On Thu, Jun 09, 2011 at 03:30:27PM -0700, Ying Han wrote: > On Thu, Jun 9, 2011 at 11:36 AM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > On Thu, Jun 09, 2011 at 10:36:47AM -0700, Ying Han wrote: > >> On Thu, Jun 9, 2011 at 1:35 AM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > >> > On Wed, Jun 08, 2011 at 08:52:03PM -0700, Ying Han wrote: > >> >> On Wed, Jun 8, 2011 at 8:32 AM, Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > >> >> > I guess it would make much more sense to evaluate if reclaiming from > >> >> > memcgs while there are others exceeding their soft limit is even a > >> >> > problem. Otherwise this discussion is pretty pointless. > >> >> > >> >> AFAIK it is a problem since it changes the spec of kernel API > >> >> memory.soft_limit_in_bytes. That value is set per-memcg which all the > >> >> pages allocated above that are best effort and targeted to reclaim > >> >> prior to others. > >> > > >> > That's not really true. Quoting the documentation: > >> > > >> > When the system detects memory contention or low memory, control groups > >> > are pushed back to their soft limits. If the soft limit of each control > >> > group is very high, they are pushed back as much as possible to make > >> > sure that one control group does not starve the others of memory. > >> > > >> > I am language lawyering here, but I don't think it says it won't touch > >> > other memcgs at all while there are memcgs exceeding their soft limit. > >> > >> Well... :) I would say that the documentation of soft_limit needs lots > >> of work especially after lots of discussions we have after the LSF. > >> > >> The RFC i sent after our discussion has the following documentation, > >> and I only cut & paste the content relevant to our conversation here: > >> > >> What is "soft_limit"? > >> The "soft_limit was introduced in memcg to support over-committing the > >> memory resource on the host. Each cgroup can be configured with > >> "hard_limit", where it will be throttled or OOM killed by going over > >> the limit. However, the allocation can go above the "soft_limit" as > >> long as there is no memory contention. The "soft_limit" is the kernel > >> mechanism for re-distributing spare memory resource among cgroups. > >> > >> What we have now? > >> The current implementation of softlimit is based on per-zone RB tree, > >> where only the cgroup exceeds the soft_limit the most being selected > >> for reclaim. > >> > >> It makes less sense to only reclaim from one cgroup rather than > >> reclaiming all cgroups based on calculated propotion. This is required > >> for fairness. > >> > >> Proposed design: > >> round-robin across the cgroups where they have memory allocated on the > >> zone and also exceed the softlimit configured. > >> > >> there was a question on how to do zone balancing w/o global LRU. This > >> could be solved by building another cgroup list per-zone, where we > >> also link cgroups under their soft_limit. We won't scan the list > >> unless the first list being exhausted and > >> the free pages is still under the high_wmark. > >> > >> Since the per-zone memcg list design is being replaced by your > >> patchset, some of the details doesn't apply. But the concept still > >> remains where we would like to scan some memcgs first (above > >> soft_limit) . > > > > I think the most important thing we wanted was to round-robin scan all > > soft limit excessors instead of just the biggest one. I understood > > this is the biggest fault with soft limits right now. > > > > We came up with maintaining a list of excessors, rather than a tree, > > and from this particular implementation followed naturally that this > > list is scanned BEFORE we look at other memcgs at all. > > > > This is a nice to have, but it was never the primary problem with the > > soft limit implementation, as far as I understood. > > > >> > It would be a lie about the current code in the first place, which > >> > does soft limit reclaim and then regular reclaim, no matter the > >> > outcome of the soft limit reclaim cycle. It will go for the soft > >> > limit first, but after an allocation under pressure the VM is likely > >> > to have reclaimed from other memcgs as well. > >> > > >> > I saw your patch to fix that and break out of reclaim if soft limit > >> > reclaim did enough. But this fix is not much newer than my changes. > >> > >> My soft_limit patch was developed in parallel with your patchset, and > >> most of that wouldn't apply here. > >> Is that what you are referring to? > > > > No, I meant that the current behaviour is old and we are only changing > > it only now, so we are not really breaking backward compatibility. > > > >> > The second part of this is: > >> > > >> > Please note that soft limits is a best effort feature, it comes with > >> > no guarantees, but it does its best to make sure that when memory is > >> > heavily contended for, memory is allocated based on the soft limit > >> > hints/setup. Currently soft limit based reclaim is setup such that > >> > it gets invoked from balance_pgdat (kswapd). > >> > >> We had patch merged which add the soft_limit reclaim also in the global ttfp. > >> > >> memcg-add-the-soft_limit-reclaim-in-global-direct-reclaim.patch > >> > >> > It's not the pages-over-soft-limit that are best effort. It says that > >> > it tries its best to take soft limits into account while reclaiming. > >> Hmm. Both cases are true. The best effort pages I referring to means > >> "the page above the soft_limit are targeted to reclaim first under > >> memory contention" > > > > I really don't know where you are taking this from. That is neither > > documented anywhere, nor is it the current behaviour. > > I got the email from andrew on may 27 and you were on the cc-ed :) > Anyway, i just forwarded you that one. I wasn't asking about this patch at all... This is the conversation: Me: > >> > It's not the pages-over-soft-limit that are best effort. It says that > >> > it tries its best to take soft limits into account while reclaiming. You: > >> Hmm. Both cases are true. The best effort pages I referring to means > >> "the page above the soft_limit are targeted to reclaim first under > >> memory contention" Me: > > I really don't know where you are taking this from. That is neither > > documented anywhere, nor is it the current behaviour. And this is still my question. Current: scan up to all pages of the biggest soft limit offender, then reclaim from random memcgs (because of the global LRU). After my patch: scan all memcgs according to their size, with double the pressure on those over their soft limit. Please tell me exactly how my patch regresses existing behaviour, a user interface, a documented feature, etc. If you have an even better idea, please propose it. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>