On 09/04/2013 04:27 AM, Xavier Hernandez wrote:
I would also like to note that each node can store multiple elements. Current implementation creates a node for each byte in the key. In my implementation I only create a node if there is a prefix coincidence between 2 or more keys. This reduces the number of nodes and the number of indirections.
Whatever we do, we should try to make sure that the changes are profiled against real usage. When I was making my own dict optimizations back in March of last year, I started by looking at how they're actually used. At that time, a significant majority of dictionaries contained just one item. That's why I only implemented a simple mechanism to pre-allocate the first data_pair instead of doing something more ambitious. Even then, the difference in actual performance or CPU usage was barely measurable. Dict usage has certainly changed since then, but I think you'd still be hard pressed to find a case where a single dict contains more than a handful of entries, and approaches that are optimized for dozens to hundreds might well perform worse than simple ones (e.g. because of cache aliasing or branch misprediction). If you're looking for other optimization opportunities that might provide even bigger "bang for the buck" then I suggest that stack-frame or frame->local allocations are a good place to start. Or string copying in places like loc_copy. Or the entire fd_ctx/inode_ctx subsystem. Let me know and I'll come up with a few more. To put a bit of a positive spin on things, the GlusterFS code offers many opportunities for improvement in terms of CPU and memory efficiency (though it's surprisingly still way better than Ceph in that regard).
