Re: [RFC] cfq: adapt slice to number of processes doing I/O

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Sep 03 2009, Jeff Moyer wrote:
> Corrado Zoccolo <czoccolo@gmail.com> writes:
> 
> > When the number of processes performing I/O concurrently increases,  a
> > fixed time slice per process will cause large latencies.
> > In the patch, if there are more than 3 processes performing concurrent
> > I/O, we scale the time slice down proportionally.
> > To safeguard sequential bandwidth, we impose a minimum time slice,
> > computed from cfq_slice_idle (the idea is that cfq_slice_idle
> > approximates the cost for a seek).
> >
> > I performed two tests, on a rotational disk:
> > * 32 concurrent processes performing random reads
> > ** the bandwidth is improved from 466KB/s to 477KB/s
> > ** the maximum latency is reduced from 7.667s to 1.728
> > * 32 concurrent processes performing sequential reads
> > ** the bandwidth is reduced from 28093KB/s to 24393KB/s
> > ** the maximum latency is reduced from 3.781s to 1.115s
> >
> > I expect numbers to be even better on SSDs, where the penalty to
> > disrupt sequential read is much less.
> 
> Interesting approach.  I'm not sure what the benefits will be on SSDs,
> as the idling logic is disabled for them (when nonrot is set and they
> support ncq).  See cfq_arm_slice_timer.

Also, the problem with scaling the slice a lot is that throughput has a
tendency to fall off a cliff at some point. Have you tried benchmarking
buffered writes with reads?

-- 
Jens Axboe