Re: using tree as attribute source is slow, was Re: Help troubleshoot performance regression cloning with depth: git 2.44 vs git 2.42

Jeff King <peff@peff.net> writes:

It should be per tree_oid or you will get a stale and incorrect result when you read paths from a different tree. But thanks for that "something simple and stupid" code to clearly demonstrate that repeated reading of the attributes data is the problem.

Given a tree with a name, the result of reading a path from that tree does not depend on the repository the tree appears in, so the cache does not have a reason to be tied to a particular repository. Generally we work only inside a single repository, so attaching the cache to that single repository would be a good way to make it available globally without adding another global variable, as the_repository can serve as the starting point for the global state, but other than that there is no reason.

I agree that the attribute layer may be a better place to cache this data. As you pointed out, it already has a caching behaviour in its attr_stack data structure that is optimized for local walk that visits every path in a tree in depth first order, but it is likely that a different caching scheme that is more suitable for random access may need to be introduced. The cache eviction strategy may need some thought (the attr_stack based caching has an obviously optimal eviction strategy---to evict the attribute data read from a directory when the traversal leaves that directory) in order to avoid unbounded bloat of the cached data.