> Tridge and I tried out the postgresql benchmark you used here and this
> contention is due to a bug in postgres. From a quick strace, we found
> the threads do a load of select(0, NULL, NULL, NULL, {0,0}). Basically all
> threads are pounding on schedule().
...
> Our guess is that the app has some form of userspace synchronisation
> (semaphores/spinlocks). I'd argue that the app needs to be fixed not the
> kernel, or a more valid test case is put forwards. :)
...
> PS: I just looked at the postgresql source and the spinlocks (s_lock() etc)
> are in a tight loop doing select(0, NULL, NULL, NULL, {0,0}).
Anton,
Thanks for looking into postgresql/pgbench related locking. Yes,
apparently postgresql uses a synchronization scheme that uses select()
to effect delays for backing off while attempting to acquire a lock.
However, it seems to me that runqueue lock contention was not entirely due
to postgresql code, since it was largely alleviated by the multiqueue
scheduler patch.
In using postgresql/pgbench to measure lock contention, I was attempting
to apply a typical server workload to measure scalability using only open
software. My goal is to load and measure the kernel for server performance,
so I need to ensure that the software I use represents likely real world
server configurations. I did not use mysql, because it cannot perform
transactions which I considered important. Any pointers to other open
database software or benchmarks that might be suitable for this effort
would be appreciated.
Jonathan
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