From: jw schultz <jw@pegasys.ws>
To: linux-kernel@vger.kernel.org
Subject: Re: Strange IDE performance change in 2.6.1-rc1 (again)
Date: Sat, 3 Jan 2004 19:02:16 -0800 [thread overview]
Message-ID: <20040104030216.GB7758@pegasys.ws> (raw)
In-Reply-To: <200401030415.i034FJoc006230@turing-police.cc.vt.edu>
On Fri, Jan 02, 2004 at 11:15:18PM -0500, Valdis.Kletnieks@vt.edu wrote:
> On Sat, 03 Jan 2004 04:33:28 +0100, Tobias Diedrich <ranma@gmx.at> said:
>
> > Very interesting tidbit:
> >
> > with 2.6.1-rc1 and "dd if=/dev/hda of=/dev/null" I get stable 28 MB/s,
> > but with "cat < /dev/hda > /dev/null" I get 48 MB/s according to "vmstat
> > 5".
>
> 'cat' is probably doing a stat() on stdout and seeing it's connected to /dev/null
> and not even bothering to do the write() call. I've seen similar behavior in other
> GNU utilities.
That is unlikely.
However, i have seen some versions of cat check the input
file and if it is mappable mmap it instead of read. Given
that a write to /dev/null returns count without
copy_from_user the mapped page never faults so there is no
disk io.
--
________________________________________________________________
J.W. Schultz Pegasystems Technologies
email address: jw@pegasys.ws
Remember Cernan and Schmitt
prev parent reply other threads:[~2004-01-04 3:02 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-01-02 16:02 Strange IDE performance change in 2.6.1-rc1 (again) Paolo Ornati
2004-01-02 18:08 ` Ed Sweetman
2004-01-02 21:04 ` Paolo Ornati
2004-01-02 21:27 ` Valdis.Kletnieks
2004-01-03 10:20 ` Paolo Ornati
2004-01-02 21:32 ` Mike Fedyk
2004-01-02 22:34 ` Martin Josefsson
2004-01-03 11:13 ` Paolo Ornati
2004-01-03 22:40 ` Andrew Morton
2004-01-04 14:30 ` Paolo Ornati
2004-01-05 23:19 ` Ram Pai
2004-01-07 14:59 ` Paolo Ornati
2004-01-07 19:23 ` Ram Pai
2004-01-07 20:12 ` Paolo Ornati
2004-01-07 23:57 ` Andrew Morton
2004-01-08 7:31 ` Ram Pai
2004-01-09 1:05 ` Ram Pai
2004-01-09 1:17 ` Andrew Morton
2004-01-09 19:15 ` Ram Pai
2004-01-09 19:44 ` Andrew Morton
2004-01-10 14:48 ` Paolo Ornati
2004-01-10 16:00 ` Ed Sweetman
2004-01-10 16:19 ` Ed Sweetman
2004-01-10 17:29 ` Paolo Ornati
2004-01-10 17:29 ` Paolo Ornati
2004-03-29 15:45 ` Ram Pai
2004-01-04 17:15 ` Buffer and Page cache coherent? was: " Mike Fedyk
2004-01-04 22:10 ` Andrew Morton
2004-01-04 23:22 ` Mike Fedyk
2004-01-04 23:32 ` Andrew Morton
2004-01-04 23:45 ` Mike Fedyk
2004-01-05 0:23 ` Andrew Morton
2004-01-03 10:20 ` Paolo Ornati
2004-01-03 3:33 ` Tobias Diedrich
2004-01-03 4:15 ` Valdis.Kletnieks
2004-01-03 13:39 ` Tobias Diedrich
2004-01-03 20:56 ` Tobias Diedrich
2004-01-04 3:02 ` jw schultz [this message]
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