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

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 21 Apr 2009 22:49:06 +0200
Andrea Righi <righi.andrea@gmail.com> wrote:
> yep! right. Anyway, it's not completely wrong to account dirty pages in
> this way. The dirty pages actually belong to cgroup A and providing per
> cgroup upper limits of dirty pages could help to equally distribute
> dirty pages, that are hard/slow to reclaim, among cgroups.
> 
> But this is definitely another problem.
> 
Hmm, my motivation for dirty accounting in memcg is for supporting dirty_ratio
to do smooth page reclaiming and to kick background-write-out.


> And it doesn't help for the problem described by Ted, expecially for the
> IO controller. The only way I see to correctly handle that case is to
> limit the rate of dirty pages per cgroup, accounting the dirty activity
> to the cgroup that firstly touched the page (and not the owner as
> intended by the memory controller).
> 
Owner of the page should know dirty ratio, too.

> And this should be probably strictly connected to the IO controller. If
> we throttle or delay the dispatching/submission of some IO requests
> without throttling the dirty pages rate a cgroup could completely waste
> its own available memory with dirty (hard and slow to reclaim) pages.
> 
> That is in part the approach I used in io-throttle v12, adding a hook in
> balance_dirty_pages_ratelimited_nr() to throttle the current task when
> cgroup's IO limit are exceeded. Argh!
> 
> So, another proposal could be to re-add in io-throttle v14 the old hook
> also in balance_dirty_pages_ratelimited_nr().
> 
> In this way io-throttle would:
> 
> - use page_cgroup infrastructure and page_cgroup->flags to encode the
>   cgroup id that firstly dirtied a generic page
> - account and opportunely throttle sync and writeback IO requests in
>   submit_bio()
> - at the same time throttle the tasks in
>   balance_dirty_pages_ratelimited_nr() if the cgroup they belong has
>   exhausted the IO BW (or quota, share, etc. in case of proportional BW
>   limit)
> 

IMHO, io-controller should just work as I/O subsystem as bdi. Now, per-bdi dirty_ratio
is suppoted and it seems to work well.  

Can't we write a function like  bdi_writeout_fraction() ?;
It will be a simple choice.

Thanks,
-Kame