xnn_weights_cache_provider look_up doesn't work?

[mcr229]

I've recently been experimenting with XNNPACK's weight cache to reduce load time by caching packed weights and also reduce memory pressure for repeated weights across the same kernels.

I was experiementing with fully-connected operator and found that the weight cache was never being hit. I noticed that when using the apis to create the xnn_weights_cache_t we set the look up function to be xnn_internal_weights_cache_look_up:

XNNPACK/src/runtime.c

Line 148 in 85071b8

cache_provider->look_up = (size_t(*)(void*, const struct xnn_weights_cache_look_up_key*))xnn_internal_weights_cache_look_up;

looking at this function, it looks like a placeholder function which would always return XNN_CACHE_NOT_FOUND:

XNNPACK/src/cache.c

Lines 491 to 496 in 85071b8

	size_t xnn_internal_weights_cache_look_up(
	struct xnn_internal_weights_cache* cache, const struct xnn_weights_cache_look_up_key* cache_key)
	{
	// The default implementation does not support this query.
	return XNN_CACHE_NOT_FOUND;
	}

Now when I'm using the weights cache to create a runtime_t with only a fully connected operator, in the flow of creating the fully-connected operator, we look up the cache to see if the weights have been packed before, using xnn_weights_cache_look_up:

XNNPACK/src/operators/fully-connected-nc.c

Lines 154 to 157 in 85071b8

	if (use_weights_cache(fully_connected_op)) {
	cache_offset = xnn_weights_cache_look_up(
	fully_connected_op->weights_cache, &cache_key);
	}

However this just uses the the placeholder function above, returning XNN_CACHE_NOT_FOUND:

XNNPACK/src/cache.c

Lines 530 to 534 in 85071b8

	size_t xnn_weights_cache_look_up(
	xnn_weights_cache_t cache, const struct xnn_weights_cache_look_up_key* cache_key)
	{
	return cache->look_up(cache->context, cache_key);
	}

As a result, every look up would then fall to XNN_CACHE_NOT_FOUND, in which weights have to be repacked, and memory has to be allocated for the newly packed weights:

XNNPACK/src/operators/fully-connected-nc.c

Lines 159 to 179 in 85071b8

	if (cache_offset == XNN_CACHE_NOT_FOUND) {
	void* weights_ptr = xnn_get_pointer_to_write_weights(
	fully_connected_op, aligned_total_weights_size, packed_weights_padding_byte);
	if (weights_ptr == NULL) {
	xnn_log_error(
	"failed to allocate %zu bytes for %s operator packed weights",
	packed_weights_size, xnn_operator_type_to_string(operator_type));
	goto error;
	}
	xnn_log_debug("allocated %zu bytes for packed weights in %s operator",
	aligned_total_weights_size, xnn_operator_type_to_string(operator_type));
	if (flags & XNN_FLAG_TRANSPOSE_WEIGHTS) {
	pack_gemm_gio_w(
	/*groups=*/1, output_channels, input_channels,
	nr, kr, sr,
	output_channels,
	kernel, bias, /*scale=*/NULL,
	weights_ptr,
	gemm_config->nr * extra_weights_bytes,
	packing_params);

Am I looking at this incorrectly? Or is this a feature that is still a wip? Or is this a bug that is meant to be fixed in the future?

[digantdesai]

@mcr229 - Is it possible to not use the default constructor i.e. xnn_create_weights_cache_with_size and write a custom constructor which populates struct xnn_weights_cache_provider with your methods?

[mcr229]

@digantdesai hmm I'm wondering if this suggests that XNNPACK doesn't offer a default operator for looking up weights then?