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



* Linus Torvalds <torvalds linux-foundation org> wrote:

> On Fri, 2 Oct 2009, Jens Axboe wrote:
> > 
> > It's really not that simple, if we go and do easy latency bits, then 
> > throughput drops 30% or more.
> 
> Well, if we're talking 500-950% improvement vs 30% deprovement, I 
> think it's pretty clear, though. Even the server people do care about 
> latencies.
> 
> Often they care quite a bit, in fact.

The other thing is that latency is basically a given property in any 
system - as an app writer you have to live with it, there's not much you 
can do to improve it.

Bandwidth on the other hand is a lot more engineerable, as it tends to 
be about batching things and you can batch in user-space too. Batching 
is often easier to do than getting good latencies.

Then there's also the fact that the range of apps that care about 
bandwidth is a lot smaller than the range of apps which care about 
latencies. The default should help more apps - i.e. latencies.

	Ingo


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