Re: [RFC][PATCH -mm 0/5] cgroup: block device i/o controller (v9)

[Andrea Righi]

Vivek Goyal wrote:
> On Wed, Sep 17, 2008 at 10:47:54AM +0200, Andrea Righi wrote:
>> Hirokazu Takahashi wrote:
>>> Hi,
>>>
>>>> TODO:
>>>>
>>>> * Try to push down the throttling and implement it directly in the I/O
>>>>   schedulers, using bio-cgroup (http://people.valinux.co.jp/~ryov/bio-cgroup/)
>>>>   to keep track of the right cgroup context. This approach could lead to more
>>>>   memory consumption and increases the number of dirty pages (hard/slow to
>>>>   reclaim pages) in the system, since dirty-page ratio in memory is not
>>>>   limited. This could even lead to potential OOM conditions, but these problems
>>>>   can be resolved directly into the memory cgroup subsystem
>>>>
>>>> * Handle I/O generated by kswapd: at the moment there's no control on the I/O
>>>>   generated by kswapd; try to use the page_cgroup functionality of the memory
>>>>   cgroup controller to track this kind of I/O and charge the right cgroup when
>>>>   pages are swapped in/out
>>> FYI, this also can be done with bio-cgroup, which determine the owner cgroup
>>> of a given anonymous page.
>>>
>>> Thanks,
>>> Hirokazu Takahashi
>> That would be great! FYI here is how I would like to proceed:
>>
>> - today I'll post a new version of my cgroup-io-throttle patch rebased
>>   to 2.6.27-rc5-mm1 (it's well tested and seems to be stable enough).
>>   To keep the things light and simpler I've implemented custom
>>   get_cgroup_from_page() / put_cgroup_from_page() in the memory
>>   controller to retrieve the owner of a page, holding a reference to the
>>   corresponding memcg, during async writes in submit_bio(); this is not
>>   probably the best way to proceed, and a more generic framework like
>>   bio-cgroup sounds better, but it seems to work quite well. The only
>>   problem I've found is that during swap_writepage() the page is not
>>   assigned to any page_cgroup (page_get_page_cgroup() returns NULL), and
>>   so I'm not able to charge the cost of this I/O operation to the right
>>   cgroup. Does bio-cgroup address or even resolve this issue?
>> - begin to implement a new branch of cgroup-io-throttle on top of
>>   bio-cgroup
>> - also start to implement an additional request queue to provide first a
>>   control at the cgroup level and a dispatcher to pass the request to
>>   the elevator (as suggested by Vivek)
>>
> 
> Hi Andrea,
> 
> So if we maintain and rb-tree per request queue and implement the cgroup
> rules there, then that will take care of io-throttling also. (One can
> control the release of bio/requests to elevator based on any kind of
> rules. proportional weight/max-bandwidth).
> 
> If that's the case, I was wondering what do you mean by "begin to
> implement new branch of cgroup-io-throttle" on top of bio-cgroup".

Correct, with the rb-tree per request queue solution there's no need to
keep track of the context in the struct bio, since the i/o control
based on per cgroup rules has been already performed by the first i/o
dispatcher. And I would really like to dedicate all my efforts to move
in this direction, but it would be interesting as well to test the
bio-cgroup functionality since it's working from now, it's a generic
framework and used by another project (dm-ioband). This is the reason
because I put it there, specifying to open a new branch, because it
would be an alternative solution to the following point.

-Andrea
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