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

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Mar 11, 2016 at 11:34:40AM +0300, 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.

While I do expect memory.high to be adjusted every once in a while, I
can't see anybody doing it by a significant fraction of the cgroup
every couple of seconds - or tighter than the workingset; and dropping
use-once cache is cheap. What kind of usecase would that be?

But even if we're wrong about it and this becomes a scalability issue,
the knob - even when reclaiming synchroneously - makes no guarantees
about the target being met once the write finishes. It's a best effort
mechanism. What would break if we made it async later on?

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