On Wed, Nov 21, 2012 at 12:27:28PM +0400, Glauber Costa wrote:

On 11/20/2012 10:23 PM, David Rientjes wrote:

quoted

Anton can correct me if I'm wrong, but I certainly don't think this is 
where mempressure is headed: I don't think any accounting needs to be done

Yup, I'd rather not do any accounting, at least not in bytes.

quoted

and, if it is, it's a design issue that should be addressed now rather 
than later.  I believe notifications should occur on current's mempressure 
cgroup depending on its level of reclaim: nobody cares if your memcg has a 
limit of 64GB when you only have 32GB of RAM, we'll want the notification.

My main concern is that to trigger those notifications, one would have
to first determine whether or not the particular group of tasks is under
pressure.

As far as I understand, the notifications will be triggered by a process
that tries to allocate memory. So, effectively that would be a per-process
pressure.

So, if one process in a group is suffering, we notify that "a process in a
group is under pressure", and the notification goes to a cgroup listener

And to do that, we need to somehow know how much memory we are
using, and how much we are reclaiming, etc. On a system-wide level, we
have this information. On a grouplevel, this is already accounted by memcg.

In fact, the current code already seems to rely on memcg:

+	vmpressure(sc->target_mem_cgroup,
+		   sc->nr_scanned - nr_scanned, nr_reclaimed);

Well, I'm yet unsure about the details, but I guess in "mempressure"
cgroup approach, this will be derived from the current->, i.e. a task.

But note that we won't report pressure to a memcg cgroup, we will notify
only mempressure cgroup. But a process can be in both of them
simultaneously. In the code, the mempressure and memcg will not depend on
each other.

Now, let's start simple: Assume we will have a different cgroup.
We want per-group pressure notifications for that group. How would you
determine that the specific group is under pressure?

If a process that tries to allocate memory & causes reclaim is a part of
the cgroup, then cgroup has a pressure.

At least that's very brief understanding of the idea, details to be
investigated... But I welcome David to comment whether I got everything
correctly. :)

Thanks,
Anton.

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