Re: [PATCH v8 11/12] writeback: make background writeback cgroup aware

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jun 08, 2011 at 01:03:15PM +0900, KAMEZAWA Hiroyuki wrote:
> On Tue, 7 Jun 2011 21:02:21 -0700
> Greg Thelen <gthelen@google.com> wrote:
> 
> > On Tue, Jun 7, 2011 at 5:18 PM, KAMEZAWA Hiroyuki
> > <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > > On Tue, 7 Jun 2011 17:05:40 -0400
> > > Vivek Goyal <vgoyal@redhat.com> wrote:
> > >
> > >> On Tue, Jun 07, 2011 at 01:43:08PM -0700, Greg Thelen wrote:
> > >> > Vivek Goyal <vgoyal@redhat.com> writes:
> > >> >
> > >> > > On Fri, Jun 03, 2011 at 09:12:17AM -0700, Greg Thelen wrote:
> > >> > >> When the system is under background dirty memory threshold but a cgroup
> > >> > >> is over its background dirty memory threshold, then only writeback
> > >> > >> inodes associated with the over-limit cgroup(s).
> > >> > >>
> > >> > >
> > >> > > [..]
> > >> > >> -static inline bool over_bground_thresh(void)
> > >> > >> +static inline bool over_bground_thresh(struct bdi_writeback *wb,
> > >> > >> +                                       struct writeback_control *wbc)
> > >> > >>  {
> > >> > >>          unsigned long background_thresh, dirty_thresh;
> > >> > >>
> > >> > >>          global_dirty_limits(&background_thresh, &dirty_thresh);
> > >> > >>
> > >> > >> -        return (global_page_state(NR_FILE_DIRTY) +
> > >> > >> -                global_page_state(NR_UNSTABLE_NFS) > background_thresh);
> > >> > >> +        if (global_page_state(NR_FILE_DIRTY) +
> > >> > >> +            global_page_state(NR_UNSTABLE_NFS) > background_thresh) {
> > >> > >> +                wbc->for_cgroup = 0;
> > >> > >> +                return true;
> > >> > >> +        }
> > >> > >> +
> > >> > >> +        wbc->for_cgroup = 1;
> > >> > >> +        wbc->shared_inodes = 1;
> > >> > >> +        return mem_cgroups_over_bground_dirty_thresh();
> > >> > >>  }
> > >> > >
> > >> > > Hi Greg,
> > >> > >
> > >> > > So all the logic of writeout from mem cgroup works only if system is
> > >> > > below background limit. The moment we cross background limit, looks
> > >> > > like we will fall back to existing way of writting inodes?
> > >> >
> > >> > Correct.  If the system is over its background limit then the previous
> > >> > cgroup-unaware background writeback occurs.  I think of the system
> > >> > limits as those of the root cgroup.  If the system is over the global
> > >> > limit than all cgroups are eligible for writeback.  In this situation
> > >> > the current code does not distinguish between cgroups over or under
> > >> > their dirty background limit.
> > >> >
> > >> > Vivek Goyal <vgoyal@redhat.com> writes:
> > >> > > If yes, then from design point of view it is little odd that as long
> > >> > > as we are below background limit, we share the bdi between different
> > >> > > cgroups. The moment we are above background limit, we fall back to
> > >> > > algorithm of sharing the disk among individual inodes and forget
> > >> > > about memory cgroups. Kind of awkward.
> > >> > >
> > >> > > This kind of cgroup writeback I think will atleast not solve the problem
> > >> > > for CFQ IO controller, as we fall back to old ways of writting back inodes
> > >> > > the moment we cross dirty ratio.
> > >> >
> > >> > It might make more sense to reverse the order of the checks in the
> > >> > proposed over_bground_thresh(): the new version would first check if any
> > >> > memcg are over limit; assuming none are over limit, then check global
> > >> > limits.  Assuming that the system is over its background limit and some
> > >> > cgroups are also over their limits, then the over limit cgroups would
> > >> > first be written possibly getting the system below its limit.  Does this
> > >> > address your concern?
> > >>
> > >> Do you treat root group also as any other cgroup? If no, then above logic
> > >> can lead to issue of starvation of root group inode. Or unfair writeback.
> > >> So I guess it will be important to treat root group same as other groups.
> > >>
> > >
> > > As far as I can say, you should not place programs onto ROOT cgroups if you need
> > > performance isolation.
> > 
> > Agreed.
> > 
> > > From the code, I think if the system hits dirty_ratio, "1" bit of bitmap should be
> > > set and background writeback can work for ROOT cgroup seamlessly.
> > >
> > > Thanks,
> > > -Kame
> > 
> > Not quite.  The proposed patches do not set the "1" bit (css_id of
> > root is 1).  mem_cgroup_balance_dirty_pages() (from patch 10/12)
> > introduces the following balancing loop:
> > +       /* balance entire ancestry of current's mem. */
> > +       for (; mem_cgroup_has_dirty_limit(mem); mem =
> > parent_mem_cgroup(mem)) {
> > 
> > The loop terminates when mem_cgroup_has_dirty_limit() is called for
> > the root cgroup.  The bitmap is set in the body of the loop.  So the
> > root cgroup's bit (bit 1) will never be set in the bitmap.  However, I
> > think the effect is the same.  The proposed changes in this patch
> > (11/12) have background writeback first checking if the system is over
> > limit and if yes, then b_dirty inodes from any cgroup written.  This
> > means that a small system background limit with an over-{fg or
> > bg}-limit cgroup could cause other cgroups that are not over their
> > limit to have their inodes written back.  In an system-over-limit
> > situation normal system-wide bdi writeback is used (writing inodes in
> > b_dirty order).  For those who want isolation, a simple rule to avoid
> > this is to ensure that that sum of all cgroup background_limits is
> > less than the system background limit.
> > 
> 
> Hmm, should we add the rule ? 
> How about disallowing to set dirty_ratio bigger than system's one ?

I guess in common case people will use a common dirty ratio for all cgroups
(same as system dirty ratio). So it might not be of much value.

Thanks
Vivek

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