Re: new group commit behavior not helping?

On 1 April 2012 01:10, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Hoping to demonstrate the wonders of our new group commit code, I ran
> some benchmarks on the IBM POWER7 machine with synchronous_commit =
> on.  But, it didn't come out much better than 9.1.  pgbench, scale
> factor 300, median of 3 30-minute test runs, # clients = #threads,
> shared_buffers = 8GB, maintenance_work_mem = 1GB, synchronous_commit =
> on, checkpoint_segments = 300, checkpoint_timeout = 15min,
> checkpoint_completion_target = 0.9, wal_writer_delay = 20ms.

Why the low value for wal_writer_delay?

> master:
> 01 tps = 118.968446 (including connections establishing)
> 02 tps = 120.666865 (including connections establishing)
> 04 tps = 209.624437 (including connections establishing)
> 08 tps = 377.387029 (including connections establishing)
> 16 tps = 695.172899 (including connections establishing)
> 32 tps = 1318.468375 (including connections establishing)
>
> REL9_1_STABLE:
> 01 tps = 117.037056 (including connections establishing)
> 02 tps = 119.393871 (including connections establishing)
> 04 tps = 205.958750 (including connections establishing)
> 08 tps = 365.464735 (including connections establishing)
> 16 tps = 673.379394 (including connections establishing)
> 32 tps = 1101.324865 (including connections establishing)

(presumably s/tps/clients/ was intended here)

> Is this expected behavior?  Is this not the case where it's supposed
> to help?  I thought Peter G. posted results showing a huge improvement
> on this kind of workload, and I thought Heikki reproduced them on a
> different server, so I'm confused why I can't.

The exact benchmark that I ran was the update.sql pgbench-tools
benchmark, on my laptop. The idea was to produce a sympathetic
benchmark with a workload that was maximally commit-bound. Heikki
reproduced similar numbers on his laptop, iirc. Presumably the default
TPC-B-like transaction test has been used here.

You didn't mention what kind of disks this server has - I'm not sure
if that information is available elsewhere. That could be highly
pertinent.

--
Peter Geoghegan       http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training and Services