* Jens Axboe <jens.axboe@xxxxxxxxxx> wrote: > 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. Good, so we are in violent agreement :-) Ingo -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
