Re: [PATCH 14/14] mm: memcontrol: hook up vmpressure to socket pressure

Vladimir Davydov
2015-11-15T13:55:20Z

On Thu, Nov 12, 2015 at 06:41:33PM -0500, Johannes Weiner wrote:
> Let the networking stack know when a memcg is under reclaim pressure
> so that it can clamp its transmit windows accordingly.
>
> Whenever the reclaim efficiency of a cgroup's LRU lists drops low
> enough for a MEDIUM or HIGH vmpressure event to occur, assert a
> pressure state in the socket and tcp memory code that tells it to curb
> consumption growth from sockets associated with said control group.
>
> vmpressure events are naturally edge triggered, so for hysteresis
> assert socket pressure for a second to allow for subsequent vmpressure
> events to occur before letting the socket code return to normal.

AFAICS, in contrast to v1, now you don't modify vmpressure behavior,
which means socket_pressure will only be set when cgroup hits its
high/hard limit. On tightly packed system, this is unlikely IMO -
cgroups will mostly experience pressure due to memory shortage at the
global level and/or their low limit configuration, in which case no
vmpressure events will be triggered and therefore tcp window won't be
clamped accordingly.

May be, we could use a per memcg slab shrinker to detect memory
pressure? This looks like abusing shrinkers API though.

Thanks,
Vladimir