[dm-devel] Re: IO scheduler based IO controller V10

[Ingo Molnar]

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

> On Fri, Oct 02 2009, Ingo Molnar wrote:
> > 
> > * Jens Axboe <jens.axboe at oracle.com> wrote:
> > 
> > > On Fri, Oct 02 2009, Ingo Molnar wrote:
> > > > 
> > > > * Jens Axboe <jens.axboe at 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 :-)
> 
> Yes, perhaps that last sentence didn't provide enough evidence of 
> which category I put Mike's test into :-)
> 
> So to kick things off, I added an 'interactive' knob to CFQ and 
> defaulted it to on, along with re-enabling slice idling for hardware 
> that does tagged command queuing. This is almost completely identical 
> to what Vivek Goyal originally posted, it's just combined into one and 
> uses the term 'interactive' instead of 'fairness'. I think the former 
> is a better umbrella under which to add further tweaks that may 
> sacrifice throughput slightly, in the quest for better latency.
> 
> It's queued up in the for-linus branch.

i'd say 'latency' describes it even better. 'interactivity' as a term is 
a bit overladen.

	Ingo