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

[Fabio Checconi]

> From: Jens Axboe <jens.axboe at oracle.com>
> Date: Tue, Nov 18, 2008 08:12:08PM +0100
>
> On Tue, Nov 18 2008, Fabio Checconi wrote:
> > BFQ started from CFQ, extending it in the way you correctly describe,
> > so it is indeed very similar.  There are also some minor changes to
> > locking, cic handling, hw_tag detection and to the CIC_SEEKY heuristic.
> > 
...
> My preferred approach here would be, in order or TODO:
> 
> - Create and test the smallish patches for seekiness, hw_tag checking,
>   and so on for CFQ.
> - Create and test a WF2Q+ service dispatching patch for CFQ.
> 
> and if there are leftovers after that, we could even conditionally
> enable some of those if appropriate. I think the WF2Q+ is quite cool and
> could be easily usable as the default, so it's definitely a viable
> alternative.
> 
> My main goal here is basically avoiding addition of Yet Another IO
> scheduler, especially one that is so closely tied to CFQ already.
> 
> I'll start things off by splitting cfq into a few files similar to what
> bfq has done, as I think it makes a lot of sense. Fabio, if you could
> create patches for the small behavioural changes you made, we can
> discuss and hopefully merge those next.
> 

Ok, I can do that, I need just a little bit of time to organize
the work.

About these small (some of them are really small) changes, a mixed list
of things that they will touch and/or things that I'd like to have clear
before starting to write the patches (maybe we can start another thread
for them):

  - In cfq_exit_single_io_context() and in changed_ioprio(), cic->key
    is dereferenced without holding any lock.  As I reported in [1]
    this seems to be a problem when an exit() races with a cfq_exit_queue()
    and in a few other cases.  In BFQ we used a somehow involved
    mechanism to avoid that, abusing rcu (of course we'll have to wait
    the patch to talk about it :) ), but given my lack of understanding
    of some parts of the block layer, I'd be interested in knowing if
    the race is possible and/or if there is something more involved
    going on that can cause the same effects.

  - set_task_ioprio() in fs/ioprio.c doesn't seem to have a write
    memory barrier to pair with the dependent read one in
    cfq_get_io_context().

  - CFQ_MIN_TT is 2ms, this can result, depending on the value of
    HZ in timeouts of one jiffy, that may expire too early, so we are
    just wasting time and do not actually wait for the task to present
    its new request.  Dealing with seeky traffic we've seen a lot of
    early timeouts due to one jiffy timers expiring too early, is
    it worth fixing or can we live with that?

  - 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.

  - Initially, cic->last_request_pos is zero, so the sdist charged
    to a task for its first seek depends on the position on the disk
    that is accessed first, independently from its seekiness.  Even
    if there is a cap on that value, we choose to not charge the first
    seek to processes; that resulted in less wrong predictions for
    purely sequential loads.

  - From my understanding, with shared I/O contexts, two different
    tasks may concurrently lookup for a cfqd into the same ioc.
    This may result in cfq_drop_dead_cic() being called two times
    for the same cic.  Am I missing something that prevents that from
    happening?

Regarding the code splitup, do you think you'll go for the CFS(BFQ) way,
using a single compilation unit and including the .c files, or a layout
with different compilation units (like the ll_rw_blk.c splitup)?


[1]: http://lkml.org/lkml/2008/8/18/119
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