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

[Ingo Molnar]

* Mike Galbraith <efault at gmx.de> wrote:

> On Fri, 2009-10-02 at 11:24 +0200, Ingo Molnar wrote:
> > * Jens Axboe <jens.axboe at oracle.com> wrote:
> > 
> > > It's not hard to make the latency good, the hard bit is making sure we 
> > > also perform well for all other scenarios.
> > 
> > Looking at the numbers from Mike:
> > 
> >  | dd competing against perf stat -- konsole -e exec timings, 5 back to 
> >  | back runs
> >  |                                                         Avg
> >  | before         9.15    14.51     9.39    15.06     9.90   11.6
> >  | after [+patch] 1.76     1.54     1.93     1.88     1.56    1.7
> > 
> > _PLEASE_ make read latencies this good - the numbers are _vastly_ 
> > better. We'll worry about the 'other' things _after_ we've reached good 
> > latencies.
> > 
> > I thought this principle was a well established basic rule of Linux 
> > IO scheduling. Why do we have to have a 'latency vs. bandwidth' 
> > discussion again and again? I thought latency won hands down.
> 
> Just a note: In the testing I've done so far, we're better off today 
> than ever, [...]

Definitely so, and a couple of months ago i've sung praises of that 
progress on the IO/fs latencies front:

   http://lkml.org/lkml/2009/4/9/461

... but we are greedy bastards and dont define excellence by how far 
down we have come from but by how high we can still climb ;-)

	Ingo