[Devel] Re: [PATCH 9/9] ext3: do not throttle metadata and journal IO

[KAMEZAWA Hiroyuki]

On Wed, 22 Apr 2009 12:22:41 +0200
Andrea Righi <righi.andrea at gmail.com> wrote:
 
> Actually I was proposing something quite similar, if I've understood
> well. Just add a hook in balance_dirty_pages() to throttle tasks in
> cgroups that exhausted their IO BW.
> 
> The way to do so will be similar to the per-bdi write throttling, taking
> in account the IO requests previously submitted per cgroup, the pages
> dirtied per cgroup (considering that are not necessarily dirtied by the
> owner of the page) and apply something like congestion_wait() to
> throttle the tasks in the cgroups that exceeded the BW limit.
> 
> Maybe we can just introduce cgroup_dirty_limit() simply replicating what
> we're doing for task_dirty_limit(), but using per cgroup statistics of
> course.
> 
> I can change the io-throttle controller to do so. This feature should be
> valid also for the proportional BW approach.
> 
> BTW Vivek's proposal to also dispatch IO requests according to cgroup
> proportional BW limits can be still valid and it is worth to be tested
> IMHO. But we must also find a way to say to the right cgroup: hey! stop
> to waste the memory with dirty pages, because you've directly or
> indirectly generated too much IO in the system and I'm throttling and/or
> not scheduling your IO requests.
> 
> Objections?
> 
No objections. plz let me know my following understanding is right.

  1. dirty_ratio should be supported per cgroup.
     - Memory cgroup should support dirty_ratio or dirty_ratio cgroup should be implemented.
       For doing this, we can make use of page_cgroup.

       One good point of dirty-ratio cgroup is that dirty-ratio accounting is done
       against a cgroup which made pages dirty not against a owner of the page. But
       if dirty_ratio cgroup is completely independent from mem_cgroup, it cannot
       be a help for memory reclaiming.
       Then,
         - memcg itself should have dirty_ratio check.
         - like bdi/task_dirty_limit(), a cgroup (which is not memcg) can be used
           another filter for dirty_ratio.

  2. dirty_ratio is not I/O BW control.

  3. I/O BW(limit) control cgroup should be implemented and it should be exsiting
     in I/O scheduling layer or somewhere around. But it's not easy.

  4. To track bufferred I/O, we have to add "tag" to pages which tell us who
     generated the I/O. Now it's called blockio-cgroup and implementation details
     are still under discussion.

So, current status is.

  A. memcg should support dirty_ratio for its own memory reclaim.
     in plan.

  B. another cgroup can be implemnted to support cgroup_dirty_limit().
     But relationship with "A" should be discussed.
     no plan yet.

  C. I/O cgroup and bufferred I/O tracking system.
     Now under patch review.

And this I/O throttle is mainly for "C" discussion. 

Right ?

-Kame



Regards,
-Kame




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