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perf(profiling): subsample heap allocations to reduce hash-table lookups #15138

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77e0748
perf(profiling): Subsample heap allocations to reduce hash-table lookups
danielsn Nov 4, 2025
22905de
Merge branch 'main' into dsn/subsample_heap
taegyunkim Nov 4, 2025
748f29b
add frozen check for heap tracker
danielsn Nov 4, 2025
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44 changes: 37 additions & 7 deletions ddtrace/profiling/collector/_memalloc_heap.c
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Original file line number Diff line number Diff line change
Expand Up @@ -75,6 +75,11 @@ typedef struct
uint64_t allocated_memory;
/* True if we are exporting the current heap profile */
bool frozen;
/* The fraction of allocation samples that should be tracked for live heap. Expressed as 1/N:
* i.e. if the value is 16, we will track 1/16 of the samples.
* For now, use a prime number to get decent hash behaviour using %
*/
uint64_t live_heap_subsampling_fraction;
/* Contains the ongoing heap allocation/deallocation while frozen */
struct
{
Expand Down Expand Up @@ -116,6 +121,7 @@ heap_tracker_init(heap_tracker_t* heap_tracker)
traceback_array_init(&heap_tracker->unreported_samples);
heap_tracker->allocated_memory = 0;
heap_tracker->frozen = false;
heap_tracker->live_heap_subsampling_fraction = 1;
heap_tracker->sample_size = 0;
heap_tracker->current_sample_size = 0;
memalloc_gil_debug_check_init(&heap_tracker->gil_guard);
Expand Down Expand Up @@ -183,12 +189,19 @@ heap_tracker_thaw(heap_tracker_t* heap_tracker)
free(to_free);
}

static bool should_track_pointer(void* ptr) {
return ((uintptr_t) ptr % global_heap_tracker.live_heap_subsampling_fraction == 0);
}

/* Public API */

void
memalloc_heap_tracker_init(uint32_t sample_size)
{
heap_tracker_init(&global_heap_tracker);
//TODO, take the subsampling_fraction as an argument and use it here
// Use a prime number to get a decent hash using %.
global_heap_tracker.live_heap_subsampling_fraction = 17;
global_heap_tracker.sample_size = sample_size;
global_heap_tracker.current_sample_size = heap_tracker_next_sample_size(sample_size);
}
Expand Down Expand Up @@ -218,6 +231,13 @@ memalloc_heap_untrack_no_cpython(heap_tracker_t* heap_tracker, void* ptr)
MEMALLOC_GIL_DEBUG_CHECK_RELEASE(&heap_tracker->gil_guard);
return NULL;
}

// If we're not tracking pointers of this hash, don't nothing to do here.
if (!should_track_pointer(ptr)) {
MEMALLOC_GIL_DEBUG_CHECK_RELEASE(&heap_tracker->gil_guard);
return NULL;
}

if (!heap_tracker->frozen) {
traceback_t* tb = memalloc_heap_map_remove(heap_tracker->allocs_m, ptr);
if (tb && !tb->reported) {
Expand Down Expand Up @@ -306,10 +326,18 @@ memalloc_heap_add_sample_no_cpython(heap_tracker_t* heap_tracker, traceback_t* t
}

traceback_t* old = NULL;
if (heap_tracker->frozen) {
old = memalloc_heap_map_insert(heap_tracker->freezer.allocs_m, tb->ptr, tb);

if (should_track_pointer(tb->ptr)) {
if (heap_tracker->frozen) {
old = memalloc_heap_map_insert(heap_tracker->freezer.allocs_m, tb->ptr, tb);
} else {
old = memalloc_heap_map_insert(heap_tracker->allocs_m, tb->ptr, tb);
}
} else {
old = memalloc_heap_map_insert(heap_tracker->allocs_m, tb->ptr, tb);
if (!heap_tracker->frozen) {
// TODO: figure out if there is anything to do if we're frozen.
traceback_array_append(&heap_tracker->unreported_samples, tb);
}
}

/* Reset the counter to 0 */
Expand Down Expand Up @@ -385,7 +413,7 @@ memalloc_heap_track(uint16_t max_nframe, void* ptr, size_t size, PyMemAllocatorD
}

PyObject*
memalloc_sample_to_tuple(traceback_t* tb, bool is_live)
memalloc_sample_to_tuple(traceback_t* tb, bool is_live, uint64_t live_heap_scaling_factor)
{
PyObject* tb_and_info = PyTuple_New(4);
if (tb_and_info == NULL) {
Expand All @@ -396,10 +424,12 @@ memalloc_sample_to_tuple(traceback_t* tb, bool is_live)
size_t alloc_size;

if (is_live) {
/* in_use_size is scaled */
in_use_size = tb->size * live_heap_scaling_factor;
/* alloc_size tracks new allocations since the last heap snapshot. Once
* we report it (tb->reported == true), we set the value to 0 to avoid
* double-counting allocations across multiple snapshots. */
in_use_size = tb->size;

alloc_size = tb->reported ? 0 : tb->size;
} else {
in_use_size = 0;
Expand Down Expand Up @@ -445,7 +475,7 @@ memalloc_heap(void)
traceback_t* tb;

while (memalloc_heap_map_iter_next(it, &key, &tb)) {
PyObject* tb_and_info = memalloc_sample_to_tuple(tb, true);
PyObject* tb_and_info = memalloc_sample_to_tuple(tb, true, global_heap_tracker.live_heap_subsampling_fraction);

PyList_SET_ITEM(heap_list, list_index, tb_and_info);
list_index++;
Expand All @@ -460,7 +490,7 @@ memalloc_heap(void)
for (size_t i = 0; i < global_heap_tracker.unreported_samples.count; i++) {
traceback_t* tb = global_heap_tracker.unreported_samples.tab[i];

PyObject* tb_and_info = memalloc_sample_to_tuple(tb, false);
PyObject* tb_and_info = memalloc_sample_to_tuple(tb, false, global_heap_tracker.live_heap_subsampling_fraction);

PyList_SET_ITEM(heap_list, list_index, tb_and_info);
list_index++;
Expand Down
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