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use atomic compare exchange when setting the GC mark-bit #50576

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merged 3 commits into from
Jul 18, 2023
Merged

use atomic compare exchange when setting the GC mark-bit #50576

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21941a3
Revert "Don't use exchange in the hot path of the GC (#50021)"
d-netto Jul 17, 2023
3725a53
Revert "relax assertion involving pg->nold to reflect that it may be …
d-netto Jul 17, 2023
509eb5c
suggestion from Gabriel's review
d-netto Jul 18, 2023
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47 changes: 22 additions & 25 deletions src/gc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -799,7 +799,7 @@ STATIC_INLINE void gc_queue_big_marked(jl_ptls_t ptls, bigval_t *hdr,
FORCE_INLINE int gc_try_setmark_tag(jl_taggedvalue_t *o, uint8_t mark_mode) JL_NOTSAFEPOINT
{
assert(gc_marked(mark_mode));
uintptr_t tag = jl_atomic_load_relaxed((_Atomic(uintptr_t)*)&o->header);
uintptr_t tag = o->header;
if (gc_marked(tag))
return 0;
if (mark_reset_age) {
Expand All @@ -813,9 +813,13 @@ FORCE_INLINE int gc_try_setmark_tag(jl_taggedvalue_t *o, uint8_t mark_mode) JL_N
tag = tag | mark_mode;
assert((tag & 0x3) == mark_mode);
}
jl_atomic_store_relaxed((_Atomic(uintptr_t)*)&o->header, tag); //xchg here was slower than
verify_val(jl_valueof(o)); //potentially redoing work because of a stale tag.
return 1;
// XXX: note that marking not only sets the GC bits but also updates the
// page metadata for pool allocated objects.
// The second step is **not** idempotent, so we need a compare exchange here
// (instead of a pair of load&store) to avoid marking an object twice
tag = jl_atomic_exchange_relaxed((_Atomic(uintptr_t)*)&o->header, tag);
verify_val(jl_valueof(o));
return !gc_marked(tag);
}

// This function should be called exactly once during marking for each big
Expand Down Expand Up @@ -854,7 +858,7 @@ STATIC_INLINE void gc_setmark_pool_(jl_ptls_t ptls, jl_taggedvalue_t *o,
if (mark_mode == GC_OLD_MARKED) {
ptls->gc_cache.perm_scanned_bytes += page->osize;
static_assert(sizeof(_Atomic(uint16_t)) == sizeof(page->nold), "");
page->nold++;
jl_atomic_fetch_add_relaxed((_Atomic(uint16_t)*)&page->nold, 1);
}
else {
ptls->gc_cache.scanned_bytes += page->osize;
Expand Down Expand Up @@ -1377,27 +1381,20 @@ static jl_taggedvalue_t **gc_sweep_page(jl_gc_pool_t *p, jl_gc_pagemeta_t **allo
nfree = (GC_PAGE_SZ - GC_PAGE_OFFSET) / osize;
goto done;
}
// note that `pg->nold` may not be accurate with multithreaded marking since
// two threads may race when trying to set the mark bit in `gc_try_setmark_tag`.
// We're basically losing a bit of precision in the sweep phase at the cost of
// making the mark phase considerably cheaper.
// See issue #50419
if (jl_n_markthreads == 0) {
// For quick sweep, we might be able to skip the page if the page doesn't
// have any young live cell before marking.
if (!sweep_full && !pg->has_young) {
assert(!prev_sweep_full || pg->prev_nold >= pg->nold);
if (!prev_sweep_full || pg->prev_nold == pg->nold) {
// the position of the freelist begin/end in this page
// is stored in its metadata
if (pg->fl_begin_offset != (uint16_t)-1) {
*pfl = page_pfl_beg(pg);
pfl = (jl_taggedvalue_t**)page_pfl_end(pg);
}
freedall = 0;
nfree = pg->nfree;
goto done;
// For quick sweep, we might be able to skip the page if the page doesn't
// have any young live cell before marking.
if (!sweep_full && !pg->has_young) {
assert(!prev_sweep_full || pg->prev_nold >= pg->nold);
if (!prev_sweep_full || pg->prev_nold == pg->nold) {
// the position of the freelist begin/end in this page
// is stored in its metadata
if (pg->fl_begin_offset != (uint16_t)-1) {
*pfl = page_pfl_beg(pg);
pfl = (jl_taggedvalue_t**)page_pfl_end(pg);
}
freedall = 0;
nfree = pg->nfree;
goto done;
}
}

Expand Down