From: "Richard B. Johnson" <root@chaos.analogic.com>
To: Andrew Morton <akpm@osdl.org>
Cc: Olaf Hering <olh@suse.de>, linux-kernel@vger.kernel.org
Subject: Re: very low performance on SCSI disks if device node is in tmpfs
Date: Tue, 25 May 2004 17:57:59 -0400 (EDT) [thread overview]
Message-ID: <Pine.LNX.4.53.0405251754440.1448@chaos> (raw)
In-Reply-To: <20040525144836.1af59a96.akpm@osdl.org>
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On Tue, 25 May 2004, Andrew Morton wrote:
> Olaf Hering <olh@suse.de> wrote:
> >
> > Any ideas why the location of the device node makes such a big
> > difference? I always wondered why my firewire is so dog slow with 2.6.
> > Now I know the reason: /dev is in tmpfs.
> > I dont see that with IDE disks, only with SCSI.
>
> This is truly bizarre. Reading /dev/sda I get 24MB/sec at 700 context
> switches/sec. Reading /mnt/tmpfs/sda it's 14MB/sec, 7000 switches/sec.
> /mnt/ramfs/sda is slow too. /mnt/hda5/sda is fast.
>
>
> I'd assumed that the kernel got the backing_dev_info's screwed up and the
> tmpfs node isn't doing readahead but that appears to not be the case
> (/dev/sda is still fast with zero readahead).
>
>
> You really, really get the weird-bug-of-the-month award for this one. I'll
> poke at it some more later on.
> -
Try the attached utility to compare the open time on devfs and on
the regular fs.
I note that opening a file on a RAM disk seems to take a lot
longer than on a physical device. This might be paging time.
Anyway, this might give some hints of what's causing the
problem.
Cheers,
Dick Johnson
Penguin : Linux version 2.4.26 on an i686 machine (5570.56 BogoMips).
Note 96.31% of all statistics are fiction.
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next prev parent reply other threads:[~2004-05-25 22:01 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-05-25 18:47 Olaf Hering
2004-05-25 19:14 ` Richard B. Johnson
2004-05-25 19:34 ` Olaf Hering
2004-05-25 19:44 ` Richard B. Johnson
2004-05-25 20:02 ` Oliver Neukum
2004-05-25 21:48 ` Andrew Morton
2004-05-25 21:57 ` Richard B. Johnson [this message]
2004-05-25 21:59 ` Andrew Morton
2004-05-25 22:41 ` Andrew Morton
2004-05-26 0:05 ` Linus Torvalds
2004-05-26 0:10 ` viro
2004-05-26 0:14 ` Andrew Morton
2004-05-26 9:53 ` Olaf Hering
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