Re: Avoiding repeated snapshot computation

On Sat, Nov 26, 2011 at 6:51 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:

> On Saturday, November 26, 2011 11:39:23 PM Robert Haas wrote:
> > On Sat, Nov 26, 2011 at 5:28 PM, Andres Freund <andres(at)anarazel(dot)de>
> wrote:
> > > On Saturday, November 26, 2011 09:52:17 PM Tom Lane wrote:
> > >> I'd just as soon keep the fields in a logical order.
> > >
> > > Btw, I don't think the new order is necessarily worse than the old one.
> >
> > You forget to attach the benchmark results.
> >
> > My impression is that cache lines on modern hardware are 64 or 128
> > *bytes*, in which case this wouldn't matter a bit.
> >
> > But testing is even better than guessing.
> Being prodded like that I ran a very quick benchmark on my workstation.
> Unfortunately that means I cannot work during the time which is why I kept
> it
> rather short...
>
> That machine has 2 E5520(at)2(dot)27GHz which means 2(sockets) * 4(cores) *
> 2(threads) and 24GB of ram.
>
> Data was initialized with: pgbench -h /tmp/ --unlogged-tables -i -s 20
> pgbench
>
>
> pgbench -h /tmp/ pgbench -S -j 16 -c 16 -T 60
>
> origsnap: 92825.743958 93145.110901 93389.915474 93175.482351
> reordersnap: 93560.183272 93613.333494 93495.263012 93523.368489
>
> pgbench -h /tmp/ pgbench -S -j 32 -c 32 -T 60
>
> origsnap: 81846.743329 81545.175672 81702.755226 81576.576435
> 81228.154119 81546.047708 81421.436262
> reordersnap: 81823.479196 81787.784508 81820.242145 81790.263415
> 81762.421592 81496.333144 81732.088876
>
> At that point I noticed I had accidentally run with a nearly stock
> config...
> An even shorter run with a more approrioate config yielded:
>
>
> pgbench -h /tmp/ pgbench -S -j 32 -c 32 -T 20
>
> origsnap: 102234.664020 102003.449741 102119.509053 101722.410387
> 101973.651318 102056.440561
> reordersnap: 103444.877879 103385.888808 103302.318923 103372.659486
> 103330.157612 103313.833821
>
>
>
> Looks like a win to me. Even on this comparatively small machine.
>

This may not be necessary, but can you please share the modified config you
used for the last run.

I tabulated your last results to make it more readable, and added columns
to show the improvement.

origsnap reordersnap diff %age improvement
------------------------------------------------------------------
102234.66402 103444.877879 1210.213859 1.1837607827
102003.449741 103385.888808 1382.439067 1.3552865815
102119.509053 103302.318923 1182.80987 1.1582604352
101722.410387 103372.659486 1650.249099 1.6223063263
101973.651318 103330.157612 1356.506294 1.3302517626
102056.440561 103313.833821 1257.39326 1.2320567454

That looks like a win to me too. We're getting a little over 1% improvement
for free!

Maybe submitting this patch to the commitfest might help get some serious
consideration from a reviewer.

Regards,
--
Gurjeet Singh
EnterpriseDB Corporation
The Enterprise PostgreSQL Company