On Jul7, 2011, at 18:09 , Robert Haas wrote: > On Thu, Jul 7, 2011 at 5:54 AM, Florian Pflug <f...@phlo.org> wrote: >> In effect, the resulting thing is an LWLock with a partitioned shared >> counter. The partition one backend operates on for shared locks is >> determined by its backend id. >> >> I've added the implementation to the lock benchmarking tool at >> https://github.com/fgp/lockbench >> and also pushed a patched version of postgres to >> https://github.com/fgp/postgres/tree/lwlock_part >> >> The number of shared counter partitions is current 4, but can easily >> be adjusted in lwlock.h. The code uses GCC's atomic fetch and add >> intrinsic if available, otherwise it falls back to using a per-partition >> spin lock. > > I think this is probably a good trade-off for locks that are most > frequently taken in shared mode (like SInvalReadLock), but it seems > like it could be a very bad trade-off for locks that are frequently > taken in both shared and exclusive mode (e.g. ProcArrayLock, > BufMappingLocks).
That's definitely a concern. One idea I had to alleviate that is to add a per-partition "used" flag to the LWLock struct, and set that to true (if not already set) before incrementing a partition's shared counter. Exclusive lockers would then only need to inspect those partitions for which the flag is set, and would clear all flags after having acquires the lock. I actually tried to do that initially, but figuring out where and how to safely clear the flag again turned out to be a bit hairy. So I decided to keep it simple first, and wait to see whether that problem manifests itself in pratice. > I don't want to fiddle with your git repo, but if you attach a patch > that applies to the master branch I'll give it a spin if I have time. Patch attached. Beware that it needs at least GCC 4.1, otherwise it'll use a per-partition spin lock instead of "locked xadd" to increment the shared counters. best regards, Florian Pflug
pg_lwlock_part.patch
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