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    SubjectRe: IO scheduler based IO controller V10
    On Fri, Oct 02 2009, Ingo Molnar wrote:
    >
    > * Jens Axboe <jens.axboe@oracle.com> wrote:
    >
    > > It's not _that_ easy, it depends a lot on the access patterns. A good
    > > example of that is actually the idling that we already do. Say you
    > > have two applications, each starting up. If you start them both at the
    > > same time and just care for the dumb low latency, then you'll do one
    > > IO from each of them in turn. Latency will be good, but throughput
    > > will be aweful. And this means that in 20s they are both started,
    > > while with the slice idling and priority disk access that CFQ does,
    > > you'd hopefully have both up and running in 2s.
    > >
    > > So latency is good, definitely, but sometimes you have to worry about
    > > the bigger picture too. Latency is more than single IOs, it's often
    > > for complete operation which may involve lots of IOs. Single IO
    > > latency is a benchmark thing, it's not a real life issue. And that's
    > > where it becomes complex and not so black and white. Mike's test is a
    > > really good example of that.
    >
    > To the extent of you arguing that Mike's test is artificial (i'm not
    > sure you are arguing that) - Mike certainly did not do an artificial
    > test - he tested 'konsole' cache-cold startup latency, such as:

    [snip]

    I was saying the exact opposite, that Mike's test is a good example of a
    valid test. It's not measuring single IO latencies, it's doing a
    sequence of valid events and looking at the latency for those. It's
    benchmarking the bigger picture, not a microbenchmark.

    --
    Jens Axboe



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