Messages in this thread | | | Date | Mon, 14 Apr 2014 09:11:25 +0100 | From | Glyn Normington <> | Subject | Re: Kernel scanning/freeing to relieve cgroup memory pressure |
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Johannes/Michal
What are your thoughts on this matter? Do you see this as a valid requirement?
Regards, Glyn
On 02/04/2014 19:00, Tejun Heo wrote: > (cc'ing memcg maintainers and cgroup ML) > > On Wed, Apr 02, 2014 at 02:08:04PM +0100, Glyn Normington wrote: >> Currently, a memory cgroup can hit its oom limit when pages could, in >> principle, be reclaimed by the kernel except that the kernel does not >> respond directly to cgroup-local memory pressure. > So, ummm, it does. > >> A use case where this is important is running a moderately large Java >> application in a memory cgroup in a PaaS environment where cost to the >> user depends on the memory limit ([1]). Users need to tune the memory >> limit to reduce their costs. During application initialisation large >> numbers of JAR files are opened (read-only) and read while loading the >> application code and its dependencies. This is reflected in a peak of >> file cache usage which can push the memory cgroup memory usage >> significantly higher than the value actually needed to run the application. >> >> Possible approaches include (1) automatic response to cgroup-local >> memory pressure in the kernel, and (2) a kernel API for reclaiming >> memory from a cgroup which could be driven under oom notification (with >> the oom killer disabled for the cgroup - it would be enabled if the >> cgroup was still oom after calling the kernel to reclaim memory). >> >> Clearly (1) is the preferred approach. The closest facility in the >> kernel to (2) is to ask the kernel to free pagecache using `echo 1 > >> /proc/sys/vms/drop_caches`, but that is too wide-ranging, especially in >> a PaaS environment hosting multiple applications. A similar facility >> could be provided for a cgroup via a cgroup pseudo-file >> `memory.drop_caches`. >> >> Other approaches include a mempressure cgroup ([2]) which would not be >> suitable for PaaS applications. See [3] for Andrew Morton's response. A >> related workaround ([4]) was included in the 3.6 kernel. >> >> Related discussions: >> [1] https://groups.google.com/a/cloudfoundry.org/d/topic/vcap-dev/6M8BDV_tq7w/discussion >> [2]https://lwn.net/Articles/531077/ <https://lwn.net/Articles/531077/> >> [3]https://lwn.net/Articles/531138/ <https://lwn.net/Articles/531138/> >> [4]https://lkml.org/lkml/2013/6/6/462 <https://lkml.org/lkml/2013/6/6/462>& >> https://github.com/torvalds/linux/commit/e62e384e >> <https://github.com/torvalds/linux/commit/e62e384e>.
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