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Date: Fri, 19 Jul 2002 17:03:59 -0400
From: Johannes Erdfelt <johannes@erdfelt.com>
To: David Rees <dbr@greenhydrant.com>, linux-kernel@vger.kernel.org
Subject: Re: 2.4.19rc2aa1 VM too aggressive?
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On Fri, Jul 19, 2002, David Rees <dbr@greenhydrant.com> wrote:
> On Fri, Jul 19, 2002 at 04:33:50PM -0400, Johannes Erdfelt wrote:
> > I recently upgraded a web server I run to a the 2.4.19rc2aa1 kernel to
> > see how much better the VM is.
> > 
> > It seems to be better than the older 2.4 kernels used on this machine,
> > but there seems to be lots of motion in the cache for all of the free
> > memory that exists:
> > 
> >    procs                      memory    swap          io     system  cpu
> >  3  0  0 106036 502288  10812  67236   0   0     0     0  802   494  46  37  17
> >  5  0  2 106032 476188  10844  91496   0   0     4   316  905   573  54  37   8
> > 16  0  2 106032 355400  10844 203880   0   0     4     0  909   540  51  49   0
> > 10  0  2 106024 340108  10852 221548   0   0    28     0  975   659  36  64   0
> >  0  0  0 106024 528340  10852  43572   0   0     4     0  569   426  17  17  67
> >  0  1  0 106024 531304  10852  43612   0   0     4     0  542   342   9  14  77
> >
> > This is with a 1 second interval. Why is it that most of the time I have
> > ~400MB of memory free (this machine has 1GB of memory). Why does the
> > cache size vary so wildly?
> > 
> > This machine is busy, as you can see, but it looks like the VM is trying
> > to be a bit too aggressive here.
> 
> What type of workload?  This looks fairly typicaly of a workload which
> writes/deletes large files.

Web server. The only writing is for the log files, which is relatively
minimal.

You can see from the io, that writes are relatively infrequent, while
reads happen regularly to fetch various documents from disk.

One thing also, is there is lots of process creation in this example.
For a variety of reasons, PHP programs are forked often from the Apache
server.

The systems running an older kernel (like RedHat's 2.4.9-21) are much
more consistent in their usage of memory. There are no 150MB swings in
cache utiliziation, etc.

What's really odd in the vmstat output is the fact that there is no disk
I/O that follows these wild swings. Where is this cache memory coming
from? Or is the accounting just wrong?

JE

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