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List:       linux-poweredge
Subject:    Re: RHEL3 and performance issues
From:       James Bourne <jbourne () mtroyal ! ca>
Date:       2004-09-28 15:16:02
Message-ID: Pine.LNX.4.44.0409280854260.20646-100000 () skuld ! mtroyal ! ca
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On Tue, 28 Sep 2004, Seth Mos wrote:

> James Bourne wrote:
> > Hi,
> > We have a PE6400 running RHEL3 and Domino.  After migrating from RH
> > 7.2/7.3ish system about a week ago we've been experiencing unreasonable
> > performance issues (magnitude of about 10x slower).
> 
> The default read ahead settings changes somewhere around 2.4.20-20 or so 
> (redhat errata kernel for 7.3).
> 
> You need too manually adjust these. This also valid for all kernel.org 
> kernels > 2.4.20.
> 
> Just stick them into /etc/rc.d/rc.local or the sysctl.conf config file.
> 
> echo 0 > /proc/sys/net/ipv4/tcp_ecn
> echo 511 > /proc/sys/vm/max-readahead
> echo 127 > /proc/sys/vm/min-readahead

I had tried the max/min-readahead but it wasn't disk buffering that caused
the problem.  The vm.pagecache setting was the single change that did solve
the issue (to this point, let's get through a full day of usage first then
I'll know for sure..).  I originally thought it was disk I/O due to the
large number of procs in uninterruptable wait but, what was happening was
that there was so much data in swap, the server was thrashing badly...

With those settings, the OS will actually start buffering larger sections of
disk (127 blocks min in a read).  This works if you can guarentee that your
data is sequentially written to disk, otherwise if it's fragmented then the
sequential read of 127 or more blocks may actually waste more memory then it
saves...  Although, I know when we did change those numbers here it did
help as well, which may be telling us that our disk data is sequential...

A good document to read is
http://people.redhat.com/nhorman/papers/rhel3_vm.pdf
(thanks to seth vidal at duke for that!) as it contains everything you
wanted to know and more about the VM in RHEL3....

Thanks and regards
James

> Cheers,
> Seth

-- 
James Bourne, Senior Systems Administrator
Mount Royal College, Calgary, AB, CA
www.mtroyal.ca

"There are only 10 types of people in this world: those who
understand binary and those who don't."

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