Messages in this thread | | | Date | Wed, 30 Dec 2009 21:31:03 +0100 | Subject | Re: [PATCH] cfq-iosched: non-rot devices do not need queue merging | From | Corrado Zoccolo <> |
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On Wed, Dec 30, 2009 at 7:45 PM, Jens Axboe <jens.axboe@oracle.com> wrote: > On Wed, Dec 30 2009, Corrado Zoccolo wrote: >> Non rotational devices' performances are not affected by >> distance of requests, so there is no point in having overhead >> to merge queues of nearby requests. > > If the distance is zero, it may still make a big difference (at least > for writes). This check would be better as "ncq and doesn't suck", ala > > blk_queue_nonrot(q) && tagged > > like we do elsewhere.
For reads, though, even flash cards and netbook ssds are completely unaffected. I have done few experiments on my available disks: * http://dl.dropbox.com/u/3525644/service_time.png (I used the program: http://dl.dropbox.com/u/3525644/stride.c to get the graphs).
For distance 0, I think request merging will be more effective than queue merging, moreover I think the multi-thread trick to have large I/O depth is used for reads, not writes (where simply issuing buffered writes already achieves a similar effect), so I think it is safe to disable it for all non-rotational devices.
Thanks, Corrado > > -- > Jens Axboe > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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