| From: | "Luke Lonergan" <llonergan(at)greenplum(dot)com> |
|---|---|
| To: | stange(at)rentec(dot)com, "Greg Stark" <gsstark(at)mit(dot)edu> |
| Cc: | "Dave Cramer" <pg(at)fastcrypt(dot)com>, "Joshua Marsh" <icub3d(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org |
| Subject: | Re: Hardware/OS recommendations for large databases ( |
| Date: | 2005-11-19 16:13:09 |
| Message-ID: | BFA48F15.14182%llonergan@greenplum.com |
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| Lists: | pgsql-performance |
Alan,
On 11/18/05 11:39 AM, "Alan Stange" <stange(at)rentec(dot)com> wrote:
> Yes and no. The one cpu is clearly idle. The second cpu is 40% busy
> and 60% idle (aka iowait in the above numbers).
The "aka iowait" is the problem here - iowait is not idle (otherwise it
would be in the "idle" column).
Iowait is time spent waiting on blocking io calls. As another poster
pointed out, you have a two CPU system, and during your scan, as predicted,
one CPU went 100% busy on the seq scan. During iowait periods, the CPU can
be context switched to other users, but as I pointed out earlier, that's not
useful for getting response on decision support queries.
Thanks for your data, it exemplifies many of the points brought up:
- Lots of disks and expensive I/O hardware does not help improve performance
on large table queries because I/O bandwidth does not scale beyond
110-120MB/s on the fastest CPUs
- OLTP performance optimizations are different than decision support
Regards,
- Luke
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