[Devel] Re: memrlimit controller merge to mainline

[Hugh Dickins]

On Fri, 25 Jul 2008, Paul Menage wrote:
> On Fri, Jul 25, 2008 at 5:06 AM, Hugh Dickins <hugh at veritas.com> wrote:
> >
> > (Different topic, but one day I ought to get around to saying again
> > how absurd I think a swap controller; whereas a mem+swap controller
> > makes plenty of sense.  I think Rik and others said the same.)
> 
> Agreed that a swap controller without a memory controller doesn't make
> much sense, but a memory controller without a swap controller can make
> sense on machines that don't intend to use swap.

I agree that a memory controller without a swap controller can
make sense: I hope so, anyway, since that's what's in mainline.
Even if swap is used, memory is a more precious resource than swap,
and you were right to go about controlling memory first.

> 
> So if they were separate controllers, we'd use the proposed cgroup
> dependency features to make the swap controller depend on the memory
> controller - in which case you'd only be able to mount the swap
> controller on a hierarchy that also had the memory controller, and the
> swap controller would be able to make use of the page ownership
> information.
> 
> It's more of a modularity issue than a functionality issue, I think -
> the swap controller and memory controller are tracking fundamentally
> different things (space on disk versus pages in memory), and the only
> dependency between them is the "memory controller" tracking the
> ownership of a page and providing it to the "swap controller".

It sounds as if you're interpreting my "mem+swap controller" as a
mem controller and a swap controller and the swap controller makes
use of some of the mem controller infrastructure.

No, I'm trying to say something stronger than that.  I'm saying,
as I've said before, that I cannot imagine why anyone would want
to control swap itself - what they want to control is the total
of mem+swap.  Swap is a second-class citizen, nobody wants swap
if they can have mem, so why control it separately?

IIRC Rik expressed the same by pointing out that a cgroup at its
swap limit would then be forced to grow in mem (until it hits its
mem limit): so controlling the less precious resource would increase
pressure on the more precious resource.  (Actually, that probably
bears little relation to what he said - sorry, Rik!)  I don't recall
what answer he got, perhaps I'd be persuaded if I heard it again.

Hugh
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