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Re: dbt2 performance

From: Yu-Ju Hong <yuru(dot)hong(at)gmail(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: dbt2 performance
Date: 2010-02-25 23:29:42
Message-ID: 516a4a601002251529y7d445a19w15750315db8b5bd8@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
Thanks for the reply.

On Thu, Feb 25, 2010 at 5:48 PM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:

> Yu-Ju Hong wrote:
>
>> 2. Moreover, the disk utilization was high and the "await" time from
>> iostat is around 500 ms. Could disk I/O limit the overall throughput? The
>> server has 2 SATA disks, one for system and postgresql and the other is
>> dedicated to logging (pg_xlog). As far as I understand, modern database
>> systems should be CPU-bound rather than I/O-bound, is it because I did not
>> perform adequate performance tuning?
>>
>
> dbt2 is almost exclusively disk I/O bound once the data set gets big
> enough.  There are some applications where most of the data fits in RAM and
> therefore CPU performance is the limiter.  dbt2 is exactly the opposite of
> such an application though, and the idea that "modern database systems
> should be CPU bound" is not really true at all.  That's only the case if the
> data you're operating on fits in RAM.  Otherwise, databases are just as I/O
> bound as they've always been.  Main thing that's changed is there's a lot
> more RAM in systems nowadays.
>

In my test, there was almost no disk reads (mostly disk writes), so I
assumed the size of the database didn't cause the performance bottleneck.
Maybe I was wrong. If so, should I increase shared_buffer?

Assuming that dbt2 was limited by disk I/O in my experiments, do you think
the numbers I got with my server configuration are reasonable?

Also, would you mind giving some examples where the applications are CPU
bound? That could be useful information to me.

>
> By the way:  a large increase in checkpoint_segments is the first thing you
> should do.  If you check the database logs, they're probably filled with
> complaints about it being too low.  32 would be a useful starting value,
> going much higher for a test that's only 10 minutes long is probably
> cheating.
>
>
I increased the checkpoint_segments to 10 when I ran the tests. I'll
certainly increase it to 32 and give it a try.



>  --
> Greg Smith  2ndQuadrant US  Baltimore, MD
> PostgreSQL Training, Services and Support
> greg(at)2ndQuadrant(dot)com   www.2ndQuadrant.us <http://www.2ndquadrant.us/>
>
>
Thanks,
Yu-Ju

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