Re: [gentoo-user] Performance problem at writing big files and Multitasking

[Kellystewart00@yahoo.com.au]

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-----Original Message-----
From: Matthias Bethke <matthias@towiski.de>

Date: Mon, 26 Feb 2007 17:28:35 
To:gentoo-user@lists.gentoo.org
Subject: Re: [gentoo-user] Performance problem at writing big files and Multitasking


Hi Daniel,
on Saturday, 2007-02-10 at 12:49:14, you wrote:
> I will give short overview what i have tried so far.
> 
> 1. Trying different I/O Scheduler ( cfq anticipatory and deadline)
> 2. Enabling Low latency kernel and Preemptible kernel
> 3. Setting 1000 HZ for timer frequency
> 4. Tried the new kernel 2.6.19-gentoo-r6 and even the testing version
> 2.6.20-gentoo with core 2 enabled in processor type

Oh, so it is a multicore CPU---sorry if you mentioned it already, I
had deleted the start of the thread already when I read Benno's advice.
In that case, try 100 Hz scheduling period as well. I've had very bad
experiences with I/O and 250 Hz or higher on a dual Xeon. My guess is
that it was a cache effect and therefore shouldn't happen on the
Core2Duo, but it might still be worth a try.

> As i am using Xfce i installed the diskperf-plugin which monitors disk
> I/O. The monitoring is divided in disk-read and disk-write.
> I recognized that every time when reading stops writing starts. So is
> this staggering of writing to disk normal as the programs have to read
> data they want to write to disk? On my previous machine i didn't
> recognize such a behaviour.

So you're reading and writing from/to the same disk? I'd expect that
behavior then, because the I/O scheduler tries to satisfy requests with
as little thrashing as possible. So if there are enough write requests
queued up it may keep the HD busy writing for a while before reading the
next chunk from somewhere else.

cheers!
	Matthias
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