Date: Sat, 3 Jan 2004 19:02:16 -0800
From: jw schultz <jw@pegasys.ws>
To: linux-kernel@vger.kernel.org
Subject: Re: Strange IDE performance change in 2.6.1-rc1 (again)
Message-ID: <20040104030216.GB7758@pegasys.ws>
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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
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