Re: [RFC PATCH 0/5] IO-less balance dirty pages

[Boaz Harrosh <bharrosh@panasas.com>]

On 02/10/2011 01:30 AM, Jan Kara wrote:
> On Sun 06-02-11 19:54:37, Boaz Harrosh wrote:
>> On 02/04/2011 03:38 AM, Jan Kara wrote:
>>> The basic idea (implemented in the third patch) is that processes throttled
>>> in balance_dirty_pages() wait for enough IO to complete. The waiting is
>>> implemented as follows: Whenever we decide to throttle a task in
>>> balance_dirty_pages(), task adds itself to a list of tasks that are throttled
>>> against that bdi and goes to sleep waiting to receive specified amount of page
>>> IO completions. Once in a while (currently HZ/10, in patch 5 the interval is
>>> autotuned based on observed IO rate), accumulated page IO completions are
>>> distributed equally among waiting tasks.
>>>
>>> This waiting scheme has been chosen so that waiting time in
>>> balance_dirty_pages() is proportional to
>>>   number_waited_pages * number_of_waiters.
>>> In particular it does not depend on the total number of pages being waited for,
>>> thus providing possibly a fairer results.
>>>
>>> I gave the patches some basic testing (multiple parallel dd's to a single
>>> drive) and they seem to work OK. The dd's get equal share of the disk
>>> throughput (about 10.5 MB/s, which is nice result given the disk can do
>>> about 87 MB/s when writing single-threaded), and dirty limit does not get
>>> exceeded. Of course much more testing needs to be done but I hope it's fine
>>> for the first posting :).
>>
>> So what is the disposition of Wu's patches in light of these ones?
>> * Do they replace Wu's, or Wu's just get rebased ontop of these at a
>>   later stage?
>   They are meant as a replacement.
> 
>> * Did you find any hard problems with Wu's patches that delay them for
>>   a long time?
>   Wu himself wrote that the current patchset probably won't fly because it
> fluctuates too much. So he decided to try to rewrite patches from per-bdi
> limits to global limits when he has time...
> 
>> * Some of the complicated stuff in Wu's patches are the statistics and
>>   rate control mechanics. Are these the troubled area? Because some of
>>   these are actually some things that I'm interested in, and that appeal
>>   to me the most.
>   Basically yes, this logic seems to be the problematic one.
> 
> 								Honza

Thanks dear Jan for you reply. I would love to talk about all this
in LSF, and other writeback issues. Keep us posted with results of
of your investigations.

Cheers
Boaz

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