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SubjectRe: [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v7)
On Mon, 15 Mar 2010 00:26:37 +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 (v6 -> v7)
> ~~~~~~~~~~~~~~~~~~~~~~
> * introduce trylock_page_cgroup() to guarantee that lock_page_cgroup()
> is never called under tree_lock (no strict accounting, but better overall
> performance)
> * do not account file cache statistics for the root cgroup (zero
> overhead for the root cgroup)
> * fix: evaluate cgroup free pages as at the minimum free pages of all
> its parents
>
> Results
> ~~~~~~~
> The testcase is a kernel build (2.6.33 x86_64_defconfig) on a Intel Core 2 @
> 1.2GHz:
>
> <before>
> - root cgroup: 11m51.983s
> - child cgroup: 11m56.596s
>
> <after>
> - root cgroup: 11m51.742s
> - child cgroup: 12m5.016s
>
> In the previous version of this patchset, using the "complex" locking scheme
> with the _locked and _unlocked version of mem_cgroup_update_page_stat(), the
> child cgroup required 11m57.896s and 12m9.920s with lock_page_cgroup()+irq_disabled.
>
> With this version there's no overhead for the root cgroup (the small difference
> is in error range). I expected to see less overhead for the child cgroup, I'll
> do more testing and try to figure better what's happening.
>
Okay, thanks. This seems good result. Optimization for children can be done under
-mm tree, I think. (If no nack, this seems ready for test in -mm.)

> In the while, it would be great if someone could perform some tests on a larger
> system... unfortunately at the moment I don't have a big system available for
> this kind of tests...
>
I hope, too.

Thanks,
-Kame



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