Re: [gentoo-performance] preformance problem

[Ed Grimm]

On Fri, 15 Oct 2004, Istvan Soos wrote:

>> Stage 3, so your system is not fully compiled and thus not fully
>> optimised, however it should not impact your disk I/O ;p
>
> sorry, I've emerge -pv system (recompile gcc and others...)

I hope you made a typo there; -p is for pretend.

I know this is rather unfashionable, but I'd look to see what top said
was going on while the system was being sluggish.  I'd probably have top
on a 15 second or more update, so you have a chance to really look at
its output.  If you have more io wait (wa%) when certain processes are
nearly the top CPU users, they may have some specific issues.  Note that
some processes can do vast amounts of disk I/O with almost no CPU usage,
so it can be difficult to find the culprit in this manner.

I/O wait issues can also be caused by failing hardware.  Normally, there
would be a fair amount of kernel syslog complaints (these could be in
/var/adm/messages, /var/log/syslog, or a variety of other places.  I
can't remember where Gentoo's standard location is; all I remember for
certain was that its default syslog put too many facilities in one
location for my preference; so I fixed it.)

Finally - just to verify, you are actually seeing visible slowdowns, and
not merely noticing the io wait stat?  I have seen some people get very
concerned about io wait climbing when they did io intensive commands;
this is something that will always happen on a system which has more CPU
bandwidth than disk bandwidth (i.e. almost every system around today.)
This can be even more pronounced on systems where people've decided to
do RAID on IDE or ATA devices (including, apparently, SATA devices,
although I've not had a chance to play with these myself.)

Ed

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