Message-ID: <4D39D07B.1070802@squeakycode.net>
Date: Fri, 21 Jan 2011 12:29:15 -0600
From: Andy Colson <andy@squeakycode.net>
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To: grant@amadensor.com
CC: pgsql-performance@postgresql.org
Subject: Re: Fun little performance IMPROVEMENT...
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On 1/21/2011 12:12 PM, grant@amadensor.com wrote:
> I was doing a little testing to see how machine load affected the
> performance of different types of queries, index range scans, hash joins,
> full scans, a mix, etc.
>
> In order to do this, I isolated different performance hits, spinning only
> CPU, loading the disk to create high I/O wait states, and using most of
> the physical memory.   This was on a 4 CPU Xen virtual machine running
> 8.1.22 on CENTOS.
>
>
> Here is the fun part.   When running 8 threads spinning calculating square
> roots (using the stress package), the full scan returned consistently 60%
> faster than the machine with no load.   It was returning 44,000 out of
> 5,000,000 rows.   Here is the explain analyze.   I am hoping that this
> triggers something (I can run more tests as needed) that can help us make
> it always better.
>
> Idling:
>                                                           QUERY PLAN
> ----------------------------------------------------------------------------------------------------------------------------
>   Seq Scan on schedule_details  (cost=0.00..219437.90 rows=81386 width=187)
> (actual time=0.053..2915.966 rows=44320 loops=1)
>     Filter: (schedule_type = '5X'::bpchar)
>   Total runtime: 2986.764 ms
>
> Loaded:
>                                                           QUERY PLAN
> ----------------------------------------------------------------------------------------------------------------------------
>   Seq Scan on schedule_details  (cost=0.00..219437.90 rows=81386 width=187)
> (actual time=0.034..1698.068 rows=44320 loops=1)
>     Filter: (schedule_type = '5X'::bpchar)
>   Total runtime: 1733.084 ms
>

Odd.  Did'ja by chance run the select more than once... maybe three or 
four times, and always get the same (or close) results?

Is the stress package running niced?

-Andy