Re: dbt2 performance

On Thu, Feb 25, 2010 at 6:29 PM, Yu-Ju Hong <yuru(dot)hong(at)gmail(dot)com> wrote:
> 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?

Well if you're writing a lot of stuff to disk you could easily be I/O limited.

> 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?

Since you've provided no details on your hardware configuration I'm
not sure how anyone could express an educated opinion on this
(personally I wouldn't know anyway, but others here would).

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

You'll typically be CPU bound when you're not I/O bound - i.e. when
the data that your accessing is small enough to fit in memory.

...Robert