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. > > Yeah, currently the soft limit reclaim cycle preceeds the generic > reclaim cycle. But the end result is that other memcgs are reclaimed > from as well in both cases. The exact timing is irrelevant. > > And this has been the case for a long time, so I don't think my rework > breaks existing users in that regard. > >> > My code does that, so I don't think we are breaking any promises >> > currently made in the documentation. >> > >> > But much more important than keeping documentation promises is not to >> > break actual users. So if you are yourself a user of soft limits, >> > test the new code pretty please and complain if it breaks your setup! >> >> Yes, I've been running tests on your patchset, but not getting into >> specific configurations yet. But I don't think it is hard to generate >> the following scenario: >> >> on 32G machine, under root I have three cgroups with 20G hard_limit and >> cgroup-A: soft_limit 1g, usage 20g with clean file pages >> cgroup-B: soft_limit 10g, usage 5g with clean file pages >> cgroup-C: soft_limit 10g, usage 5g with clean file pages >> >> I would assume reclaiming from cgroup-A should be sufficient under >> global memory pressure, and no pages needs to be reclaimed from B or >> C, especially both of them have memory usage under their soft_limit. > > Keep in mind that memcgs are scanned proportionally to their size, > that we start out with relatively low scan counts, and that the > priority levels are a logarithmic scale. > > The formula is essentially this: > > (usage / PAGE_SIZE) >> priority > > which means that we would scan as follows, with decreased soft limit > priority for A: > > A: ((20 << 30) >> 12) >> 11 = 2560 pages > B: (( 5 << 30) >> 12) >> 12 = 320 pages > C: = 320 pages. > > So even if B and C are scanned, they are only shrunk by a bit over a > megabyte tops. For decreasing levels (if they are reached at all if > there is clean cache around): > > A: 20M 40M 80M 160M ... > B: 2M 4M 8M 16M ... > > While it would be sufficient to reclaim only from A, actually > reclaiming from B and C is not a big deal in practice, I would > suspect. One way to think about how the user will set the soft_limit ( in our case for example) is to set the soft_limit to be the working_set_size of the cgroup (by doing working set estimation). The soft_limit will be readjust at run time based on the workload. In that case, shrinking the memory from B/C has potential performance impact on the application. While it doesn't mean we can never reclaim pages from them, but shrinking from A ( usage is 19G above its soft_limit ) first will provide better predictability of performance. --Ying > -- 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/ . 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