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SubjectRe: Kernel scanning/freeing to relieve cgroup memory pressure
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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