Re: [PATCH/RFC 0/4] cfq: implement merging and breaking up of cfq_queues

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

On Sat, Oct 24 2009, Corrado Zoccolo wrote:
> Hi Jeff,
> this series looks good.
> I like in particular the fact that you move seekiness detection in the cfqq.
> This can help with processes that issue sequential reads and seeky
> writes, or vice versa.
> Probably, also the think time could be made per-cfqq, so that the
> decision whether we should idle for a given cfqq is more precise.
> 
> On Fri, Oct 23, 2009 at 11:14 PM, Jeff Moyer <jmoyer@redhat.com> wrote:
> > Hi,
> >
> > This is a follow-up patch to the original close cooperator support for
> > CFQ.  The problem is that some programs (NFSd, dump(8), iscsi target
> > mode driver, qemu) interleave sequential I/Os between multiple threads
> > or processes.  The result is that there are large delays due to CFQ's
> > idling logic that leads to very low throughput.
> 
> You identified the problem in the idling logic, that reduces the
> throughput in this particular scenario, in which various threads or
> processes issue (in random order) the I/O requests with different I/O
> contexts on behalf of a single entity.
> In this case, any idling between those threads is detrimental.
> Ideally, such cases should be already spotted, since think time should
> be high for such processes, so I wonder if this indicates a problem in
> the current think time logic.

That isn't necessarily true, it may just as well be that there's very
little think time (don't see the connection here). A test case to
demonstrate this would be a number of processes/threads splitting a
sequential read of a file between them.

-- 
Jens Axboe