Messages in this thread | | | Date | Thu, 2 Dec 2004 20:52:36 +0100 | From | Jens Axboe <> | Subject | Re: Time sliced CFQ io scheduler |
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On Thu, Dec 02 2004, Andrew Morton wrote: > Jens Axboe <axboe@suse.de> wrote: > > > > as: > > Reader: 27985KiB/sec (max_lat=34msec) > > Writer: 64KiB/sec (max_lat=1042msec) > > > > cfq: > > Reader: 12703KiB/sec (max_lat=108msec) > > Writer: 9743KiB/sec (max_lat=89msec) > > > > If you look at vmstat while running these tests, cfq and deadline give > > equal bandwidth for the reader and writer all the time, while as > > basically doesn't give anything to the writer (a single block per second > > only). Nick, is the write batching broken or something? > > Looks like it. We used to do 2/3rds-read, 1/3rd-write in that testcase.
But 'as' has had no real changes in about 9 months time, it's really strange. Twiddling with write expire and write batch expire settings make no real difference. Upping the ante to 4 clients, two readers and two writers work about the same: 27MiB/sec aggregate read bandwidth, ~100KiB/sec write.
At least something needs to be done about it. I don't know what kernel this is a regression against, but at least it means that current 2.6 with its default io scheduler has basically zero write performance in presence of reads.
-- Jens Axboe
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