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

[Jens Axboe <jens axboe oracle com>]

On Fri, Oct 02 2009, Ingo Molnar wrote:
> 
> * Jens Axboe <jens axboe 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.

It's really not that simple, if we go and do easy latency bits, then
throughput drops 30% or more. You can't say it's black and white latency
vs throughput issue, that's just not how the real world works. The
server folks would be most unpleased.

-- 
Jens Axboe