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Triton Response Cache

Overview

In this document an inference request is the model name, model version, and input tensors (name, shape, datatype and tensor data) that make up a request submitted to Triton. An inference result is the output tensors (name, shape, datatype and tensor data) produced by an inference execution. The response cache is used by Triton to hold inference results generated for previous executed inference requests. Triton will maintain the response cache so that inference requests that hit in the cache will not need to execute a model to produce results and will instead extract their results from the cache. For some use cases this can significantly reduce the inference request latency.

Triton accesses the response cache with a hash of the inference request that includes the model name, model version and model inputs. If the hash is found in the cache, the corresponding inference result is extracted from the cache and used for the request. When this happens there is no need for Triton to execute the model to produce the inference result. If the hash is not found in the cache, Triton executes the model to produce the inference result, and then records that result in the cache so that subsequent inference requests can (re)use those results.

Usage

In order for caching to be used on a given model, it must be enabled on both the server-side, and in the model's model config. See the following sections below for more details.

Enable Caching on Server-side

The response cache is enabled on the server-side by specifying a <cache_implementation> and corresponding configuration when starting the Triton server.

Through the CLI, this translates to setting tritonserver --cache-config <cache_implementation>,<key>=<value> .... For example:

tritonserver --cache-config local,size=1048576

For in-process C API applications, this translates to calling TRITONSERVER_SetCacheConfig(const char* cache_implementation, const char* config_json).

This allows users to enable/disable caching globally on server startup.

Enable Caching for a Model

By default, no model uses response caching even if the response cache is enabled globally with the --cache-config flag.

For a given model to use response caching, the model must also have response caching enabled in its model configuration:

# config.pbtxt

response_cache {
  enable: true
}

This allows users to enable/disable caching for specific models.

For more information on enabling the response cache for each model, see the model configuration docs.

Cache Implementations

Starting in the 23.03 release, Triton has a set of TRITONCACHE APIs that are used to communicate with a cache implementation of the user's choice.

A cache implementation is a shared library that implements the required TRITONCACHE APIs and is dynamically loaded on server startup, if enabled.

Triton's most recent tritonserver release containers come with the following cache implementations out of the box:

  • local: /opt/tritonserver/caches/local/libtritoncache_local.so
  • redis: /opt/tritonserver/caches/redis/libtritoncache_redis.so

With these TRITONCACHE APIs, tritonserver exposes a new --cache-config CLI flag that gives the user flexible customization of which cache implementation to use, and how to configure it. Similar to the --backend-config flag, the expected format is --cache-config <cache_name>,<key>=<value> and may be specified multiple times to specify multiple keys if the cache implementation requires it.

Local Cache

The local cache implementation is equivalent to the response cache used internally before the 23.03 release. For more implementation specific details, see the local cache implementation.

When --cache-config local,size=SIZE is specified with a non-zero SIZE, Triton allocates the requested size in CPU memory and shares the cache across all inference requests and across all models.

Redis Cache

The redis cache implementation exposes the ability for Triton to communicate with a Redis server for caching. The redis_cache implementation is essentially a Redis client that acts as an intermediary between Triton and Redis.

To list a few benefits of the redis cache compared to the local cache in the context of Triton:

  • The Redis server can be hosted remotely as long as it is accessible by Triton, so it is not tied directly to the Triton process lifetime.
    • This means Triton can be restarted and still have access to previously cached entries.
    • This also means that Triton doesn't have to compete with the cache for memory/resource usage.
  • Multiple Triton instances can share a cache by configuring each Triton instance to communicate with the same Redis server.
  • The Redis server can be updated/restarted independently of Triton, and Triton will fallback to operating as it would with no cache access during any Redis server downtime, and log appropriate errors.

In general, the Redis server can be configured/deployed as needed for your use case, and Triton's redis cache will simply act as a client of your Redis deployment. The Redis docs should be consulted for questions and details about configuring the Redis server.

For Triton-specific redis cache implementation details/configuration, see the redis cache implementation.

Custom Cache

With the TRITONCACHE API interface, it is now possible for users to implement their own cache to suit any use-case specific needs. To see the required interface that must be implemented by a cache developer, see the TRITONCACHE API header. The local or redis cache implementations may be used as reference.

Upon successfully developing and building a custom cache, the resulting shared library (ex: libtritoncache_<name>.so) must be placed in the cache directory similar to where the local and redis cache implementations live. By default, this directory is /opt/tritonserver/caches, but a custom directory may be specified with --cache-dir as needed.

To put this example together, if the custom cache were named "custom" (this name is arbitrary), by default Triton would expect to find the cache implementation at /opt/tritonserver/caches/custom/libtritoncache_custom.so.

Deprecation Notes

Note Prior to 23.03, enabling the local cache used to be done through setting a non-zero size (in bytes) when Triton was launched using the --response-cache-byte-size flag.

Starting in 23.03, the --response-cache-byte-size flag is now deprecated and --cache-config should be used instead. For backwards compatibility, --response-cache-byte-size will continue to function under the hood by being converted to the corresponding --cache-config argument, but it will default to using the local cache implementation. It is not possible to choose other cache implementations using the --response-cache-byte-size flag.

For example, --response-cache-byte-size 1048576 would be equivalent to --cache-config local,size=1048576. However, the --cache-config flag is much more flexible and should be used instead.

Warning

The local cache implementation may fail to initialize for very small values of --cache-config local,size=<small_value> or --response-cache-byte-size (ex: less than 1024 bytes) due to internal memory management requirements. If you encounter an initialization error for a relatively small cache size, try increasing it.

Similarly, the size is upper bounded by the available RAM on the system. If you encounter an initial allocation error for a very large cache size setting, try decreasing it.

Performance

The response cache is intended to be used for use cases where a significant number of duplicate requests (cache hits) are expected and therefore would benefit from caching. The term "significant" here is subjective to the use case, but a simple interpretation would be to consider the proportion of expected cache hits/misses, as well as the average time spend computing a response.

For cases where cache hits are common and computation is expensive, the cache can significantly improve overall performance.

For cases where most requests are unique (cache misses) or the compute is fast/cheap (the model is not compute-bound), the cache can negatively impact the overall performance due to the overhead of managing and communicating with the cache.

Known Limitations

  • Only input tensors located in CPU memory will be hashable for accessing the cache. If an inference request contains input tensors not in CPU memory, the request will not be hashed and therefore the response will not be cached.
  • Only responses with all output tensors located in CPU memory will be eligible for caching. If any output tensor in a response is not located in CPU memory, the response will not be cached.
  • The cache is accessed using only the inference request hash. As a result, if two different inference requests generate the same hash (a hash collision), then Triton may incorrectly use the cached result for an inference request. The hash is a 64-bit value so the likelihood of collision is small.
  • Only successful inference requests will have their responses cached. If a request fails or returns an error during inference, its response will not be cached.
  • Only requests going through the Default Scheduler or Dynamic Batch Scheduler are eligible for caching. The Sequence Batcher does not currently support response caching.
  • The response cache does not currently support decoupled models.