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? > > Stronger is the simpler definition, it's simpler code, > > The code is not really that much simpler. The one you have posted will > not work I am afraid. I haven't tested it yet but I remember I had to do > some tweaks to the reclaim path to not end up in an endless loop in the > direct reclaim (http://marc.info/?l=linux-mm&m=138677140828678&w=2 and > http://marc.info/?l=linux-mm&m=138677141328682&w=2). That's just a result of do_try_to_free_pages being stupid and using its own zonelist loop to check reclaimability by duplicating all the checks instead of properly using returned state of shrink_zones(). Something that would be worth fixing regardless of memcg guarantees. Or maybe we could add the guaranteed lru pages to sc->nr_scanned. > > your usecases are fine with it, > > my usecases do not overcommit low_limit on the available memory, so far > so good, but once we hit a corner cases when limits are set properly but > we end up not being able to reclaim anybody in a zone then OOM sounds > too brutal. What cornercases? -- 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>