Re: [Lsf] IO less throttling and cgroup aware writeback (Was: Re: Preliminary Agenda and Activities for LSF)

Jan Kara
2011-04-06T17:10:22Z

On Wed 06-04-11 12:08:05, Vivek Goyal wrote:
> On Wed, Apr 06, 2011 at 11:37:15AM -0400, Vivek Goyal wrote:
>
> [..]
> > > what kswapd is going to do writeback when the pages
> > > it's trying to writeback during a critical low memory event belong
> > > to a cgroup that is throttled at the IO level, etc.
> >
> > Throttling will move up so kswapd will not be throttled. Even today,
> > kswapd is part of root group and we do not suggest throttling root group.
> >
> > For the case of proportional disk sharing, we will probably account
> > IO to respective cgroups (pages submitted by kswapd) and that should
> > not flush to disk fairly fast and should not block for long time as it is
> > work consering mechanism.
> >
> > Do you see an issue with kswapd IO being accounted to respective cgroups
> > for proportional IO. For throttling case, all IO would go to root group
> > which is unthrottled and real issue of dirtying too many pages by
> > processes will be handled by throttling processes when they are dirtying
> > page cache.
>
> Or may be it is not a good idea to try to account pages to associated
> cgroups when memory is low and kswapd is doing IO. We can probably mark
> kswapd with some flag and account all IO to root group even for
> proportional weight mechanism. In this case isolation will be broken but
> I guess one can not do much. To avoid this situation, one should not
> have allowed too many writes and I think that's where low dirty ratio
> can come into the picture.
Well, I wouldn't bother too much with kswapd handling. MM people plan to
get rid of writeback from direct reclaim and just remove the dirty page
from LRU and recycle it once flusher thread writes it...

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR