Re: [RFC] memory cgroup: my thoughts on memsw

[Vladimir Davydov <vdavydov@parallels.com>]

Hi Johannes,

On Mon, Sep 15, 2014 at 03:14:35PM -0400, Johannes Weiner wrote:
> > Finally, my understanding (may be crazy!) how the things should be
> > configured. Just like now, there should be mem_cgroup->res accounting
> > and limiting total user memory (cache+anon) usage for processes inside
> > cgroups. This is where there's nothing to do. However, mem_cgroup->memsw
> > should be reworked to account *only* memory that may be swapped out plus
> > memory that has been swapped out (i.e. swap usage).
> 
> But anon pages are not a resource, they are a swap space liability.
> Think of virtual memory vs. physical pages - the use of one does not
> necessarily result in the use of the other.  Without memory pressure,
> anonymous pages do not consume swap space.
> 
> What we *should* be accounting and limiting here is the actual finite
> resource: swap space.  Whenever we try to swap a page, its owner
> should be charged for the swap space - or the swapout be rejected.

I've been thinking quite a bit on the problem, and finally I believe
you're right: a separate swap limit would be better than anon+swap.

Provided we make the OOM-killer kill cgroups that exceed their soft
limit and can't be reclaimed, it will solve the problem with soft limits
I described above.

Besides, comparing to anon+swap, swap limit would be more efficient (we
only need to charge one res counter, not two) and understandable to
users (it's simple to setup a limit for both kinds of resources then,
because they never mix).

Finally, we could transfer user configuration from cgroup v1 to v2
easily: just setup swap.limit to be equal to memsw.limit-mem.limit; it
won't be exactly the same, but I bet nobody will notice any difference.

So, at least for now, I vote for moving from mem+swap to swap
accounting.

Thanks,
Vladimir