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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This archive was generated by hypermail 2b29 : Tue Jul 23 2002 - 22:00:31 EST