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Re: [PATCH 7/8] blk-wbt: add general throttling mechanism

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On Wed 09-11-16 12:52:59, Jens Axboe wrote:
> On 11/09/2016 09:07 AM, Jens Axboe wrote:
> >On 11/09/2016 01:40 AM, Jan Kara wrote:
> >>>>So for devices with write cache, you will completely drain the device
> >>>>before waking anybody waiting to issue new requests. Isn't it too
> >>>>strict?
> >>>>In particular may_queue() will allow new writers to issue new writes
> >>>>once
> >>>>we drop below the limit so it can happen that some processes will be
> >>>>effectively starved waiting in may_queue?
> >>>
> >>>It is strict, and perhaps too strict. In testing, it's the only method
> >>>that's proven to keep the writeback caching devices in check. It will
> >>>round robin the writers, if we have more, which isn't necessarily a bad
> >>>thing. Each will get to do a burst of depth writes, then wait for a new
> >>>one.
> >>
> >>Well, I'm more concerned about a situation where one writer does a
> >>bursty write and blocks sleeping in may_queue(). Another writer
> >>produces a steady flow of write requests so that never causes the
> >>write queue to completely drain but that writer also never blocks in
> >>may_queue() when it starts queueing after write queue has somewhat
> >>drained because it never submits many requests in parallel. In such
> >>case the first writer would get starved AFAIU.
> >
> >I see what you are saying. I can modify the logic to ensure that if we
> >do have a waiter, we queue up others behind it. That should get rid of
> >that concern.
> 
> I added that - if we currently have a waiter, we'll add ourselves to the
> back of the waitqueue and wait.

OK, sounds good to me. If the write queue draining will show to be an
issue, it will be at least clearly visible with this logic.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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[PATCHSET] Throttled buffered writeback Jens Axboe <axboe@fb.com> - 2016-11-01 22:20 +0100
  [PATCH 3/8] writeback: mark background writeback as such Jens Axboe <axboe@fb.com> - 2016-11-01 22:20 +0100
    Re: [PATCH 3/8] writeback: mark background writeback as such Christoph Hellwig <hch@lst.de> - 2016-11-02 16:00 +0100
    Re: [PATCH 3/8] writeback: mark background writeback as such Jan Kara <jack@suse.cz> - 2016-11-05 23:30 +0100
  [PATCH 4/8] writeback: track if we're sleeping on progress in balance_dirty_pages() Jens Axboe <axboe@fb.com> - 2016-11-01 22:20 +0100
    Re: [PATCH 4/8] writeback: track if we're sleeping on progress in         balance_dirty_pages() Christoph Hellwig <hch@lst.de> - 2016-11-02 16:00 +0100
    Re: [PATCH 4/8] writeback: track if we're sleeping on progress in  balance_dirty_pages() Jan Kara <jack@suse.cz> - 2016-11-08 14:10 +0100
  [PATCH 6/8] block: add scalable completion tracking of requests Jens Axboe <axboe@fb.com> - 2016-11-01 22:20 +0100
    Re: [PATCH 6/8] block: add scalable completion tracking of requests Jan Kara <jack@suse.cz> - 2016-11-08 14:40 +0100
      Re: [PATCH 6/8] block: add scalable completion tracking of requests Jan Kara <jack@suse.cz> - 2016-11-09 10:10 +0100
        Re: [PATCH 6/8] block: add scalable completion tracking of requests Jens Axboe <axboe@kernel.dk> - 2016-11-09 21:00 +0100
          Re: [PATCH 6/8] block: add scalable completion tracking of requests Jan Kara <jack@suse.cz> - 2016-11-10 20:40 +0100
  [PATCH 2/8] writeback: add wbc_to_write_flags() Jens Axboe <axboe@fb.com> - 2016-11-01 22:20 +0100
    Re: [PATCH 2/8] writeback: add wbc_to_write_flags() Christoph Hellwig <hch@lst.de> - 2016-11-02 16:00 +0100
  [PATCH 7/8] blk-wbt: add general throttling mechanism Jens Axboe <axboe@fb.com> - 2016-11-01 22:20 +0100
    Re: [PATCH 7/8] blk-wbt: add general throttling mechanism Jan Kara <jack@suse.cz> - 2016-11-08 14:40 +0100
      Re: [PATCH 7/8] blk-wbt: add general throttling mechanism Jens Axboe <axboe@kernel.dk> - 2016-11-08 16:50 +0100
        Re: [PATCH 7/8] blk-wbt: add general throttling mechanism Jan Kara <jack@suse.cz> - 2016-11-09 09:50 +0100
          Re: [PATCH 7/8] blk-wbt: add general throttling mechanism Jens Axboe <axboe@kernel.dk> - 2016-11-09 17:10 +0100
            Re: [PATCH 7/8] blk-wbt: add general throttling mechanism Jens Axboe <axboe@kernel.dk> - 2016-11-09 21:00 +0100
              Re: [PATCH 7/8] blk-wbt: add general throttling mechanism Jan Kara <jack@suse.cz> - 2016-11-10 20:40 +0100
            Re: [PATCH 7/8] blk-wbt: add general throttling mechanism Dave Chinner <david@fromorbit.com> - 2016-11-10 01:10 +0100
  [PATCH 8/8] block: hook up writeback throttling Jens Axboe <axboe@fb.com> - 2016-11-01 22:20 +0100
    Re: [PATCH 8/8] block: hook up writeback throttling Jan Kara <jack@suse.cz> - 2016-11-08 14:50 +0100

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