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



* 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:

    sh -c "perf stat -- konsole -e exit" 2>&1|tee -a $LOGFILE

against a streaming dd.

That is a _very_ relevant benchmark IMHO and konsole's cache footprint 
is far from trivial. (In fact i'd argue it's one of the most important 
IO benchmarks on a desktop system - how does your desktop hold up to 
something doing streaming IO.)

	Ingo


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