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[dm-devel] Re: 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:


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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