Re: [RFC] writeback and cgroup

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Apr 11, 2012 at 07:05:42PM +0200, Jan Kara wrote:
> On Wed 11-04-12 11:45:31, Vivek Goyal wrote:
> > On Wed, Apr 11, 2012 at 11:40:05AM -0400, Vivek Goyal wrote:
> > > On Wed, Apr 11, 2012 at 12:24:25AM +0200, Jan Kara wrote:
> > > 
> > > [..]
> > > > > I have implemented and posted patches for per bdi per cgroup congestion
> > > > > flag. The only problem I see with that is that a group might be congested
> > > > > for a long time because of lots of other IO happening (say direct IO) and
> > > > > if you keep on backing off and never submit the metadata IO (transaction),
> > > > > you get starved. And if you go ahead and submit IO in a congested group,
> > > > > we are back to serialization issue.
> > > >   Clearly, we mustn't throttle metadata IO once it gets to the block layer.
> > > > That's why we discuss throttling of processes at transaction start after
> > > > all. But I agree starvation is an issue - I originally thought blk-throttle
> > > > throttles synchronously which wouldn't have starvation issues.
> > 
> > Current bio throttling is asynchrounous. Process can submit the bio
> > and go back and wait for bio to finish. That bio will be queued at device
> > queue in a per cgroup queue and will be dispatched to device according
> > to configured IO rate for cgroup.
> > 
> > The additional feature for buffered throttle (which never went upstream),
> > was synchronous in nature. That is we were actively putting writer to
> > sleep on a per cgroup wait queue in the request queue and wake it up when
> > it can do further IO based on cgroup limits.
>   Hmm, but then there would be similar starvation issues as with my simple
> scheme because async IO could always use the whole available bandwidth.

It depends on how the throttling logic decides to divide bandwidth between
sync and async. I had chosen a round robin policy of dispatching some
bios and then allowing some async IO etc. So async IO was not consuming
the whole available bandwidth. We could easibly tilt it in favor of sync IO
with a tunable knob.

> Mixing of sync & async throttling is really problematic... I'm wondering
> how useful the async throttling is.

If sync throttling is useful, then async throttling has to be useful too?
Especially given the fact that often async IO consumes all bandwidth
impacting sync latencies.

> Because we will block on request
> allocation once there are more than nr_requests pending requests so at that
> point throttling becomes sync anyway.

First of all flushers will block on nr_requests and not actual writers.
And secondly we thought of having per group request descriptors so that
writes of one group don't impact others. So once the writes of a group
are backlogged, then flusher can query the congestion status of group
and not submit any more writes to that group. As some writes are already
queued in that group, writes will not be starved. Well, in case of
deadline, even direct writes go in write queue so theoritically we can
hit starvation issue (flush not being able to submit writes without
risking blocking) there too.

To avoid this starvation, ideally we need per bdi per cgroup flusher. so
that flusher can simply block if there are not enough request descriptors
in the cgroup.

So trying to throttle buffered writes synchronously in balance_dirty_pages(),
atleast simlifies the implementation.  I like my implementation better
over Fengguang's approach of throttling for simple reason that buffered
writes and direct writes can be subjected to same throttling limits
instead of separate limits for buffered writes.

Thanks
Vivek