On Sat, Feb 11, 2017 at 4:17 PM, Tomas Vondra <tomas.von...@2ndquadrant.com> wrote:
> On 02/11/2017 01:21 PM, Alexander Korotkov wrote: > >> Hi, Tomas! >> >> On Sat, Feb 11, 2017 at 2:28 AM, Tomas Vondra >> <tomas.von...@2ndquadrant.com <mailto:tomas.von...@2ndquadrant.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. > Yes, but please note that pgbench runs VACUUM first, and all the tuples would be hinted. In order to xmin/xmax and clog take effect, you should run subsequent pgbench with --no-vacuum. Also usage counters of buffers make sense only when eviction happens, i.e. data don't fit shared_buffers. Also, you use pgbench -S to warmup before readonly test. I think pg_prewarm would be much better there unless your data is bigger than shared_buffers. 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). You also could run pg_prewarm before warmup pgbench for readwrite test. In my intuition you should get more stable results with shorter warmup. ------ Alexander Korotkov Postgres Professional: http://www.postgrespro.com The Russian Postgres Company
