[Devel] Re: [PATCH -mmotm 0/5] memcg: per cgroup dirty limit (v7)

[Balbir Singh]

* Andrea Righi <arighi at develer.com> [2010-03-15 00:26:37]:

> 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.

I like that the root overhead is going away.

> 
> 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'll test this, I have a small machine to test on at the moment, I'll
revert back with data. 

-- 
	Three Cheers,
	Balbir
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