Re: High CPU Utilization

Greg Smith <gsmith(at)gregsmith(dot)com> writes:

> On Mon, 16 Mar 2009, Joe Uhl wrote:
>
>> Here is vmstat 1 30. We are under peak load right now so I can gather
>> information from the real deal
>
> Quite helpful, reformatting a bit and picking an informative section:
>
> procs -----------memory---------- ---swap- ----io--- -system-- ----cpu----
> r b swpd free buff cache si so bi bo in cs us sy id wa
> 0 34 95048 11025880 56988 15020168 0 0 3852 160 3616 8614 11 1 6 82
> 3 25 95048 10996356 57044 15044796 0 0 7892 456 3126 7115 4 3 8 85
> 1 26 95048 10991692 57052 15050100 0 0 5188 176 2566 5976 3 2 12 83
>
> This says that your server is spending all its time waiting for I/O, actual CPU
> utilization is minimal. You're only achieving around 3-8MB/s of random I/O.
> That's the reality of what your disk I/O subsystem is capable of, regardless of
> what its sequential performance with dd looks like. If you were to run a more
> complicated benchmark like bonnie++ instead, I'd bet that your "seeks/second"
> results are very low, even though sequential read/write is fine.
>
> The Perc5 controllers have a pretty bad reputation for performance on this
> list, even in RAID10. Not much you can do about that beyond scrapping the
> controller and getting a better one.

Hm, well the tests I ran for posix_fadvise were actually on a Perc5 -- though
who knows if it was the same under the hood -- and I saw better performance
than this. I saw about 4MB/s for a single drive and up to about 35MB/s for 15
drives. However this was using linux md raid-0, not hardware raid.

But you shouldn't get your hopes up too much for random i/o. 3-8MB seems low
but consider the following:

$ units
2445 units, 71 prefixes, 33 nonlinear units

You have: 8kB / .5|7200min
You want: MB/s
* 1.92
/ 0.52083333

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
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