On Tue, Oct 18, 2016 at 10:14:11PM +0200, Lucas Stach wrote: > Hi all, > > this series scratches my own small itch with XFS, namely scalability of the buffer > cache in metadata intensive workloads. With a large number of cached buffers those > workloads are CPU bound with a significant amount of time spent searching the cache. > > The first commit replaces the rbtree used to index the cache with an rhashtable. The > rbtree is a bottleneck in scalability, as the data structure itself is pretty CPU > cache unfriendly. For larger numbers of cached buffers over 80% of the CPU time > is spent waiting on cache misses resulting from the inherent pointer chasing. > > rhashtables provide a fast lookup with the ability to have lookups proceed while the > hashtable is being resized. This seems to match the read dominated workload of the > buffer cache index structure pretty well. Yup, it's a good idea - I have considered doing this change for these reasons, but have never found the time. > The second patch is logical follow up. The rhashtable cache index is protected by > RCU and does not need any additional locking. By switching the buffer cache entries > over to RCU freeing the buffer cache can be operated in a completely lock-free > manner. This should help scalability in the long run. Yup, that's another reason I'd considered rhashtables :P However, this is where it gets hairy. The buffer lifecycle is intricate, subtle, and has a history of nasty bugs that just never seem to go away. This change will require a lot of verification work to ensure things like the LRU manipulations haven't been compromised by the removal of this lock... > This series survives at least a xfstests auto group run (though with the scratch > device being a ramdisk) with no regressions and didn't show any problems in my > real world testing (using the patched FS with multiple large git trees) so far. It's a performance modification - any performance/profile numbers that show the improvement? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
