Messages in this thread | | | Date | Sat, 3 Jan 2004 19:02:16 -0800 | From | jw schultz <> | Subject | Re: Strange IDE performance change in 2.6.1-rc1 (again) |
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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
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