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.