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

[Chris Mason <clm@fb.com>]

On Wed, 2014-01-08 at 15:04 +0000, Mel Gorman wrote:
> On Tue, Dec 31, 2013 at 01:19:15PM +0000, Chris Mason wrote:
> > On Tue, 2013-12-31 at 13:45 +0100, Jan Kara wrote:
> > > On Tue 31-12-13 16:49:27, Zheng Liu wrote:
> > > > Hi Chris,
> > > > 
> > > > On Mon, Dec 30, 2013 at 09:36:20PM +0000, Chris Mason wrote:
> > > > > Hi everyone,
> > > > > 
> > > > > I'd like to attend the LSF/MM conference this year.  My current
> > > > > discussion points include:
> > > > > 
> > > > > All things Btrfs!
> > > > > 
> > > > > Adding cgroups for more filesystem resources, especially to limit the
> > > > > speed dirty pages are created.
> > > > 
> > > > Interesting.  If I remember correctly, IO-less dirty throttling has been
> > > > applied into upstream kernel, which can limit the speed that dirty pages
> > > > are created.  Does it has any defect?
> > >   It works as it should. But as Jeff points out, the throttling isn't
> > > cgroup aware. So it can happen that one memcg is full of dirty pages and
> > > reclaim has problems with reclaiming pages for it. I guess what Chris asks
> > > for is that we watch number of dirty pages in each memcg and throttle
> > > processes creating dirty pages in memcg which is close to its limit on
> > > dirty pages.
> > 
> > Right, the ioless dirty throttling is fantastic, but it's based on the
> > BDI and you only get one of those per device.
> > 
> 
> It's only partially related but we'll also need to keep in mind that
> even with ioless dirty throttling that dirty_ratio and dirty_bytes have
> been showing their age for a long time.  dirty_ratio was fine when 20%
> of memory was still a few seconds of IO but it has not been the case in a
> long time. dirty_bytes is also not a great interface because it ignores the
> speed of the underlying device. While proposals to fix it have been raised
> in the past, no one (including me) has put themselves on the firing line
> to replace that interface with something like dirty_time -- do not dirty
> more pages than it takes N seconds to writeback. When/if someone clears
> their table sufficiently to tackle that problem they are likely to collide
> with any IO controller work.
> 

I'm really hoping to combine these ideas a bit.  There's a ton of
overlap between the BDI throttling and the io controller.  So, I'll
definitely poke at it with a stick, we'll see what falls out.

-chris