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Free objects with finalizer more eagerly
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* Run all the finalizers at the same time in a dead reference tree
* Reset objects that are only reachable from finalizer list as young and
  clean so that they can be collect during next quick GC without being
  promoted to old gen.

Closes JuliaLang#14127
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@yuyichao
yuyichao committed Jun 5, 2016
1 parent 9388a82 commit f5ac1f2
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Showing 3 changed files with 77 additions and 31 deletions.
14 changes: 8 additions & 6 deletions src/gc-debug.c
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Original file line number Diff line number Diff line change
Expand Up @@ -171,9 +171,10 @@ static void gc_verify_track(void)
jl_printf(JL_STDERR, "Now looking for %p =======\n", lostval);
clear_mark(GC_CLEAN);
pre_mark();
gc_mark_object_list(&to_finalize);
post_mark(&finalizer_list, 1);
post_mark(&finalizer_list_marked, 1);
gc_mark_object_list(&to_finalize, 0);
gc_mark_object_list(&finalizer_list, 0);
gc_mark_object_list(&finalizer_list_marked, 0);
visit_mark_stack();
if (lostval_parents.len == 0) {
jl_printf(JL_STDERR, "Could not find the missing link. We missed a toplevel root. This is odd.\n");
break;
Expand Down Expand Up @@ -213,9 +214,10 @@ void gc_verify(void)
clear_mark(GC_CLEAN);
gc_verifying = 1;
pre_mark();
gc_mark_object_list(&to_finalize);
post_mark(&finalizer_list, 1);
post_mark(&finalizer_list_marked, 1);
gc_mark_object_list(&to_finalize, 0);
gc_mark_object_list(&finalizer_list, 0);
gc_mark_object_list(&finalizer_list_marked, 0);
visit_mark_stack();
int clean_len = bits_save[GC_CLEAN].len;
for(int i = 0; i < clean_len + bits_save[GC_QUEUED].len; i++) {
gcval_t *v = (gcval_t*)bits_save[i >= clean_len ? GC_QUEUED : GC_CLEAN].items[i >= clean_len ? i - clean_len : i];
Expand Down
90 changes: 67 additions & 23 deletions src/gc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -262,6 +262,12 @@ static size_t max_collect_interval = 500000000UL;

// global variables for GC stats

// Resetting the object to a young object, this is used when marking the
// finalizer list to collect them the next time because the object is very
// likely dead. This also won't break the GC invariance since these objects
// are not reachable from anywhere else.
static int mark_reset_age = 0;

/*
* The state transition looks like :
*
Expand Down Expand Up @@ -354,12 +360,22 @@ static inline int gc_setmark_big(void *o, int mark_mode)
assert(find_region(o,1) == NULL);
bigval_t *hdr = bigval_header(o);
int bits = gc_bits(o);
if (bits == GC_QUEUED || bits == GC_MARKED)
mark_mode = GC_MARKED;
if ((mark_mode == GC_MARKED) & (bits != GC_MARKED)) {
// Move hdr from big_objects list to big_objects_marked list
if (mark_reset_age && !(bits & GC_MARKED)) {
// Reset the object as if it was just allocated
hdr->age = 0;
gc_big_object_unlink(hdr);
gc_big_object_link(hdr, &big_objects_marked);
gc_big_object_link(hdr, &jl_thread_heap.big_objects);
bits = GC_CLEAN;
mark_mode = GC_MARKED_NOESC;
}
else {
if (bits == GC_QUEUED || bits == GC_MARKED)
mark_mode = GC_MARKED;
if ((mark_mode == GC_MARKED) & (bits != GC_MARKED)) {
// Move hdr from big_objects list to big_objects_marked list
gc_big_object_unlink(hdr);
gc_big_object_link(hdr, &big_objects_marked);
}
}
if (!(bits & GC_MARKED)) {
if (mark_mode == GC_MARKED)
Expand All @@ -385,7 +401,17 @@ static inline int gc_setmark_pool(void *o, int mark_mode)
}
jl_gc_pagemeta_t *page = page_metadata(o);
int bits = gc_bits(o);
if (bits == GC_QUEUED || bits == GC_MARKED) {
if (mark_reset_age && !(bits & GC_MARKED)) {
// Reset the object as if it was just allocated
bits = GC_CLEAN;
mark_mode = GC_MARKED_NOESC;
page->has_young = 1;
char *page_begin = gc_page_data(o) + GC_PAGE_OFFSET;
int obj_id = (((char*)o) - page_begin) / page->osize;
uint8_t *ages = page->ages + obj_id / 8;
*ages &= ~(1 << (obj_id % 8));
}
else if (bits == GC_QUEUED || bits == GC_MARKED) {
mark_mode = GC_MARKED;
}
if (!(bits & GC_MARKED)) {
Expand Down Expand Up @@ -1192,9 +1218,9 @@ NOINLINE static void gc_mark_task(jl_task_t *ta, int d)
gc_mark_task_stack(ta, d);
}

void gc_mark_object_list(arraylist_t *list)
void gc_mark_object_list(arraylist_t *list, size_t start)
{
for (size_t i = 0;i < list->len;i++) {
for (size_t i = start;i < list->len;i++) {
gc_push_root(list->items[i], 0);
}
}
Expand Down Expand Up @@ -1385,7 +1411,7 @@ static int push_root(jl_value_t *v, int d, int bits)
return bits;
}

static void visit_mark_stack(void)
void visit_mark_stack(void)
{
while (mark_sp > 0 && !should_timeout()) {
jl_value_t *v = mark_stack[--mark_sp];
Expand Down Expand Up @@ -1444,16 +1470,16 @@ void pre_mark(void)

// find unmarked objects that need to be finalized from the finalizer list "list".
// this must happen last in the mark phase.
// if dryrun == 1, it does not schedule any actual finalization and only marks finalizers
void post_mark(arraylist_t *list, int dryrun)
static void post_mark(arraylist_t *list)
{
for(size_t i=0; i < list->len; i+=2) {
jl_value_t *v = (jl_value_t*)list->items[i];
jl_value_t *fin = (jl_value_t*)list->items[i+1];
int isfreed = !gc_marked(jl_astaggedvalue(v));
gc_push_root(fin, 0);
int isold = list == &finalizer_list && gc_bits(jl_astaggedvalue(v)) == GC_MARKED && gc_bits(jl_astaggedvalue(fin)) == GC_MARKED;
if (!dryrun && (isfreed || isold)) {
int isold = (list != &finalizer_list_marked &&
gc_bits(jl_astaggedvalue(v)) == GC_MARKED &&
gc_bits(jl_astaggedvalue(fin)) == GC_MARKED);
if (isfreed || isold) {
// remove from this list
if (i < list->len - 2) {
list->items[i] = list->items[list->len-2];
Expand All @@ -1466,19 +1492,19 @@ void post_mark(arraylist_t *list, int dryrun)
// schedule finalizer or execute right away if it is not julia code
if (gc_typeof(fin) == (jl_value_t*)jl_voidpointer_type) {
void *p = jl_unbox_voidpointer(fin);
if (!dryrun && p)
if (p)
((void (*)(void*))p)(jl_data_ptr(v));
continue;
}
gc_push_root(v, 0);
if (!dryrun) schedule_finalization(v, fin);
schedule_finalization(v, fin);
}
if (!dryrun && isold) {
if (isold) {
// The caller relies on the new objects to be pushed to the end of
// the list!!
arraylist_push(&finalizer_list_marked, v);
arraylist_push(&finalizer_list_marked, fin);
}
}
visit_mark_stack();
}

// collector entry point and control
Expand Down Expand Up @@ -1585,10 +1611,28 @@ static void _jl_gc_collect(int full, char *stack_hi)
int64_t actual_allocd = gc_num.since_sweep;
// marking is over
// 4. check for objects to finalize
post_mark(&finalizer_list, 0);
if (prev_sweep_full)
post_mark(&finalizer_list_marked, 0);
gc_mark_object_list(&to_finalize);
// Record the length of the marked list since we need to
// mark the object moved to the marked list from the
// `finalizer_list` by `post_mark`
size_t orig_marked_len = finalizer_list_marked.len;
post_mark(&finalizer_list);
if (prev_sweep_full) {
post_mark(&finalizer_list_marked);
orig_marked_len = 0;
}
gc_mark_object_list(&finalizer_list, 0);
gc_mark_object_list(&finalizer_list_marked, orig_marked_len);
// "Flush" the mark stack before flipping the reset_age bit
// so that the objects are not incorrectly resetted.
visit_mark_stack();
mark_reset_age = 1;
// Reset the age and old bit for any unmarked objects referenced by the
// `to_finalize` list. These objects are only reachable from this list
// and should not be referenced by any old objects so this won't break
// the GC invariant.
gc_mark_object_list(&to_finalize, 0);
visit_mark_stack();
mark_reset_age = 0;
gc_settime_postmark_end();

int64_t live_sz_ub = live_bytes + actual_allocd;
Expand Down
4 changes: 2 additions & 2 deletions src/gc.h
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Original file line number Diff line number Diff line change
Expand Up @@ -271,8 +271,8 @@ STATIC_INLINE void gc_big_object_link(bigval_t *hdr, bigval_t **list)
}

void pre_mark(void);
void post_mark(arraylist_t *list, int dryrun);
void gc_mark_object_list(arraylist_t *list);
void gc_mark_object_list(arraylist_t *list, size_t start);
void visit_mark_stack(void);
void gc_debug_init(void);

#define jl_thread_heap (jl_get_ptls_states()->heap)
Expand Down

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