allocator purge exposure ?

[devnexen]

jemalloc provides the ability to purge a specific arena via mallctl with the oid 'arena.[id].purge', so I was wondering if the feature closest to it would be LocalAllocator::flush() (or literally teardown in the upper level) and it was a good idea to expose such capability as C symbol ?

[mjp41]

The nearest bit is

snmalloc/src/mem/chunkallocator.h

Lines 147 to 178 in b1da339

	static void handle_decay_tick(ChunkAllocatorState* state)
	{
	auto new_epoch = (state->epoch + 1) % NUM_EPOCHS;
	// Flush old index for all threads.
	ChunkAllocatorLocalState* curr = state->all_local;
	while (curr != nullptr)
	{
	for (size_t sc = 0; sc < NUM_SLAB_SIZES; sc++)
	{
	auto& old_stack = curr->chunk_stack[sc][new_epoch];
	ChunkRecord* record = old_stack.pop_all();
	while (record != nullptr)
	{
	auto next = record->next.load();
	// Disable pages for this
	Pal::notify_not_using(
	record->meta_common.chunk.unsafe_ptr(),
	slab_sizeclass_to_size(sc));
	// Add to global state
	state->decommitted_chunk_stack[sc].push(record);
	record = next;
	}
	}
	curr = curr->next;
	}
	// Advance current index
	state->epoch = new_epoch;
	}

This on a timer pushes unused chunks of memory back into central pools and decommits them.

I want to get LowMemory notifications to call a modified version of this.

Your question makes me think that teardown should also interact with this code, and I don't think it currently does. I am deep in a concurrency issue on Verona at a moment, but can think this through tomorrow.