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
