[nSLUG] performance and schedulers

Peter Cordes peter at cordes.ca
Thu Jun 23 18:36:32 ADT 2005

On Wed, Jun 22, 2005 at 08:19:21AM -0300, Jim Haliburton wrote:
> Good Day all:
> I have read the various items on schedulers and performance and have several 
> questions.
> 1)  With a careful test of each of the various schedulers and their tunable 
> paramters, how much performance gain could one expect?   Are we talking 10. 15 
> or 30% change?
> 2)  Would a selection of higher performance components in the system give you a 
> quicker and cheaper performance gain?  In other words is it more productive to 
> just go buy a bigger box?  I am more hardware oriented and find that well 
> matched components often give much better performance gain, than hours spent 
> tweaking the OS.  Thoughts??

 Some things are limited by OS algorithms, other things are limited by
hardware.  Running one CPU bound program is limited by hardware;  Even the
most stupid OS process schedulers can't screw that up.

 As for tunables, with 2.6 I found that on a desktop machine running
bittorrent and experienceing some serious jerky behaviour and swapping out
mozilla every time I looked the other way, turning down the swappiness
helped a lot.  Bittorrent can be running all the time and you don't notice
the effect on the rest of the system.   /proc/sys/vm/swappiness is the main
VM tunable in Linux 2.6.  It was at 60, and I turned it down to 20 or 30 IIRC.

 One a machine at work with 3Ware RAID, 3w's docs recommend setting
readahead way higher than it defaults to.  They seem to be describing this
as the way to do well on a sequential I/O benchmark, though.  They were
to run  blockdev --setra 16384, but I only used 4096.  (I have a 4-disk
RAID5;  maybe a bigger array could reasonably gain from something ridiculous
like that.)

 That's about all I know or remember about kernel tuning at the moment.  I
haven't experimented with different I/O or process schedulers.  Hopefully
sometime I'll find a patch for batch-scheduling that works with a kernel I
want to run.  That way my Folding at Home processes won't get any CPU when
anything else wants it.

 Oh, and since this is my first post in a long time, I should mention that I
haven't been keeping up with my nslug email.  I haven't even been lurking...

#define X(x,y) x##y
Peter Cordes ;  e-mail: X(peter at cor , des.ca)

"The gods confound the man who first found out how to distinguish the hours!
 Confound him, too, who in this place set up a sundial, to cut and hack
 my day so wretchedly into small pieces!" -- Plautus, 200 BC
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