Re: [Lsf-pc] [LSF/MM ATTEND] Filesystems -- Btrfs, cgroups, Storage topics from Facebook

[Chris Mason <clm@fb.com>]

On Thu, 2014-01-02 at 12:10 -0500, tj@kernel.org wrote:
> Hey, Vivek.
> 
> On Thu, Jan 02, 2014 at 12:06:37PM -0500, Vivek Goyal wrote:
> > So is this a separate configuration which can be done per bdi as opposed
> > to per device? IOW throttling offered per per cgroup per bdi. This will
> > help with the case of throttling over NFS too, which some people have
> > been asking for.
> 
> Hah? No, bdi just being split per-cgroup on each device so that it can
> properly propagate congestion upwards per-blkcg, just like how we
> split request allocation per-cgroup in the block layer proper.
> 

I'm not entirely sure how well this will fit with the filesystems (who
already expect a single BDI), but it's worth trying.  I'm definitely
worried about having too many blkcgs and over-committing the dirty
memory limits.  BDI-per device setup we have now already has that
problem, but I'm not sure its a good idea to make it worse.

> > So it sounds like re-implementing throttling infrastructure at bdi level
> > now (Similar to what has been done at device level)? Of course use as
> > much code as possible. But IIUC, proposal is that effectively there will
> > can be two throttling controllers. One operating at bdi level and one
> > operating below it at device level?
> 
> Not at all.  I was arguing explicitly against that.

Keep in mind that we do already have throttling at the BDI level.  I was
definitely hoping we could consolidate some of that since it has grown
some bits of an elevator already.  I'm not saying what we have in the
bdi throttling isn't reasonable, but it is definitely replicating some
of the infrastructure down below.

We'll (Josef, me, perhaps others at FB) will do some experiments before
LSF.  The current plan is to throw some code at the wall and see what
sticks.

-chris