Re: [LUG] Unusual activiy...

[John Daragon <john@xxxxxxxxxx>]

Grant Sewell wrote:
> On Tue, 21 Jun 2005 09:39:11 +0100
> John Daragon wrote:
>
>
> > 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 :
>
>
> Cheers.  The more the merrier!
>
>
> > 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.
>
>
> Yeah, got that.  I think this had been established, but thanks anyway.  (renice can also raise the scheduling priority of a task if run as root.)
>  
>
> > 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
>
>
> Cheers John.  One quick observation though: whenever this problem occurs, kswapd is *always* put quite near the top of the, erm, "top" output, so I would guess that it's a currently running daemon rather than a dormant one.  Since I have no swap, is it possible that kswapd is actually doing something anyway, which is causing this problem?

Good question.  I don't remember exactly what the kernel does these 
days, but most unices (UNIXs ??) use the paging mechanism to load 
executable code (when you refence a page that isn't yet in memory you 
get a page fault and the page is read from the executable....)

The kernel knows where executable code is, and it's not modifiable, so 
the page fault handler can just steal a frame in the page frame table 
and schedule a read from the executable file secure in the knowledge 
that the next time the stolen page is referenced, the whole operation 
will be repeated. So kswapd *could* be cycling executables through 
available memory, I guess.  I don't think I've ever actually seen that 
happen before.

jd

--

John Daragon                                          john@xxxxxxxxxx
argv[0] limited
Lambs Lawn Cottage,  Staple Fitzpaine,  Taunton,  TA3 5SL,  UK
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