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    SubjectRe: [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v6)
    On Wed, 10 Mar 2010 00:00:31 +0100
    Andrea Righi <arighi@develer.com> wrote:

    > Control the maximum amount of dirty pages a cgroup can have at any given time.
    >
    > Per cgroup dirty limit is like fixing the max amount of dirty (hard to reclaim)
    > page cache used by any cgroup. So, in case of multiple cgroup writers, they
    > will not be able to consume more than their designated share of dirty pages and
    > will be forced to perform write-out if they cross that limit.
    >
    > The overall design is the following:
    >
    > - account dirty pages per cgroup
    > - limit the number of dirty pages via memory.dirty_ratio / memory.dirty_bytes
    > and memory.dirty_background_ratio / memory.dirty_background_bytes in
    > cgroupfs
    > - start to write-out (background or actively) when the cgroup limits are
    > exceeded
    >
    > This feature is supposed to be strictly connected to any underlying IO
    > controller implementation, so we can stop increasing dirty pages in VM layer
    > and enforce a write-out before any cgroup will consume the global amount of
    > dirty pages defined by the /proc/sys/vm/dirty_ratio|dirty_bytes and
    > /proc/sys/vm/dirty_background_ratio|dirty_background_bytes limits.
    >
    > Changelog (v5 -> v6)
    > ~~~~~~~~~~~~~~~~~~~~~~
    > * always disable/enable IRQs at lock/unlock_page_cgroup(): this allows to drop
    > the previous complicated locking scheme in favor of a simpler locking, even
    > if this obviously adds some overhead (see results below)
    > * drop FUSE and NILFS2 dirty pages accounting for now (this depends on
    > charging bounce pages per cgroup)
    >
    > Results
    > ~~~~~~~
    > I ran some tests using a kernel build (2.6.33 x86_64_defconfig) on a
    > Intel Core 2 @ 1.2GHz as testcase using different kernels:
    > - mmotm "vanilla"
    > - mmotm with cgroup-dirty-memory using the previous "complex" locking scheme
    > (my previous patchset + the fixes reported by Kame-san and Daisuke-san)
    > - mmotm with cgroup-dirty-memory using the simple locking scheme
    > (lock_page_cgroup() with IRQs disabled)
    >
    > Following the results:
    > <before>
    > - mmotm "vanilla", root cgroup: 11m51.983s
    > - mmotm "vanilla", child cgroup: 11m56.596s
    >
    > <after>
    > - mmotm, "complex" locking scheme, root cgroup: 11m53.037s
    > - mmotm, "complex" locking scheme, child cgroup: 11m57.896s
    >
    > - mmotm, lock_page_cgroup+irq_disabled, root cgroup: 12m5.499s
    > - mmotm, lock_page_cgroup+irq_disabled, child cgroup: 12m9.920s
    >
    > With the "complex" locking solution, the overhead introduced by the
    > cgroup dirty memory accounting is minimal (0.14%), compared with the overhead
    > introduced by the lock_page_cgroup+irq_disabled solution (1.90%).
    >
    Hmm....isn't this bigger than expected ?


    > The performance overhead is not so huge in both solutions, but the impact on
    > performance is even more reduced using a complicated solution...
    >
    > Maybe we can go ahead with the simplest implementation for now and start to
    > think to an alternative implementation of the page_cgroup locking and
    > charge/uncharge of pages.
    >

    maybe. But in this 2 years, one of our biggest concerns was the performance.
    So, we do something complex in memcg. But complex-locking is , yes, complex.
    Hmm..I don't want to bet we can fix locking scheme without something complex.

    Thanks,
    -Kame



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