From: Tom Sightler <ttsig@tuxyturvy.com>
To: Mark Hahn <hahn@coffee.psychology.mcmaster.ca>
Cc: Linux-Kernel <linux-kernel@vger.kernel.org>
Subject: Re: 2.4.6-pre2, pre3 VM Behavior
Date: Wed, 13 Jun 2001 22:08:17 -0400 (EDT) [thread overview]
Message-ID: <992484497.3b281c91ce043@eargle.com> (raw)
In-Reply-To: <Pine.LNX.4.10.10106140024230.980-100000@coffee.psychology.mcmaster.ca>
In-Reply-To: <Pine.LNX.4.10.10106140024230.980-100000@coffee.psychology.mcmaster.ca>
Quoting Mark Hahn <hahn@coffee.psychology.mcmaster.ca>:
> > 1. Transfer of the first 100-150MB is very fast (9.8MB/sec via 100Mb
> Ethernet,
> > close to wire speed). At this point Linux has yet to write the first
> byte to
> > disk. OK, this might be an exaggerated, but very little disk activity
> has
> > occured on my laptop.
>
> right. it could be that the VM scheduling stuff needs some way to
> tell
> whether the IO system is idle. that is, if there's no contention for
> the disk, it might as well be less lazy about writebacks.
That's exactly the way it seems.
> > 2. Suddenly it's as if Linux says, "Damn, I've got a lot of data to
> flush,
> > maybe I should do that" then the hard drive light comes on solid for
> several
> > seconds. During this time the ftp transfer drops to about 1/5 of the
> original
> > speed.
>
> such a dramatic change could be the result of IDE misconfiguration;
> is it safe to assume you have DMA or UDMA enabled?
Yes, UDMA/33 is enabled and working on the drive (using hdparm -d 0 makes the
problem way worse and my drive performs about 1/4 the speed).
> > This was much less noticeable on a server with a much faster SCSI hard
> disk
> > subsystem as it took significantly less time to flush the information
> to the
>
> is the SCSI disk actually faster (unlikley, for modern disks), or
> is the SCSI controller simply busmastering, like DMA/UDMA IDE,
> but wholly unlike PIO-mode IDE?
First, lets be fair, we're comparing a UDMA/33 IDE drive in a 1 year old laptop
(IBM Travelstar, if your interested) to a true SCSI Disk Subsystem with
mirrored/striped Ultra160 SCSI disk connected via 64bit PCI/66Mhz bus, so yes,
the SCSI subsystem is MUCH faster. Specific numbers:
Laptop with TravelStar IDE HD Max sustained read: 16.5MB/s
Server with Ultra160 SCSI disk array Max sustained read: >100MB/s
That's a big difference. The Travelstar is probably only 5400RPM and is
optimized for power savings, not speed, the SCSI subsystem has multiple 15000RPM
in a striped/mirrored configuration for speed.
Later,
Tom
next parent reply other threads:[~2001-06-14 2:08 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <Pine.LNX.4.10.10106140024230.980-100000@coffee.psychology.mcmaster.ca>
2001-06-14 2:08 ` Tom Sightler [this message]
2001-06-13 19:31 2.4.6-pre2, pre3 VM Behavior Tom Sightler
2001-06-13 20:21 ` Rik van Riel
2001-06-14 1:49 ` Tom Sightler
2001-06-14 3:16 ` Rik van Riel
2001-06-14 7:59 ` Laramie Leavitt
2001-06-14 9:24 ` Helge Hafting
2001-06-14 17:38 ` Mark Hahn
2001-06-15 8:27 ` Helge Hafting
2001-06-14 8:47 ` Daniel Phillips
2001-06-14 20:23 ` Roger Larsson
2001-06-15 6:04 ` Mike Galbraith
2001-06-14 20:39 ` John Stoffel
2001-06-14 20:51 ` Rik van Riel
2001-06-14 21:33 ` John Stoffel
2001-06-14 22:23 ` Rik van Riel
2001-06-14 15:10 ` John Stoffel
2001-06-14 18:25 ` Daniel Phillips
2001-06-14 8:30 ` Mike Galbraith
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