+// gchelper is actively marking objects it
+// may create a workbuffer that the other helpers can work on.
+// The for loop either exits when a work buffer is found
+// or when _all_ of the work.nproc GC helpers are in the loop
+// looking for work and thus not capable of creating new work.
+// This is in fact the termination condition for the STW mark
+// phase.
static Workbuf*
getfull(Workbuf *b)
{
int32 i;
if(b != nil)
- runtime·lfstackpush(&runtime·work.empty, &b->node);
+ putempty(b);
+
b = (Workbuf*)runtime·lfstackpop(&runtime·work.full);
+ if(b==nil)
+ b = (Workbuf*)runtime·lfstackpop(&runtime·work.partial);
if(b != nil || runtime·work.nproc == 1)
return b;
if(runtime·work.full != 0) {
runtime·xadd(&runtime·work.nwait, -1);
b = (Workbuf*)runtime·lfstackpop(&runtime·work.full);
- if(b != nil)
+ if(b==nil)
+ b = (Workbuf*)runtime·lfstackpop(&runtime·work.partial);
+ if(b != nil)
return b;
runtime·xadd(&runtime·work.nwait, +1);
}
}
bv = runtime·stackmapdata(stackmap, pcdata);
}
- scanblock((byte*)frame->argp, bv.n/BitsPerPointer*PtrSize, bv.bytedata);
+ scanblock((byte*)frame->argp, bv.n/BitsPerPointer*PtrSize, bv.bytedata);
}
return true;
}
case Gdead:
return;
case Grunning:
- runtime·printf("runtime: gp=%p, goid=%D, gp->atomicstatus=%d\n", gp, gp->goid, runtime·readgstatus(gp));
- runtime·throw("mark - world not stopped");
+ runtime·throw("scanstack: - goroutine not stopped");
case Grunnable:
case Gsyscall:
case Gwaiting:
runtime·tracebackdefers(gp, &fn, nil);
}
-// The gp has been moved to a gc safepoint. If there is gcphase specific
-// work it is done here.
+// If the slot is grey or black return true, if white return false.
+// If the slot is not in the known heap and thus does not have a valid GC bitmap then
+// it is considered grey. Globals and stacks can hold such slots.
+// The slot is grey if its mark bit is set and it is enqueued to be scanned.
+// The slot is black if it has already been scanned.
+// It is white if it has a valid mark bit and the bit is not set.
+static bool
+shaded(byte *slot)
+{
+ Markbits mbits;
+
+ if(!inheap(slot)) // non-heap slots considered grey
+ return true;
+
+ objectstart(slot, &mbits);
+ return (mbits.bits&bitMarked) != 0;
+}
+
+// Shade the object if it isn't already.
+// The object is not nil and known to be in the heap.
+static void
+shade(byte *b)
+{
+ byte *obj;
+ Workbuf *wbuf;
+ Markbits mbits;
+
+ if(!inheap(b))
+ runtime·throw("shade: passed an address not in the heap");
+
+ wbuf = getpartialorempty();
+ // Mark the object, return some important bits.
+ // If we combine the following two rotines we don't have to pass mbits or obj around.
+ obj = objectstart(b, &mbits);
+ wbuf = greyobject(obj, &mbits, wbuf); // augments the wbuf
+ putpartial(wbuf);
+ return;
+}
+
+// This is the Dijkstra barrier coarsened to shade grey to white whereas
+// the original Dijkstra barrier only shaded black to white.
+//
+// Shade indicates that it has seen a white pointer by adding the referent
+// to wbuf.
+void
+runtime·markwb(void **slot, void *ptr)
+{
+ // initial nil check avoids some needlesss loads
+ if(ptr != nil && inheap(ptr) && shaded((void*)slot))
+ shade(ptr);
+ *slot = ptr;
+}
+
+// The gp has been moved to a GC safepoint. GC phase specific
+// work is done here.
void
runtime·gcphasework(G *gp)
{
case GCquiesce:
case GCstw:
case GCsweep:
- // No work for now.
+ // No work.
+ break;
+ case GCscan:
+ // scan the stack, mark the objects, put pointers in work buffers
+ // hanging off the P where this is being run.
+ scanstack(gp);
break;
case GCmark:
- // Disabled until concurrent GC is implemented
- // but indicate the scan has been done.
- // scanstack(gp);
+ case GCmarktermination:
+ scanstack(gp);
+ // All available mark work will be emptied before returning.
break;
}
gp->gcworkdone = true;
}
}
+// Returns only when span s has been swept.
void
runtime·MSpan_EnsureSwept(MSpan *s)
{
sg = runtime·mheap.sweepgen;
if(runtime·atomicload(&s->sweepgen) == sg)
return;
+ // The caller must be sure that the span is a MSpanInUse span.
if(runtime·cas(&s->sweepgen, sg-2, sg-1)) {
runtime·MSpan_Sweep(s, false);
return;
return runtime·mheap.sweepdone;
}
+
void
runtime·gchelper(void)
{
g->m->traceback = 2;
gchelperstart();
- // parallel mark for over gc roots
+ // parallel mark for over GC roots
runtime·parfordo(runtime·work.markfor);
-
- // help other threads scan secondary blocks
- scanblock(nil, 0, nil);
-
- nproc = runtime·work.nproc; // runtime·work.nproc can change right after we increment runtime·work.ndone
+ if(runtime·gcphase != GCscan)
+ scanblock(nil, 0, nil); // blocks in getfull
+ nproc = runtime·work.nproc; // work.nproc can change right after we increment work.ndone
if(runtime·xadd(&runtime·work.ndone, +1) == nproc-1)
runtime·notewakeup(&runtime·work.alldone);
g->m->traceback = 0;
runtime·gcbssmask = unrollglobgcprog(runtime·gcbss, runtime·ebss - runtime·bss);
}
+// Called from malloc.go using onM, stopping and starting the world handled in caller.
void
runtime·gc_m(void)
{
a.start_time = (uint64)(g->m->scalararg[0]) | ((uint64)(g->m->scalararg[1]) << 32);
a.eagersweep = g->m->scalararg[2];
gc(&a);
+ runtime·casgstatus(gp, Gwaiting, Grunning);
+}
- if(nbadblock > 0) {
- // Work out path from root to bad block.
- for(;;) {
- gc(&a);
- if(nbadblock >= nelem(badblock))
- runtime·throw("cannot find path to bad pointer");
+void
+runtime·finishsweep_m(void)
+{
+ uint32 i, sg;
+ MSpan *s;
+
+ // The world is stopped so we should be able to complete the sweeps
+ // quickly.
+ while(runtime·sweepone() != -1)
+ runtime·sweep.npausesweep++;
+
+ // There may be some other spans being swept concurrently that
+ // we need to wait for. If finishsweep_m is done with the world stopped
+ // this code is not required.
+ sg = runtime·mheap.sweepgen;
+ for(i=0; i<runtime·work.nspan; i++) {
+ s = runtime·work.spans[i];
+ if(s->sweepgen == sg) {
+ continue;
}
+ if(s->state != MSpanInUse) // Span is not part of the GCed heap so no need to ensure it is swept.
+ continue;
+ runtime·MSpan_EnsureSwept(s);
+ }
+}
+
+// Scan all of the stacks, greying (or graying if in America) the referents
+// but not blackening them since the mark write barrier isn't installed.
+void
+runtime·gcscan_m(void)
+{
+ uint32 i, allglen, oldphase;
+ G *gp, *mastergp, **allg;
+
+ // Grab the g that called us and potentially allow rescheduling.
+ // This allows it to be scanned like other goroutines.
+ mastergp = g->m->curg;
+
+ runtime·casgstatus(mastergp, Grunning, Gwaiting);
+ mastergp->waitreason = runtime·gostringnocopy((byte*)"garbage collection scan");
+
+ // Span sweeping has been done by finishsweep_m.
+ // Long term we will want to make this goroutine runnable
+ // by placing it onto a scanenqueue state and then calling
+ // runtime·restartg(mastergp) to make it Grunnable.
+ // At the bottom we will want to return this p back to the scheduler.
+
+ oldphase = runtime·gcphase;
+
+ runtime·lock(&runtime·allglock);
+ allglen = runtime·allglen;
+ allg = runtime·allg;
+ // Prepare flag indicating that the scan has not been completed.
+ for(i = 0; i < allglen; i++) {
+ gp = allg[i];
+ gp->gcworkdone = false; // set to true in gcphasework
}
+ runtime·unlock(&runtime·allglock);
- runtime·casgstatus(gp, Gwaiting, Grunning);
+ runtime·work.nwait = 0;
+ runtime·work.ndone = 0;
+ runtime·work.nproc = 1; // For now do not do this in parallel.
+ runtime·gcphase = GCscan;
+ // ackgcphase is not needed since we are not scanning running goroutines.
+ runtime·parforsetup(runtime·work.markfor, runtime·work.nproc, RootCount + allglen, nil, false, markroot);
+ runtime·parfordo(runtime·work.markfor);
+
+ runtime·lock(&runtime·allglock);
+
+ allg = runtime·allg;
+ // Check that gc work is done.
+ for(i = 0; i < allglen; i++) {
+ gp = allg[i];
+ if(!gp->gcworkdone) {
+ runtime·throw("scan missed a g");
+ }
+ }
+ runtime·unlock(&runtime·allglock);
+
+ runtime·gcphase = oldphase;
+ runtime·casgstatus(mastergp, Gwaiting, Grunning);
+ // Let the g that called us continue to run.
}
static void
int64 t0, t1, t2, t3, t4;
uint64 heap0, heap1, obj;
GCStats stats;
-
- if(DebugPtrs)
- runtime·printf("GC start\n");
+ uint32 oldphase;
+ uint32 i;
+ G *gp;
if(runtime·debug.allocfreetrace)
runtime·tracegc();
if(runtime·debug.gctrace)
t1 = runtime·nanotime();
- // Sweep what is not sweeped by bgsweep.
- while(runtime·sweepone() != -1)
- runtime·sweep.npausesweep++;
+ runtime·finishsweep_m();
- // Cache runtime.mheap.allspans in work.spans to avoid conflicts with
+ // Cache runtime·mheap.allspans in work.spans to avoid conflicts with
// resizing/freeing allspans.
// New spans can be created while GC progresses, but they are not garbage for
// this round:
runtime·work.spans = runtime·mheap.allspans;
runtime·work.nspan = runtime·mheap.nspan;
runtime·unlock(&runtime·mheap.lock);
+ oldphase = runtime·gcphase;
runtime·work.nwait = 0;
runtime·work.ndone = 0;
- runtime·work.nproc = runtime·gcprocs();
+ runtime·work.nproc = runtime·gcprocs();
+ runtime·gcphase = GCmark;
+
+ // World is stopped so allglen will not change.
+ for(i = 0; i < runtime·allglen; i++) {
+ gp = runtime·allg[i];
+ gp->gcworkdone = false; // set to true in gcphasework
+ }
+
runtime·parforsetup(runtime·work.markfor, runtime·work.nproc, RootCount + runtime·allglen, nil, false, markroot);
if(runtime·work.nproc > 1) {
runtime·noteclear(&runtime·work.alldone);
gchelperstart();
runtime·parfordo(runtime·work.markfor);
+
scanblock(nil, 0, nil);
+ if(runtime·work.full)
+ runtime·throw("runtime·work.full != nil");
+ if(runtime·work.partial)
+ runtime·throw("runtime·work.partial != nil");
+
+ runtime·gcphase = oldphase;
t3 = 0;
if(runtime·debug.gctrace)
t3 = runtime·nanotime();
runtime·mProf_GC();
g->m->traceback = 0;
-
- if(DebugPtrs)
- runtime·printf("GC end\n");
}
extern uintptr runtime·sizeof_C_MStats;
prog = (byte*)typ->gc[1];
unrollgcprog1(mask, prog, &pos, false, true);
}
-
+
// atomic way to say mask[0] = 1
x = *(uintptr*)mask;
((byte*)&x)[0] = 1;