Hello Rik, Thanks for looking into this! On Tue, May 01, 2012 at 05:04:21PM -0400, Rik van Riel wrote: > On 05/01/2012 09:18 AM, Anton Vorontsov wrote: > >This patch implements a new event type, it will trigger whenever a > >value becomes greater than user-specified threshold, it complements > >the 'less-then' trigger type. > > > >Also, let's implement the one-shot mode for the events, when set, > >userspace will only receive one notification per crossing the > >boundaries. > > > >Now when both LT and GT are set on the same level, the event type > >works as a cross event type: it triggers whenever a value crosses > >the threshold from a lesser values side to a greater values side, > >and vice versa. > > > >We use the event types in an userspace low-memory killer: we get a > >notification when memory becomes low, so we start freeing memory by > >killing unneeded processes, and we get notification when memory hits > >the threshold from another side, so we know that we freed enough of > >memory. > > How are these vmevents supposed to work with cgroups? Currently these are independent subsystems, if you have memcg enabled, you can do almost anything* with the memory, as memg has all the needed hooks in the mm/ subsystem (it is more like "memory management tracer" nowadays :-). But cgroups have its cost, both performance penalty and memory wastage. For example, in the best case, memcg constantly consumes 0.5% of RAM to track memory usage, this is 5 MB on a 1 GB "embedded" machine. To some people it feels just wrong to waste that memory for mere notifications. Of course, this alone can be considered as a lame argument for making another subsystem (instead of "fixing" the current one). But see below, vmevent is just a convenient ABI. > What do we do when a cgroup nears its limit, and there > is no more swap space available? > > What do we do when a cgroup nears its limit, and there > is swap space available? As of now, this is all orthogonal to vmevent. Vmevent doesn't know about cgroups. If kernel has the memcg enabled, one should probably* go with it (or better, with its ABI). At least for now. > It would be nice to be able to share the same code for > embedded, desktop and server workloads... It would be great indeed, but so far I don't see much that vmevent could share. Plus, sharing the code at this point is not that interesting; it's mere 500 lines of code (comparing to more than 10K lines for cgroups, and it's not including memcg_ hooks and logic that is spread all over mm/). Today vmevent code is mostly an ABI implementation, there is very little memory management logic (in contrast to the memcg). Personally, I would rather consider sharing ABI at some point: i.e. making a memcg backend for the vmevent. That would be pretty cool. And once done, vmevent would be cgroups-aware (if memcg enabled, of course; and if not, vmevent would still work, with no memcg-related expenses). * For low memory notifications, there are still some unresolved issues with memcg. Mainly, slab accounting for the root cgroup: currently developed slab accounting doesn't account kernel's internal memory consumption, plus it doesn't account slab memory for the root cgroup at all. A few days ago I asked[1] why memcg doesn't do all this, and whether it is a design decision or just an implementation detail (so that we have a chance to fix it). But so far there were no feedback. We'll see how things turn out. [1] http://lkml.org/lkml/2012/4/30/115 Thanks! -- Anton Vorontsov Email: cbouatmailru@xxxxxxxxx -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>