On 11/16/2012 01:25 AM, David Rientjes wrote: > On Thu, 15 Nov 2012, Anton Vorontsov wrote: > >> Hehe, you're saying that we have to have cgroups=y. :) But some folks were >> deliberately asking us to make the cgroups optional. >> > > Enabling just CONFIG_CGROUPS (which is enabled by default) and no other > current cgroups increases the size of the kernel text by less than 0.3% > with x86_64 defconfig: > > text data bss dec hex filename > 10330039 1038912 1118208 12487159 be89f7 vmlinux.disabled > 10360993 1041624 1122304 12524921 bf1d79 vmlinux.enabled > > I understand that users with minimally-enabled configs for an optimized > memory footprint will have a higher percentage because their kernel is > already smaller (~1.8% increase for allnoconfig), but I think the cost of > enabling the cgroups code to be able to mount a vmpressure cgroup (which > I'd rename to be "mempressure" to be consistent with "memcg" but it's only > an opinion) is relatively small and allows for a much more maintainable > and extendable feature to be included: it already provides the > cgroup.event_control interface that supports eventfd that makes > implementation much easier. It also makes writing a library on top of the > cgroup to be much easier because of the standardization. > > I'm more concerned about what to do with the memcg memory thresholds and > whether they can be replaced with this new cgroup. If so, then we'll have > to figure out how to map those triggers to use the new cgroup's interface > in a way that doesn't break current users that open and pass the fd of > memory.usage_in_bytes to cgroup.event_control for memcg. > >> OK, here is what I can try to do: >> >> - Implement memory pressure cgroup as you described, by doing so we'd make >> the thing play well with cpusets and memcg; >> >> - This will be eventfd()-based; >> > > Should be based on cgroup.event_control, see how memcg interfaces its > memory thresholds with this in Documentation/cgroups/memory.txt. > >> - Once done, we will have a solution for pretty much every major use-case >> (i.e. servers, desktops and Android, they all have cgroups enabled); >> > > Excellent! I'd be interested in hearing anybody else's opinions, > especially those from the memcg world, so we make sure that everybody is > happy with the API that you've described. > Just CC'd them all. My personal take: Most people hate memcg due to the cost it imposes. I've already demonstrated that with some effort, it doesn't necessarily have to be so. (http://lwn.net/Articles/517634/) The one thing I missed on that work, was precisely notifications. If you can come up with a good notifications scheme that *lives* in memcg, but does not *depend* in the memcg infrastructure, I personally think it could be a big win. Doing this in memcg has the advantage that the "per-group" vs "global" is automatically solved, since the root memcg is just another name for "global". I honestly like your low/high/oom scheme better than memcg's "threshold-in-bytes". I would also point out that those thresholds are *far* from exact, due to the stock charging mechanism, and can be wrong by as much as O(#cpus). So far, nobody complained. So in theory it should be possible to convert memcg to low/high/oom, while still accepting writes in bytes, that would be thrown in the closest bucket. Another thing from one of your e-mails, that may shift you in the memcg direction: "2. The last time I checked, cgroups memory controller did not (and I guess still does not) not account kernel-owned slabs. I asked several times why so, but nobody answered." It should, now, in the latest -mm, although it won't do per-group reclaim (yet). I am also failing to see how cpusets would be involved in here. I understand that you may have free memory in terms of size, but still be further restricted by cpuset. But I also think that having multiple entry points for this buy us nothing at all. So the choices I see are: 1) If cpuset + memcg are comounted, take this into account when deciding low / high / oom. This is yet another advantage over the "threshold in bytes" interface, in which you can transparently take other issues into account while keeping the interface. 2) If they are not, just ignore this effect. The fallback in 2) sounds harsh, but I honestly think this is the price to pay for the insanity of mounting those things in different hierarchies, and we do have a plan to have all those things eventually together anyway. If you have two cgroups dealing with memory, and set them up in orthogonal ways, I really can't see how we can bring sanity to that. So just admitting and unleashing the insanity may be better, if it brings up our urge to fix it. It worked for Batman, why wouldn't it work for us? -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
