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Date: Thu, 28 Mar 2013 14:03:42 -0700 (PDT)
From: Kevin Grittner <kgrittn@ymail.com>
Reply-To: Kevin Grittner <kgrittn@ymail.com>
Subject: Re: Question about postmaster's CPU usage
To: kelphet xiong <kelphet@gmail.com>,
	"pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
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kelphet xiong <kelphet@gmail.com> wrote:

> When I use postgres and issue a simple sequential scan for a
> table inventory using query "select * from inventory;", I can see
> from "top" that postmaster is using 100% CPU, which limits the
> query execution time. My question is that, why CPU is the
> bottleneck here and what is postmaster doing? Is there any way to
> improve the performance? Thanks!

> explain analyze select * from inventory;
>=20
> Seq Scan on inventory=A0 (cost=3D0.00..180937.00 rows=3D11745000 width=3D=
16) (actual time=3D0.005..1030.403 rows=3D11745000 loops=3D1)
>=A0 Total runtime: 1750.889 ms

So it is reading and returning 11.7 million rows in about 1 second,
or about 88 nanoseconds (billionths of a second) per row.=A0 You
can't be waiting for a hard drive for many of those reads, or it
would take a lot longer, so the bottleneck is the CPU pushing the
data around in RAM.=A0 I'm not sure why 100% CPU usage would surprise
you.=A0 Are you wondering why the CPU works on the query straight
through until it is done, rather than taking a break periodically
and letting the unfinished work sit there?

--
Kevin Grittner
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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