memcg writeback (was Re: [Lsf-pc] [LSF/MM TOPIC] memcg topics.)

[Greg Thelen <gthelen@google.com>]

On Fri, Feb 3, 2012 at 1:40 AM, Wu Fengguang <fengguang.wu@intel.com> wrote:
> If moving dirty pages out of the memcg to the 20% global dirty pages
> pool on page reclaim, the above OOM can be avoided. It does change the
> meaning of memory.limit_in_bytes in that the memcg tasks can now
> actually consume more pages (up to the shared global 20% dirty limit).

This seems like an easy change, but unfortunately the global 20% pool
has some shortcomings for my needs:

1. the global 20% pool is not moderated.  One cgroup can dominate it
    and deny service to other cgroups.

2. the global 20% pool is free, unaccounted memory.  Ideally cgroups only
    use the amount of memory specified in their memory.limit_in_bytes.  The
    goal is to sell portions of a system.  Global resource like the 20% are an
    undesirable system-wide tax that's shared by jobs that may not even
    perform buffered writes.

3. Setting aside 20% extra memory for system wide dirty buffers is a lot of
    memory.  This becomes a larger issue when the global dirty_ratio is
    higher than 20%.

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