Re: s_lock() seems too aggressive for machines with many sockets

The whole thing turns out to be based on wrong baseline data, taken with
a pgbench client running from a remote machine. It all started out from
an investigation against 9.3. Curiously enough, the s_lock() problem
that existed in 9.3 has a very similar effect on throughput as a network
bottleneck has on 9.5.

Sorry for the noise, Jan

On 06/10/2015 09:18 AM, Jan Wieck wrote:
> Hi,
>
> I think I may have found one of the problems, PostgreSQL has on machines
> with many NUMA nodes. I am not yet sure what exactly happens on the NUMA
> bus, but there seems to be a tipping point at which the spinlock
> concurrency wreaks havoc and the performance of the database collapses.
>
> On a machine with 8 sockets, 64 cores, Hyperthreaded 128 threads total,
> a pgbench -S peaks with 50-60 clients around 85,000 TPS. The throughput
> then takes a very sharp dive and reaches around 20,000 TPS at 120
> clients. It never recovers from there.
>
> The attached patch demonstrates that less aggressive spinning and (much)
> more often delaying improves the performance "on this type of machine".
> The 8 socket machine in question scales to over 350,000 TPS.
>
> The patch is meant to demonstrate this effect only. It has a negative
> performance impact on smaller machines and client counts < #cores, so
> the real solution will probably look much different. But I thought it
> would be good to share this and start the discussion about reevaluating
> the spinlock code before PGCon.
>
>
> Regards, Jan
>

--
Jan Wieck
Senior Software Engineer
http://slony.info