Re: [PATCH 3/4] memcg, doc: clarify global vs. limit reclaims

[Johannes Weiner <hannes@cmpxchg.org>]

On Mon, Apr 28, 2014 at 02:26:44PM +0200, Michal Hocko wrote:
> Be explicit about global and hard limit reclaims in our documentation.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
>  Documentation/cgroups/memory.txt | 31 +++++++++++++++++--------------
>  1 file changed, 17 insertions(+), 14 deletions(-)
> 
> diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
> index 4937e6fff9b4..add1be001416 100644
> --- a/Documentation/cgroups/memory.txt
> +++ b/Documentation/cgroups/memory.txt
> @@ -236,23 +236,26 @@ it by cgroup.
>  2.5 Reclaim
>  
>  Each cgroup maintains a per cgroup LRU which has the same structure as
> -global VM. When a cgroup goes over its limit, we first try
> -to reclaim memory from the cgroup so as to make space for the new
> -pages that the cgroup has touched. If the reclaim is unsuccessful,
> -an OOM routine is invoked to select and kill the bulkiest task in the
> -cgroup. (See 10. OOM Control below.)
> -
> -The reclaim algorithm has not been modified for cgroups, except that
> -pages that are selected for reclaiming come from the per-cgroup LRU
> -list.
> -
> -NOTE: Reclaim does not work for the root cgroup, since we cannot set any
> -limits on the root cgroup.
> +global VM. Cgroups can get reclaimed basically under two conditions
> + - under global memory pressure when all cgroups are reclaimed
> +   proportionally wrt. their LRU size in a round robin fashion
> + - when a cgroup or its hierarchical parent (see 6. Hierarchical support)
> +   hits hard limit. If the reclaim is unsuccessful, an OOM routine is invoked
> +   to select and kill the bulkiest task in the cgroup. (See 10. OOM Control
> +   below.)

In the whole hierarchy, not just that cgroup.

> +Global and hard-limit reclaims share the same code the only difference
> +is the objective of the reclaim. The global reclaim aims at balancing
> +zones' watermarks while the limit reclaim frees some memory to allow new
> +charges.

This is a kswapd vs. direct reclaim issue, not global vs. memcg.
Memcg reclaim just happens to be direct reclaim.  Either way, I'd
rather not have such implementation details in the user documentation.

> +NOTE: Hard limit reclaim does not work for the root cgroup, since we cannot set
> +any limits on the root cgroup.

Not sure it's necessary to include this...