Re: [PATCH] mm: memcontrol: reclaim when shrinking memory.high below usage

From: Michal Hocko
Date: Fri Mar 11 2016 - 03:42:51 EST

On Fri 11-03-16 11:34:40, Vladimir Davydov wrote:
> On Thu, Mar 10, 2016 at 03:50:13PM -0500, Johannes Weiner wrote:
> > When setting memory.high below usage, nothing happens until the next
> > charge comes along, and then it will only reclaim its own charge and
> > not the now potentially huge excess of the new memory.high. This can
> > cause groups to stay in excess of their memory.high indefinitely.
> >
> > To fix that, when shrinking memory.high, kick off a reclaim cycle that
> > goes after the delta.
> I agree that we should reclaim the high excess, but I don't think it's a
> good idea to do it synchronously. Currently, memory.low and memory.high
> knobs can be easily used by a single-threaded load manager implemented
> in userspace, because it doesn't need to care about potential stalls
> caused by writes to these files. After this change it might happen that
> a write to memory.high would take long, seconds perhaps, so in order to
> react quickly to changes in other cgroups, a load manager would have to
> spawn a thread per each write to memory.high, which would complicate its
> implementation significantly.

Is the complication on the managing part really an issue though. Such a
manager would have to spawn a process/thread to change the .max already.

Michal Hocko