On Wed, Feb 8, 2012 at 1:31 AM, Wu Fengguang [off-list ref] wrote:

On Tue, Feb 07, 2012 at 11:55:05PM -0800, Greg Thelen wrote:

quoted

On Fri, Feb 3, 2012 at 1:40 AM, Wu Fengguang [off-list ref] wrote:

quoted

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.

It is moderated by balance_dirty_pages() -- in terms of dirty ratelimit.
And you have the freedom to control the bandwidth allocation with some
async write I/O controller.

Even though there is no direct control of dirty pages, we can roughly
get it as the side effect of rate control. Given

       ratelimit_cgroup_A = 2 * ratelimit_cgroup_B

There will naturally be more dirty pages for cgroup A to be worked by
the flusher. And the dirty pages will be roughly balanced around

       nr_dirty_cgroup_A = 2 * nr_dirty_cgroup_B

when writeout bandwidths for their dirty pages are equal.

quoted

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.

Right, it is the shortcoming.

quoted

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%.

Yeah the global pool scheme does mean that you'd better allocate at
most 80% memory to individual memory cgroups, otherwise it's possible
for a tiny memcg doing dd writes to push dirty pages to global LRU and
*squeeze* the size of other memcgs.

However I guess it should be mitigated by the fact that

- we typically already reserve some space for the root memcg

Can you give more details on that? AFAIK, we don't treat root cgroup
differently than other sub-cgroups, except root cgroup doesn't have
limit.

In general, I don't like the idea of shared pool in root for all the
dirty pages.

Imagining a system which has nothing running under root and every
application runs within sub-cgroup. It is easy to track and limit each
cgroup's memory usage, but not the pages being moved to root. We have
been experiencing difficulties of tracking pages being re-parented to
root, and this will make it even harder.

--Ying

- 20% dirty ratio is mostly an overkill for large memory systems.
 It's often enough to hold 10-30s worth of dirty data for them, which
 is 1-3GB for one 100MB/s disk. This is the reason vm.dirty_bytes is
 introduced: someone wants to do some <1% dirty ratio.

Thanks,
Fengguang

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