Re: [PATCH -mmotm 0/4] memcg: per cgroup dirty limit (v4)

[Andrea Righi <arighi@develer.com>]

On Thu, Mar 04, 2010 at 10:41:43PM +0530, Balbir Singh wrote:
> * Andrea Righi <arighi@develer.com> [2010-03-04 11:40:11]:
> 
> > Control the maximum amount of dirty pages a cgroup can have at any given time.
> > 
> > Per cgroup dirty limit is like fixing the max amount of dirty (hard to reclaim)
> > page cache used by any cgroup. So, in case of multiple cgroup writers, they
> > will not be able to consume more than their designated share of dirty pages and
> > will be forced to perform write-out if they cross that limit.
> > 
> > The overall design is the following:
> > 
> >  - account dirty pages per cgroup
> >  - limit the number of dirty pages via memory.dirty_ratio / memory.dirty_bytes
> >    and memory.dirty_background_ratio / memory.dirty_background_bytes in
> >    cgroupfs
> >  - start to write-out (background or actively) when the cgroup limits are
> >    exceeded
> > 
> > This feature is supposed to be strictly connected to any underlying IO
> > controller implementation, so we can stop increasing dirty pages in VM layer
> > and enforce a write-out before any cgroup will consume the global amount of
> > dirty pages defined by the /proc/sys/vm/dirty_ratio|dirty_bytes and
> > /proc/sys/vm/dirty_background_ratio|dirty_background_bytes limits.
> > 
> > Changelog (v3 -> v4)
> > ~~~~~~~~~~~~~~~~~~~~~~
> >  * handle the migration of tasks across different cgroups
> >    NOTE: at the moment we don't move charges of file cache pages, so this
> >    functionality is not immediately necessary. However, since the migration of
> >    file cache pages is in plan, it is better to start handling file pages
> >    anyway.
> >  * properly account dirty pages in nilfs2
> >    (thanks to Kirill A. Shutemov <kirill@shutemov.name>)
> >  * lockless access to dirty memory parameters
> >  * fix: page_cgroup lock must not be acquired under mapping->tree_lock
> >    (thanks to Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> and
> >     KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>)
> >  * code restyling
> >
> 
> This seems to be converging, what sort of tests are you running on
> this patchset? 

A very simple test at the moment, just some parallel dd's running in
different cgroups. For example:

 - cgroup A: low dirty limits (writes are almost sync)
   echo 1000 > /cgroups/A/memory.dirty_bytes
   echo 1000 > /cgroups/A/memory.dirty_background_bytes

 - cgroup B: high dirty limits (writes are all buffered in page cache)
   echo 100 > /cgroups/B/memory.dirty_ratio
   echo 50  > /cgroups/B/memory.dirty_background_ratio

Then run the dd's and look at memory.stat:
  - cgroup A: # dd if=/dev/zero of=A bs=1M count=1000
  - cgroup B: # dd if=/dev/zero of=B bs=1M count=1000

A random snapshot during the writes:

# grep "dirty\|writeback" /cgroups/[AB]/memory.stat
/cgroups/A/memory.stat:filedirty 0
/cgroups/A/memory.stat:writeback 0
/cgroups/A/memory.stat:writeback_tmp 0
/cgroups/A/memory.stat:dirty_pages 0
/cgroups/A/memory.stat:writeback_pages 0
/cgroups/A/memory.stat:writeback_temp_pages 0
/cgroups/B/memory.stat:filedirty 67226
/cgroups/B/memory.stat:writeback 136
/cgroups/B/memory.stat:writeback_tmp 0
/cgroups/B/memory.stat:dirty_pages 67226
/cgroups/B/memory.stat:writeback_pages 136
/cgroups/B/memory.stat:writeback_temp_pages 0

I plan to run more detailed IO benchmark soon.

-Andrea