When computing the size of the cache for sliding window attention,
we don't need to multiple the batch size by the number of parallel
sequences - the batch size is constant.
This also simplifies the check for whether to allocate the cache
size based on capacity or window size as the batch size is already
incorporated into the capacity when handled by the runner.
Currently sliding window attention allocates and uses the full
context size and just masks out any tokens that are outside of the
window. However, we really only need (roughly) the sliding window
size.
At large context sizes this improves two things:
- Memory allocated - since the fully context size is allocated up front,
memory requirements drop substantially. On Gemma3:4b with a 32k
context window, total memory usage (including weights and non-sliding
layers) drops from ~20GB to ~8GB.
- Computation - ranges that are completely outside of the sliding
window are now removed from the tensors that are returned from the
cache rather than simply being masked out. This results in more
efficient processing, scaling with the size of the context that
has actually been used.
Notable, this does not update the scheduler for any model to be aware of
the smaller memory requirements. This is difficult for Gemma3 because
the layers are heterogeneous between sliding and non-sliding attention.
As a result, while actual memory consumption will be reduced, the
scheduler will over-estimate the requirements of the model. This means
that splitting between GPUs or GPUs and CPUs will still be suboptimal.
Bug #9730
Currently the runner computes the kv size needed and creates a
cache of that size. This is the context size times number of
parallel sequences.
Cache implementations can make better decisions about their memory
usage, so instead pass in the required capacity, number of sequences
and maximum batch size. For now, the causal cache just uses this to
compute the size in the same way as before.
Defragging the KV cache can generate a lot of operations, so we
need to be careful that we don't overflow the number that the graph
can support. We currently account for all of the nodes that we add
to the graph for each move but we also need to include the original
cache tensors as well.
Fixes#9904
The encoder cache needs to know the position of images in the input
stream so that it knows when to delete them. Previously images didn't
have a position, so we implied one by breaking batches before an
image and then assuming the image was in the first position. However,
multimodal objects are now given explicit positions in the input
stream, so we can use that instead.
Breaking batches was also a way to simulate a cross attention mask
for mllama. However, given that it only supports a single sequence
and a single image, this mask doesn't serve any real purpose.
Removing the batch break does not appear to affect the quality of
the output.
Most of this is simply moving the input data structures to a new
package to avoid import cycles.
The GGML flash attention kernel has specific requirements for
padding and permutation. This adds support to the KV cache
for conforming to these requirements so that flash attention
can be enabled.
Flash attention can be used in the same situations as the llama
engine and is enabled by the user in the same way.
Prior to performing attention, we need to permute query, key
and value. Currently we call Contiguous after each of these
permutations, which is correct but expensive. Avoiding the
3 calls to Contiguous increases performance by over 20%.
The permutations of query and key do not violate the continuity
rules for mulmat and the Contiguous call can be simply removed.
Value requires a different permutation and does require Contiguous.
However, we can use the copy into the cache as a way to perform this
without further overhead.
To support this and avoid unexpected tensor shapes that are seen by
models, we need tighter integration between attention, cache
and backend. Future optimization will also likely need this structure
- for example, flash attention has special padding requirements in
the cache and other backends may have their own needs.
This further contains the operations that go into attention so that
these and other optimizations can be handled transparently. Models
that have special requirements for attention can still implement
their own version of it.
update Context.Forward to accept multiple tensors to match
Context.Compute signature
update Context.Forward to return Context such that it can be chained
with Context.Compute
This provides integration with the new Ollama engine
(5824541 next ollama runner (#7913)) and the rest of the Ollama
infrastructure such as the runner and Ollama server.
In addition, it also builds out the KV cache infrastructure to
support requirements of how Ollama runs models such as:
- Parallel processing
- Memory management for defragmentation and shifting
- Multi-modal modals
Both old and new engines continue to be supported. By default, only
the old engine is used. To enable the new engine:
Start the server with the OLLAMA_NEW_ENGINE environment variable set:
OLLAMA_NEW_ENGINE=1 ./ollama serve
Start a model that is supported by the Ollama engine. This one is Llama 3.1 8b Q4_K_M:
./ollama run jessegross/llama3.1
