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SubjectRe: [PATCH v6 08/10] mm: rework non-root kmem_cache lifecycle management
On Tue, Jun 04, 2019 at 07:44:52PM -0700, Roman Gushchin wrote:
> Currently each charged slab page holds a reference to the cgroup to
> which it's charged. Kmem_caches are held by the memcg and are released
> all together with the memory cgroup. It means that none of kmem_caches
> are released unless at least one reference to the memcg exists, which
> is very far from optimal.
>
> Let's rework it in a way that allows releasing individual kmem_caches
> as soon as the cgroup is offline, the kmem_cache is empty and there
> are no pending allocations.
>
> To make it possible, let's introduce a new percpu refcounter for
> non-root kmem caches. The counter is initialized to the percpu mode,
> and is switched to the atomic mode during kmem_cache deactivation. The
> counter is bumped for every charged page and also for every running
> allocation. So the kmem_cache can't be released unless all allocations
> complete.
>
> To shutdown non-active empty kmem_caches, let's reuse the work queue,
> previously used for the kmem_cache deactivation. Once the reference
> counter reaches 0, let's schedule an asynchronous kmem_cache release.
>
> * I used the following simple approach to test the performance
> (stolen from another patchset by T. Harding):
>
> time find / -name fname-no-exist
> echo 2 > /proc/sys/vm/drop_caches
> repeat 10 times
>
> Results:
>
> orig patched
>
> real 0m1.455s real 0m1.355s
> user 0m0.206s user 0m0.219s
> sys 0m0.855s sys 0m0.807s
>
> real 0m1.487s real 0m1.699s
> user 0m0.221s user 0m0.256s
> sys 0m0.806s sys 0m0.948s
>
> real 0m1.515s real 0m1.505s
> user 0m0.183s user 0m0.215s
> sys 0m0.876s sys 0m0.858s
>
> real 0m1.291s real 0m1.380s
> user 0m0.193s user 0m0.198s
> sys 0m0.843s sys 0m0.786s
>
> real 0m1.364s real 0m1.374s
> user 0m0.180s user 0m0.182s
> sys 0m0.868s sys 0m0.806s
>
> real 0m1.352s real 0m1.312s
> user 0m0.201s user 0m0.212s
> sys 0m0.820s sys 0m0.761s
>
> real 0m1.302s real 0m1.349s
> user 0m0.205s user 0m0.203s
> sys 0m0.803s sys 0m0.792s
>
> real 0m1.334s real 0m1.301s
> user 0m0.194s user 0m0.201s
> sys 0m0.806s sys 0m0.779s
>
> real 0m1.426s real 0m1.434s
> user 0m0.216s user 0m0.181s
> sys 0m0.824s sys 0m0.864s
>
> real 0m1.350s real 0m1.295s
> user 0m0.200s user 0m0.190s
> sys 0m0.842s sys 0m0.811s
>
> So it looks like the difference is not noticeable in this test.
>
> Signed-off-by: Roman Gushchin <guro@fb.com>

Acked-by: Vladimir Davydov <vdavydov.dev@gmail.com>

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