Re: Strange IDE performance change in 2.6.1-rc1 (again)

[jw schultz]

On Fri, Jan 02, 2004 at 11:15:18PM -0500, Valdis.Kletnieks@xxxxxx wrote:
> On Sat, 03 Jan 2004 04:33:28 +0100, Tobias Diedrich <ranma@xxxxxx>  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@xxxxxxxxxx

		Remember Cernan and Schmitt
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