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runtime: do not scan maps when k/v do not contain pointers

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Currently we scan maps even if k/v does not contain pointers.
This is required because overflow buckets are hanging off the main table.
This change introduces a separate array that contains pointers to all
overflow buckets and keeps them alive. Buckets themselves are marked
as containing no pointers and are not scanned by GC (if k/v does not
contain pointers).

This brings maps in line with slices and chans -- GC does not scan
their contents if elements do not contain pointers.

Currently scanning of a map[int]int with 2e8 entries (~8GB heap)
takes ~8 seconds. With this change scanning takes negligible time.

Update #9477.

Change-Id: Id8a04066a53d2f743474cad406afb9f30f00eaae
Reviewed-on: https://go-review.googlesource.com/3288
Reviewed-by: Keith Randall <khr@golang.org>
This commit is contained in:
Dmitry Vyukov 2015-01-26 21:04:41 +03:00
parent 561ce92fa0
commit 85e7bee19f
3 changed files with 110 additions and 35 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

58
src/cmd/gc/reflect.c
View File

@ -181,6 +181,10 @@ mapbucket(Type *t)
valuesfield->down = overflowfield;
overflowfield->down = T;
// See comment on hmap.overflow in ../../runtime/hashmap.go.
if(!haspointers(t->type) && !haspointers(t->down))
bucket->haspointers = 1; // no pointers
bucket->width = offset;
bucket->local = t->local;
t->bucket = bucket;
@ -197,7 +201,7 @@ static Type*
hmap(Type *t)
{
Type *h, *bucket;
Type *bucketsfield, *oldbucketsfield;
Type *bucketsfield, *oldbucketsfield, *overflowfield;
int32 offset;
if(t->hmap != T)
@ -208,9 +212,10 @@ hmap(Type *t)
h->noalg = 1;
offset = widthint; // count
offset += 4; // flags
offset += 4; // hash0
offset += 1; // flags
offset += 1; // B
offset += 2; // padding
offset += 4; // hash0
offset = (offset + widthptr - 1) / widthptr * widthptr;
bucketsfield = typ(TFIELD);
@ -227,12 +232,20 @@ hmap(Type *t)
oldbucketsfield->sym->name = "oldbuckets";
offset += widthptr;
offset += widthptr; // nevacuate (last field in Hmap)
offset += widthptr; // nevacuate
overflowfield = typ(TFIELD);
overflowfield->type = types[TUNSAFEPTR];
overflowfield->width = offset;
overflowfield->sym = mal(sizeof(Sym));
overflowfield->sym->name = "overflow";
offset += widthptr;
// link up fields
h->type = bucketsfield;
bucketsfield->down = oldbucketsfield;
oldbucketsfield->down = T;
oldbucketsfield->down = overflowfield;
overflowfield->down = T;
h->width = offset;
h->local = t->local;
@ -245,7 +258,7 @@ Type*
hiter(Type *t)
{
int32 n, off;
Type *field[7];
Type *field[9];
Type *i;
if(t->hiter != T)
@ -259,6 +272,7 @@ hiter(Type *t)
// h *Hmap
// buckets *Bucket
// bptr *Bucket
// overflow unsafe.Pointer
// other [4]uintptr
// }
// must match ../../runtime/hashmap.c:hash_iter.
@ -292,29 +306,39 @@ hiter(Type *t)
field[5]->sym = mal(sizeof(Sym));
field[5]->sym->name = "bptr";
// all other non-pointer fields
field[6] = typ(TFIELD);
field[6]->type = typ(TARRAY);
field[6]->type->type = types[TUINTPTR];
field[6]->type->bound = 4;
field[6]->type->width = 4 * widthptr;
field[6]->type = types[TUNSAFEPTR];
field[6]->sym = mal(sizeof(Sym));
field[6]->sym->name = "other";
field[6]->sym->name = "overflow0";
field[7] = typ(TFIELD);
field[7]->type = types[TUNSAFEPTR];
field[7]->sym = mal(sizeof(Sym));
field[7]->sym->name = "overflow1";
// all other non-pointer fields
field[8] = typ(TFIELD);
field[8]->type = typ(TARRAY);
field[8]->type->type = types[TUINTPTR];
field[8]->type->bound = 4;
field[8]->type->width = 4 * widthptr;
field[8]->sym = mal(sizeof(Sym));
field[8]->sym->name = "other";
// build iterator struct holding the above fields
i = typ(TSTRUCT);
i->noalg = 1;
i->type = field[0];
off = 0;
for(n = 0; n < 6; n++) {
for(n = 0; n < nelem(field)-1; n++) {
field[n]->down = field[n+1];
field[n]->width = off;
off += field[n]->type->width;
}
field[6]->down = T;
off += field[6]->type->width;
if(off != 10 * widthptr)
yyerror("hash_iter size not correct %d %d", off, 10 * widthptr);
field[nelem(field)-1]->down = T;
off += field[nelem(field)-1]->type->width;
if(off != 12 * widthptr)
yyerror("hash_iter size not correct %d %d", off, 11 * widthptr);
t->hiter = i;
i->map = t;
return i;

12
src/reflect/type.go
View File

@ -1469,9 +1469,8 @@ func MapOf(key, elem Type) Type {
// Make a map type.
var imap interface{} = (map[unsafe.Pointer]unsafe.Pointer)(nil)
prototype := *(**mapType)(unsafe.Pointer(&imap))
mt := new(mapType)
*mt = *prototype
*mt = **(**mapType)(unsafe.Pointer(&imap))
mt.string = &s
mt.hash = fnv1(etyp.hash, 'm', byte(ktyp.hash>>24), byte(ktyp.hash>>16), byte(ktyp.hash>>8), byte(ktyp.hash))
mt.key = ktyp
@ -1575,7 +1574,7 @@ func (gc *gcProg) appendProg(t *rtype) {
for i := 0; i < c; i++ {
gc.appendProg(t.Field(i).Type.common())
}
if gc.size > oldsize + t.size {
if gc.size > oldsize+t.size {
panic("reflect: struct components are larger than the struct itself")
}
gc.size = oldsize + t.size
@ -1650,6 +1649,12 @@ const (
)
func bucketOf(ktyp, etyp *rtype) *rtype {
// See comment on hmap.overflow in ../runtime/hashmap.go.
var kind uint8
if ktyp.kind&kindNoPointers != 0 && etyp.kind&kindNoPointers != 0 {
kind = kindNoPointers
}
if ktyp.size > maxKeySize {
ktyp = PtrTo(ktyp).(*rtype)
}
@ -1679,6 +1684,7 @@ func bucketOf(ktyp, etyp *rtype) *rtype {
b := new(rtype)
b.size = gc.size
b.kind = kind
b.gc[0], _ = gc.finalize()
s := "bucket(" + *ktyp.string + "," + *etyp.string + ")"
b.string = &s

75
src/runtime/hashmap.go
View File

@ -106,13 +106,24 @@ type hmap struct {
// Note: the format of the Hmap is encoded in ../../cmd/gc/reflect.c and
// ../reflect/type.go. Don't change this structure without also changing that code!
count int // # live cells == size of map. Must be first (used by len() builtin)
flags uint32
hash0 uint32 // hash seed
flags uint8
B uint8 // log_2 of # of buckets (can hold up to loadFactor * 2^B items)
hash0 uint32 // hash seed
buckets unsafe.Pointer // array of 2^B Buckets. may be nil if count==0.
oldbuckets unsafe.Pointer // previous bucket array of half the size, non-nil only when growing
nevacuate uintptr // progress counter for evacuation (buckets less than this have been evacuated)
// If both key and value do not contain pointers, then we mark bucket
// type as containing no pointers. This avoids scanning such maps.
// However, bmap.overflow is a pointer. In order to keep overflow buckets
// alive, we store pointers to all overflow buckets in hmap.overflow.
// Overflow is used only if key and value do not contain pointers.
// overflow[0] contains overflow buckets for hmap.buckets.
// overflow[1] contains overflow buckets for hmap.oldbuckets.
// The first indirection allows us to reduce static size of hmap.
// The second indirection allows to store a pointer to the slice in hiter.
overflow *[2]*[]*bmap
}
// A bucket for a Go map.
@ -135,6 +146,7 @@ type hiter struct {
h *hmap
buckets unsafe.Pointer // bucket ptr at hash_iter initialization time
bptr *bmap // current bucket
overflow [2]*[]*bmap // keeps overflow buckets alive
startBucket uintptr // bucket iteration started at
offset uint8 // intra-bucket offset to start from during iteration (should be big enough to hold bucketCnt-1)
wrapped bool // already wrapped around from end of bucket array to beginning
@ -152,10 +164,24 @@ func evacuated(b *bmap) bool {
func (b *bmap) overflow(t *maptype) *bmap {
return *(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize)-regSize))
}
func (b *bmap) setoverflow(t *maptype, ovf *bmap) {
func (h *hmap) setoverflow(t *maptype, b, ovf *bmap) {
if t.bucket.kind&kindNoPointers != 0 {
h.createOverflow()
*h.overflow[0] = append(*h.overflow[0], ovf)
}
*(**bmap)(add(unsafe.Pointer(b), uintptr(t.bucketsize)-regSize)) = ovf
}
func (h *hmap) createOverflow() {
if h.overflow == nil {
h.overflow = new([2]*[]*bmap)
}
if h.overflow[0] == nil {
h.overflow[0] = new([]*bmap)
}
}
func makemap(t *maptype, hint int64) *hmap {
if sz := unsafe.Sizeof(hmap{}); sz > 48 || sz != uintptr(t.hmap.size) {
throw("bad hmap size")
@ -463,7 +489,7 @@ again:
memstats.next_gc = memstats.heap_alloc
}
newb := (*bmap)(newobject(t.bucket))
b.setoverflow(t, newb)
h.setoverflow(t, b, newb)
inserti = &newb.tophash[0]
insertk = add(unsafe.Pointer(newb), dataOffset)
insertv = add(insertk, bucketCnt*uintptr(t.keysize))
@ -548,6 +574,8 @@ func mapiterinit(t *maptype, h *hmap, it *hiter) {
it.h = nil
it.buckets = nil
it.bptr = nil
it.overflow[0] = nil
it.overflow[1] = nil
if raceenabled && h != nil {
callerpc := getcallerpc(unsafe.Pointer(&t))
@ -560,7 +588,7 @@ func mapiterinit(t *maptype, h *hmap, it *hiter) {
return
}
if unsafe.Sizeof(hiter{})/ptrSize != 10 {
if unsafe.Sizeof(hiter{})/ptrSize != 12 {
throw("hash_iter size incorrect") // see ../../cmd/gc/reflect.c
}
it.t = t
@ -569,6 +597,14 @@ func mapiterinit(t *maptype, h *hmap, it *hiter) {
// grab snapshot of bucket state
it.B = h.B
it.buckets = h.buckets
if t.bucket.kind&kindNoPointers != 0 {
// Allocate the current slice and remember pointers to both current and old.
// This preserves all relevant overflow buckets alive even if
// the table grows and/or overflow buckets are added to the table
// while we are iterating.
h.createOverflow()
it.overflow = *h.overflow
}
// decide where to start
r := uintptr(fastrand1())
@ -585,14 +621,8 @@ func mapiterinit(t *maptype, h *hmap, it *hiter) {
// Remember we have an iterator.
// Can run concurrently with another hash_iter_init().
for {
old := h.flags
if old == old|iterator|oldIterator {
break
}
if cas(&h.flags, old, old|iterator|oldIterator) {
break
}
if old := h.flags; old&(iterator|oldIterator) != iterator|oldIterator {
atomicor8(&h.flags, iterator|oldIterator)
}
mapiternext(it)
@ -753,6 +783,15 @@ func hashGrow(t *maptype, h *hmap) {
h.buckets = newbuckets
h.nevacuate = 0
if h.overflow != nil {
// Promote current overflow buckets to the old generation.
if h.overflow[1] != nil {
throw("overflow is not nil")
}
h.overflow[1] = h.overflow[0]
h.overflow[0] = nil
}
// the actual copying of the hash table data is done incrementally
// by growWork() and evacuate().
}
@ -836,7 +875,7 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
memstats.next_gc = memstats.heap_alloc
}
newx := (*bmap)(newobject(t.bucket))
x.setoverflow(t, newx)
h.setoverflow(t, x, newx)
x = newx
xi = 0
xk = add(unsafe.Pointer(x), dataOffset)
@ -863,7 +902,7 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
memstats.next_gc = memstats.heap_alloc
}
newy := (*bmap)(newobject(t.bucket))
y.setoverflow(t, newy)
h.setoverflow(t, y, newy)
y = newy
yi = 0
yk = add(unsafe.Pointer(y), dataOffset)
@ -899,6 +938,12 @@ func evacuate(t *maptype, h *hmap, oldbucket uintptr) {
if oldbucket+1 == newbit { // newbit == # of oldbuckets
// Growing is all done. Free old main bucket array.
h.oldbuckets = nil
// Can discard old overflow buckets as well.
// If they are still referenced by an iterator,
// then the iterator holds a pointers to the slice.
if h.overflow != nil {
h.overflow[1] = nil
}
}
}
}