Re: [Bug 192981] New: page allocation stalls

[Dave Chinner <david@fromorbit.com>]

On Fri, Feb 17, 2017 at 08:11:09PM +0900, Tetsuo Handa wrote:
> On 2017/02/17 7:21, Dave Chinner wrote:
> > FWIW, the major problem with removing the blocking in inode reclaim
> > is the ease with which you can then trigger the OOM killer from
> > userspace.  The high level memory reclaim algorithms break down when
> > there are hundreds of direct reclaim processes hammering on reclaim
> > and reclaim stops making progress because it's skipping dirty
> > objects.  Direct reclaim ends up insufficiently throttled, so rather
> > than blocking it winds up reclaim priority and then declares OOM
> > because reclaim runs out of retries before sufficient memory has
> > been freed.
> > 
> > That, right now, looks to be an unsolvable problem without a major
> > rework of direct reclaim.  I've pretty much given up on ever getting
> > the unbound direct reclaim concurrency problem that is causing us
> > these problems fixed, so we are left to handle it in the subsystem
> > shrinkers as best we can. That leaves us with an unfortunate choice: 
> > 
> > 	a) throttle excessive concurrency in the shrinker to prevent
> > 	   IO breakdown, thereby causing reclaim latency bubbles
> > 	   under load but having a stable, reliable system; or
> > 	b) optimise for minimal reclaim latency and risk userspace
> > 	   memory demand triggering the OOM killer whenever there
> > 	   are lots of dirty inodes in the system.
> > 
> > Quite frankly, there's only one choice we can make in this
> > situation: reliability is always more important than performance.
> 
> Is it possible to get rid of direct reclaim and let allocating thread
> wait on queue? I wished such change in context of __GFP_KILLABLE at
> http://lkml.kernel.org/r/201702012049.BAG95379.VJFFOHMStLQFOO@I-love.SAKURA.ne.jp .

Yup, that's similar to what I've been suggesting - offloading the
direct reclaim slowpath to a limited set of kswapd-like workers
and blocking the allocating processes until there is either memory
for them or OOM is declared...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com