Re: High context switches occurring - Mailing list pgsql-performance

From Tom Lane
Subject Re: High context switches occurring
Date
Msg-id 22673.1133912703@sss.pgh.pa.us
Whole thread Raw
In response to Re: High context switches occurring  ("Anjan Dave" <adave@vantage.com>)
Responses Re: High context switches occurring
List pgsql-performance
"Anjan Dave" <adave@vantage.com> writes:
> -bash-3.00$ time pgbench -c 1000 -t 30 pgbench
> starting vacuum...end.
> transaction type: TPC-B (sort of)
> scaling factor: 1
> number of clients: 1000
> number of transactions per client: 30
> number of transactions actually processed: 30000/30000
> tps = 45.871234 (including connections establishing)
> tps = 46.092629 (excluding connections establishing)

I can hardly think of a worse way to run pgbench :-(.  These numbers are
about meaningless, for two reasons:

1. You don't want number of clients (-c) much higher than scaling factor
(-s in the initialization step).  The number of rows in the "branches"
table will equal -s, and since every transaction updates one
randomly-chosen "branches" row, you will be measuring mostly row-update
contention overhead if there's more concurrent transactions than there
are rows.  In the case -s 1, which is what you've got here, there is no
actual concurrency at all --- all the transactions stack up on the
single branches row.

2. Running a small number of transactions per client means that
startup/shutdown transients overwhelm the steady-state data.  You should
probably run at least a thousand transactions per client if you want
repeatable numbers.

Try something like "-s 10 -c 10 -t 3000" to get numbers reflecting test
conditions more like what the TPC council had in mind when they designed
this benchmark.  I tend to repeat such a test 3 times to see if the
numbers are repeatable, and quote the middle TPS number as long as
they're not too far apart.

            regards, tom lane

pgsql-performance by date:

Previous
From: Ameet Kini
Date:
Subject: Re: postgresql performance tuning
Next
From: Michael Stone
Date:
Subject: Re: postgresql performance tuning