Re: [PATCH v6 8/9] memcg: check memcg dirty limits in page writeback

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Mar 14, 2011 at 10:10:03PM +0100, Jan Kara wrote:
> On Mon 14-03-11 13:54:08, Vivek Goyal wrote:
> > On Fri, Mar 11, 2011 at 10:43:30AM -0800, Greg Thelen wrote:
> > > If the current process is in a non-root memcg, then
> > > balance_dirty_pages() will consider the memcg dirty limits as well as
> > > the system-wide limits.  This allows different cgroups to have distinct
> > > dirty limits which trigger direct and background writeback at different
> > > levels.
> > > 
> > > If called with a mem_cgroup, then throttle_vm_writeout() queries the
> > > given cgroup for its dirty memory usage limits.
> > > 
> > > Signed-off-by: Andrea Righi <arighi@develer.com>
> > > Signed-off-by: Greg Thelen <gthelen@google.com>
> > > Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > > Acked-by: Wu Fengguang <fengguang.wu@intel.com>
> > > ---
> > > Changelog since v5:
> > > - Simplified this change by using mem_cgroup_balance_dirty_pages() rather than
> > >   cramming the somewhat different logic into balance_dirty_pages().  This means
> > >   the global (non-memcg) dirty limits are not passed around in the
> > >   struct dirty_info, so there's less change to existing code.
> > 
> > Yes there is less change to existing code but now we also have a separate
> > throttlig logic for cgroups. 
> > 
> > I thought that we are moving in the direction of IO less throttling
> > where bdi threads always do the IO and Jan Kara also implemented the
> > logic to distribute the finished IO pages uniformly across the waiting
> > threads.
>   Yes, we'd like to avoid doing IO from balance_dirty_pages(). But if the
> logic in cgroups specific part won't get too fancy (which it doesn't seem
> to be the case currently), it shouldn't be too hard to convert it to the new
> approach.
> 
> We can talk about it at LSF but at least with my approach to IO-less
> balance_dirty_pages() it would be easy to convert cgroups throttling to
> the new way. With Fengguang's approach it might be a bit harder since he
> computes a throughput and from that necessary delay for a throttled task
> but with cgroups that is impossible to compute so he'd have to add some
> looping if we didn't write enough pages from the cgroup yet. But still it
> would be reasonable doable AFAICT.
> 
> > Keeping it separate for cgroups, reduces the complexity but also forks
> > off the balancing logic for root and other cgroups. So if Jan Kara's
> > changes go in, it automatically does not get used for memory cgroups.
> > 
> > Not sure how good a idea it is to use a separate throttling logic for
> > for non-root cgroups. 
>   Yeah, it looks a bit odd. I'd think that we could just cap
> task_dirty_limit() by a value computed from a cgroup limit and be done
> with that but I probably miss something...

I think previous implementation did something similar. Currently dirty
limit is per_bdi/per_task. They made it per_cgroup/per_bdi/per_task. This
new version tries to simplify the things by keeping mem cgroup throttling
logic separate.

> Sure there is also a different
> background limit but that's broken anyway because a flusher thread will
> quickly stop doing writeback if global background limit is not exceeded.
> But that's a separate topic so I'll reply with this to a more appropriate
> email ;)

I think last patch in the series (patch 9) takes care of that. In case of
mem_cgroup writeback, it forces flusher thread to write till we are
below the background ratio of cgroup (and not global background ratio).

Thanks
Vivek

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