Re: Proposal for CSN based snapshots

On 08/23/2016 06:18 PM, Heikki Linnakangas wrote:
> On 08/22/2016 08:38 PM, Andres Freund wrote:
>> On 2016-08-22 20:32:42 +0300, Heikki Linnakangas wrote:
>>> I
>>> remember seeing ProcArrayLock contention very visible earlier, but I can't
>>> hit that now. I suspect you'd still see contention on bigger hardware,
>>> though, my laptop has oly 4 cores. I'll have to find a real server for the
>>> next round of testing.
>>
>> Yea, I think that's true. I can just about see ProcArrayLock contention
>> on my more powerful laptop, to see it really bad you need bigger
>> hardware / higher concurrency.
>
> As soon as I sent my previous post, Vladimir Borodin kindly offered
> access to a 32-core server for performance testing. Thanks Vladimir!
>
> I installed Greg Smith's pgbench-tools kit on that server, and ran some
> tests. I'm seeing some benefit on "pgbench -N" workload, but only after
> modifying the test script to use "-M prepared", and using Unix domain
> sockets instead of TCP to connect. Apparently those things add enough
> overhead to mask out the little difference.
>
> Attached is a graph with the results. Full results are available at
> https://hlinnaka.iki.fi/temp/csn-4-results/. In short, the patch
> improved throughput, measured in TPS, with >= 32 or so clients. The
> biggest difference was with 44 clients, which saw about 5% improvement.
>
> So, not phenomenal, but it's something. I suspect that with more cores,
> the difference would become more clear.
>
> Like on a cue, Alexander Korotkov just offered access to a 72-core
> system :-). Thanks! I'll run the same tests on that.

And here are the results on the 72 core machine (thanks again,
Alexander!). The test setup was the same as on the 32-core machine,
except that I ran it with more clients since the system has more CPU
cores. In summary, in the best case, the patch increases throughput by
about 10%. That peak is with 64 clients. Interestingly, as the number of
clients increases further, the gain evaporates, and the CSN version
actually performs worse than unpatched master. I don't know why that is.
One theory that by eliminating one bottleneck, we're now hitting another
bottleneck which doesn't degrade as gracefully when there's contention.

Full results are available at
https://hlinnaka.iki.fi/temp/csn-4-72core-results/.

- Heikki