On Wed, May 28 2014, Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > On Wed, May 28, 2014 at 04:21:44PM +0200, Michal Hocko wrote: >> On Wed 28-05-14 09:49:05, Johannes Weiner wrote: >> > On Wed, May 28, 2014 at 02:10:23PM +0200, Michal Hocko wrote: >> > > Hi Andrew, Johannes, >> > > >> > > On Mon 28-04-14 14:26:41, Michal Hocko wrote: >> > > > This patchset introduces such low limit that is functionally similar >> > > > to a minimum guarantee. Memcgs which are under their lowlimit are not >> > > > considered eligible for the reclaim (both global and hardlimit) unless >> > > > all groups under the reclaimed hierarchy are below the low limit when >> > > > all of them are considered eligible. >> > > > >> > > > The previous version of the patchset posted as a RFC >> > > > (http://marc.info/?l=linux-mm&m=138677140628677&w=2) suggested a >> > > > hard guarantee without any fallback. More discussions led me to >> > > > reconsidering the default behavior and come up a more relaxed one. The >> > > > hard requirement can be added later based on a use case which really >> > > > requires. It would be controlled by memory.reclaim_flags knob which >> > > > would specify whether to OOM or fallback (default) when all groups are >> > > > bellow low limit. >> > > >> > > It seems that we are not in a full agreement about the default behavior >> > > yet. Johannes seems to be more for hard guarantee while I would like to >> > > see the weaker approach first and move to the stronger model later. >> > > Johannes, is this absolutely no-go for you? Do you think it is seriously >> > > handicapping the semantic of the new knob? >> > >> > Well we certainly can't start OOMing where we previously didn't, >> > that's called a regression and automatically limits our options. >> > >> > Any unexpected OOMs will be much more acceptable from a new feature >> > than from configuration that previously "worked" and then stopped. >> >> Yes and we are not talking about regressions, are we? >> >> > > My main motivation for the weaker model is that it is hard to see all >> > > the corner case right now and once we hit them I would like to see a >> > > graceful fallback rather than fatal action like OOM killer. Besides that >> > > the usaceses I am mostly interested in are OK with fallback when the >> > > alternative would be OOM killer. I also feel that introducing a knob >> > > with a weaker semantic which can be made stronger later is a sensible >> > > way to go. >> > >> > We can't make it stronger, but we can make it weaker. >> >> Why cannot we make it stronger by a knob/configuration option? > > Why can't we make it weaker by a knob? Why should we design the > default for unforeseeable cornercases rather than make the default > make sense for existing cases and give cornercases a fallback once > they show up? My 2c... The following works for my use cases: 1) introduce memory.low_limit_in_bytes (default=0 thus no default change from older kernels) 2) interested users will set low_limit_in_bytes to non-zero value. Memory protected by low limit should be as migratable/reclaimable as mlock memory. If a zone full of mlock memory causes oom kills, then so should the low limit. If we find corner cases where low_limit_in_bytes is too strict, then we could discuss a new knob to relax it. But I think we should start with a strict low-limit. If the oom killer gets tied in knots due to low limit, then I'd like to explore fixing the oom killer before relaxing low limit. Disclaimer: new use cases will certainly appear with various requirements. But an oom-killing low_limit_in_bytes seems like a generic opt-in feature, so I think it's worthwhile. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>