[Devel] Re: [patch 0/4] [RFC] Another proportional weight IO controller

[Fabio Checconi]

> From: Aaron Carroll <aaronc at gelato.unsw.edu.au>
> Date: Wed, Nov 19, 2008 12:52:42PM +1100
>
> Fabio Checconi wrote:
> >   - To detect hw tagging in BFQ we consider a sample valid iff the
> >     number of requests that the scheduler could have dispatched (given
> >     by cfqd->rb_queued + cfqd->rq_in_driver, i.e., the ones still into
> >     the scheduler plus the ones into the driver) is higher than the
> >     CFQ_HW_QUEUE_MIN threshold.  This obviously caused no problems
> >     during testing, but the way CFQ uses now seems a little bit
> >     strange.
> 
> BFQ's tag detection logic is broken in the same way that CFQ's used to
> be.  Explanation is in this patch:
> 

If you look at bfq_update_hw_tag(), the logic introduced by the patch
you mention is still there; BFQ starts with ->hw_tag = 1, and updates it
every 32 valid samples.  What changed WRT your patch, apart from the
number of samples, is that the condition for a sample to be valid is:

  bfqd->rq_in_driver + bfqd->queued >= 5

while in your patch it is:

  cfqd->rq_queued > 5 || cfqd->rq_in_driver > 5

We preferred the first one because that sum better reflects the number
of requests that could have been dispatched, and I don't think that this
is wrong.

There is a problem, but it's not within the tag detection logic itself.
>From some quick experiments, what happens is that when a process starts,
CFQ considers it seeky (*), BFQ doesn't.  As a side effect BFQ does not
always dispatch enough requests to correctly detect tagging.

At the first seek you cannot tell if the process is going to bee seeky
or not, and we have chosen to consider it sequential because it improved
fairness in some sequential workloads (the CIC_SEEKY heuristic is used
also to determine the idle_window length in [bc]fq_arm_slice_timer()).

Anyway, we're dealing with heuristics, and they tend to favor some
workload over other ones.  If recovering this thoughput loss is more
important than a transient unfairness due to short idling windows assigned
to sequential processes when they start, I've no problems in switching
the CIC_SEEKY logic to consider a process seeky when it starts.

Thank you for testing and for pointing out this issue, we missed it
in our testing.


(*) to be correct, the initial classification depends on the position
    of the first accessed sector.
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