Re: Should we cacheline align PGXACT?

On 02/11/2017 01:21 PM, Alexander Korotkov wrote:
> Hi, Tomas!
>
> On Sat, Feb 11, 2017 at 2:28 AM, Tomas Vondra
> <tomas(dot)vondra(at)2ndquadrant(dot)com <mailto:tomas(dot)vondra(at)2ndquadrant(dot)com>> wrote:
>
> As discussed at the Developer meeting ~ a week ago, I've ran a
> number of benchmarks on the commit, on a small/medium-size x86
> machines. I currently don't have access to a machine as big as used
> by Alexander (with 72 physical cores), but it seems useful to verify
> the patch does not have negative impact on smaller machines.
>
> In particular I've ran these tests:
>
> * r/o pgbench
> * r/w pgbench
> * 90% reads, 10% writes
> * pgbench with skewed distribution
> * pgbench with skewed distribution and skipping
>
>
> Thank you very much for your efforts!
> I took a look at these tests. One thing catch my eyes. You warmup
> database using pgbench run. Did you consider using pg_prewarm instead?
>
> SELECT sum(x.x) FROM (SELECT pg_prewarm(oid) AS x FROM pg_class WHERE
> relkind IN ('i', 'r') ORDER BY oid) x;
>
> In my experience pg_prewarm both takes less time and leaves less
> variation afterwards.
>

I've considered it, but the problem I see in using pg_prewarm for
benchmarking purposes is that it only loads the data into memory, but it
does not modify the tuples (so all tuples have the same xmin/xmax, no
dead tuples, ...), it does not set usage counters on the buffers and
also does not generate any clog records.

I don't think there's a lot of variability in the results I measured. If
you look at (max-min) for each combination of parameters, the delta is
generally within 2% of average, with a very few exceptions, usually
caused by the first run (so perhaps the warmup should be a bit longer).

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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