Re: I/O or Threading Suffer

From: Michael Riexinger <michael.riexinger_at_de.clara.net>
Date: Wed, 21 Jul 2004 10:23:52 +0200
On Tue Jul 20 23:48:17 2004, Dan Nelson wrote:
> In the last episode (Jul 21), jesk said:
> > i figured out that the performanceloss only really occur if the
> > process is heavily writing on the filesystem. dd if=/dev/zero
> > of=/dev/null bs=128k doesnt hurt much in responsetime of parallel
> > processes, but when dd operates on the filesystem with of=foo every
> > process will be affect in executiontime. a simple ps or ls meanwhile
> > dding onto the disk will be hang for dozen of seconds.
> 
> Ah. now that's a different story.  You're out of the control of the
> process scheduler and into the disk.  I don't suppose you're using an
> IDE/ATA disk with no tagged queueing? :) Run "dmesg | grep depth.queue"
> to see how many requests can be queued up on your disk at once.
> 
> That dd is stuffing lots of dirty data into the disk cache, and all the
> other processes have to wait in line to get their I/Os done.  You'll
> see much better results from a SCSI disk (with usual queue depths
> between 32 and 64), and even better results from a multi-disk hardware
> RAID array (which will have a large write cache).

Same here. 
I tried both with IDE and SCSI. With IDE it's terrible, ps for example
takes seconds to execute. With an 2 cpu machine with scsi hdd's, it's
better but there's some delay, too. I tested on a FreeBSD 4.9 machine
with ide disks and there is no delay, it works perfect. That makes 
FreeBSD-5 really unusable for me.


Best regards,

Michael Riexinger
systems engineer

-- 
claranet gmbh   internet service provider
tel   +49 (0) 69  - 40 80 18 - 300
email: michael.riexinger_at_de.clara.net  http://www.claranet.de/
Received on Wed Jul 21 2004 - 06:23:40 UTC

This archive was generated by hypermail 2.4.0 : Wed May 19 2021 - 11:38:02 UTC