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SubjectRe: IO scheduler based IO controller V10

* Jens Axboe <jens.axboe@oracle.com> wrote:

> 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.

Good, so we are in violent agreement :-)

Ingo


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