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    SubjectRe: Time sliced CFQ io scheduler
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    On Wed, 2004-12-08 at 08:20 +0100, Jens Axboe wrote:
    > On Wed, Dec 08 2004, Nick Piggin wrote:
    > > On Wed, 2004-12-08 at 07:58 +0100, Jens Axboe wrote:
    > > > On Wed, Dec 08 2004, Nick Piggin wrote:
    > > > > On Tue, 2004-12-07 at 18:25 -0800, Andrew Morton wrote:
    > >
    > > > > I think we could detect when a disk asks for more than, say, 4
    > > > > concurrent requests, and in that case turn off read anticipation
    > > > > and all the anti-starvation for TCQ by default (with the option
    > > > > to force it back on).
    > > >
    > > > CFQ only allows a certain depth a the hardware level, you can control
    > > > that. I don't think you should drop the AS behaviour in that case, you
    > > > should look at when the last request comes in and what type it is.
    > > >
    > > > With time sliced cfq I'm seeing some silly SCSI disk behaviour as well,
    > > > it gets harder to get good read bandwidth as the disk is trying pretty
    > > > hard to starve me. Maybe killing write back caching would help, I'll
    > > > have to try.
    > > >
    > >
    > > I "fixed" this in AS. It gets (or got, last time we checked, many months
    > > ago) pretty good read latency even with a big write and a very large
    > > tag depth.
    > >
    > > What were the main things I had to do... hmm, I think the main one was
    > > to not start on a new batch until all requests from a previous batch
    > > are reported to have completed. So eg. you get all reads completing
    > > before you start issuing any more writes. The write->read side of things
    > > isn't so clear cut with your "smart" write caches on the IO systems, but
    > > no doubt that helps a bit.
    >
    > I can see the read/write batching being helpful there, at least to
    > prevent writes starving reads if you let the queue drain completely
    > before starting a new batch.
    >
    > CFQ does something similar, just not batched together. But it does let
    > the depth build up a little and drain out. In fact I think I'm missing
    > a little fix there thinking about it, that could be why the read
    > latencies hurt on write intensive loads (the dispatch queue is drained,
    > the hardware queue is not fully).
    >

    OK, you should look into that, because I found it was quite effective.
    Maybe you have a little bug or oversight somewhere if you read latencies
    are really bad. Note that AS read latencies at 256 tags aren't so good
    as at 2 tags... but I think they're an order of magnitude better than
    with deadline on the hardware we were testing.

    > > Of course, after you do all that your database performance has well and
    > > truly gone down the shitter. It is also hampered by the more fundamental
    > > issue that read anticipating can block up the pipe for IO that is cached
    > > on the controller/disks and would get satisfied immediately.
    >
    > I think we need to end up with something that sets the machine profile
    > for the interesting disks. Some things you can check for at runtime
    > (like the writes being extremely fast is a good indicator of write
    > caching), but it is just not possible to cover it all. Plus, you end up
    > with 30-40% of the code being convoluted stuff added to detect it.
    >

    Ideally maybe we would have a userspace program that is run to detect
    various disk parameters and ask the user / config file what sort of
    workloads we want to do, and spits out a recommended IO scheduler and
    /sys configuration to accompany it.

    That at least could be made quite sophisticated than a kernel solution,
    and could gather quite a lot of "static" disk properties.

    Of course there will be also some things that need to be done in
    kernel...


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