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

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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.

-- 
Jens Axboe

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Thread

[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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