[Devel] Re: [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v6)

[KAMEZAWA Hiroyuki]

On Wed, 10 Mar 2010 00:00:31 +0100
Andrea Righi <arighi at 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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