Re: [LUG] Unusual activiy...

[John Daragon <john@xxxxxxxxxx>]

Grant Sewell wrote:
> On Sun, 19 Jun 2005 10:33:30 +0100
> Darke, Clive wrote:
>
>
> > Try running sar (system activity reporter) for the period in question.  My
> > guess is that paging/swapping will be peaking.  Do you have enough swap
> > space?
> >
> > Clive
>
>
> Ah!  You may well have hit the nail on the head there.  I completely forgot that I 
> initially used the swap partition (about 1 gig) on hda (system is install on hdb)... 
> which has since been removed to put in another machine, and I didn't bother 
> rejigging my hdb partitions to create another swap partition.  Do I have enough 
> swap?  Probably not since I'm not running with any :D  However, I have 640Mb of RAM 
> and I never run anything more intensive than OpenOffice, so I don't even run out of 
> RAM during normal operations.  It would explain why kswapd is always present with 
> the machine does this, though.
>
> Since the machine that is housing the original hda harddrive is no longer performing any useful functions, I may well put it back in this one and have a play around... and create a swap partition :D


I thought I'd just chip in here because a couple of preconceptions are 
getting in the way of finding out what's happening to your system :

1) nice. nice is a mechanism for decreasing task priority. This 
effectively decreases the probability that a task (if runnable) will be 
scheduled on return from interrupt. It's of marginal use in the control 
of tasks which perform much I/O. In this case, with low CPU usage on the 
machine, you can reduce the priority of this task as much as you like, 
and it'll still get run when it's not in iowait.

2) Paging. If you have no swap space you will not be performing any page 
out operations. If you perform no page outs, then you will be performing 
no page ins, either (with the exception of loading executable code). So 
the I/O you are observing is not paging.

Of course, given that you can't page out, the amount of space you have 
for block buffers may be limited. What you *may* be observing is a 
(relative) failure of the block cache to perform efficiently because it 
has limited space to work with.  The block/paging cache scheme varies 
from kernel to kernel, and I haven't looked at the Linux one for a 
while, so I'm disinclined to offer a concrete mechanism.

jd

--

John Daragon                                          john@xxxxxxxxxx
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