Tuesday, 23 August 2011

On the limitations of caching

I've been spending a little time thinking about caching lately. Irradiance caching, optimising some caching in production code, and hardware caches. Here's a little brain-dump.

I've come to the conclusion that if you ever need anything more than a trivially small cache, your algorithm is wrong. This is particularly so for parallel programming, as caches are often a difficult to handle piece of interdependent common state.

My reasoning goes a little something like this. If you are relying on a cache for performance, it is usually to re-use the results of some expensive transformation a -> b. Crucially, there needs to be 1 a to one or more b (otherwise there is no cache reuse!). For an efficient cache, you need:

  • To be caching the results of a fairly complex computation,
  • To have an efficient means of storing the results of the computation,
  • To have a well-behaved cache indexing function.
Now, what often ends up happening is that you iterate over the many objects 'b', and populate the cache on-demand from a common ancestor piece of data, a. And here's the first problem. You are primarily iterating over the wrong dataset, and filtering and culling parts of the other dataset by proxy. You have an implicit data filtering or reduction operation that could be better handled more explicitly.

Ok, so, we switch our iteration round to iterate over the source objects 'a'. We iterate over each 'a' object, and produce the resulting 1 or more 'b' objects. At this point, do we really have a cache? We've turned our cache into a data transformation kernel. All of sudden our cache is no longer some intermediate state used in a computation, but it is now simply a piece of input data. Since we now have a piece of code transforming [a] -> [b], we have a piece of code that is very amenable to data parallelism. Since we've decoupled the code from a cache, it's also much easier to parallelise.

I think ultimately an extensive caching scheme is evidence that the data domain hasn't been adequately structured and managed. If you need a cache, you probably need more data conditioning, filtering, and reduction.

A good counter-example is the vertex stream processing unit of a modern GPU. These units read huge streams of data, transform them, and maintain minimal cached state. Vertex caches are typically miniscule in comparison to the volume of data they process. Yet a post-transform cache can often maintain a very high level of efficiency. The key to this cache's success is the pre-processing (ie structuring) of the data fed into it. It is not random. It is well conditioned, which yields efficiency at a number of levels, and enables a vertex cache to be used only as a relatively modest addition to the pipeline. The cache is the icing on the cake, not a fundamental component of the algorithm.

I think this is an important lesson when transitioning from writing single-threaded to parallel code. Don't dream up increasingly elaborate caching schemes. Instead think up separable, well-structured data-parallel transformations. When you feel the need for a cache, you probably need to carefully re-examine your data flow structure.

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