[dm-devel] Re: IO scheduler based IO controller V10
Jens Axboe
jens.axboe at oracle.com
Fri Oct 2 09:28:39 UTC 2009
On Fri, Oct 02 2009, 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.
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
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