On Fri, Oct 02 2009, Ingo Molnar wrote: > > * Jens Axboe <jens.axboe@xxxxxxxxxx> 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 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
