Re: [RFC] Add mempressure cgroup

[Anton Vorontsov <anton.vorontsov@linaro.org>]

On Wed, Nov 28, 2012 at 05:27:51PM -0800, Anton Vorontsov wrote:
> On Wed, Nov 28, 2012 at 03:14:32PM -0800, Andrew Morton wrote:
> [...]
> > Compare this with the shrink_slab() shrinkers.  With these, the VM can
> > query and then control the clients.  If something goes wrong or is out
> > of balance, it's the VM's problem to solve.
> > 
> > So I'm thinking that a better design would be one which puts the kernel
> > VM in control of userspace scanning and freeing.  Presumably with a
> > query-and-control interface similar to the slab shrinkers.
> 
> Thanks for the ideas, Andrew.
> 
> Query-and-control scheme looks very attractive, and that's actually
> resembles my "balance" level idea, when userland tells the kernel how much
> reclaimable memory it has. Except the your scheme works in the reverse
> direction, i.e. the kernel becomes in charge.
> 
> But there is one, rather major issue: we're crossing kernel-userspace
> boundary. And with the scheme we'll have to cross the boundary four times:
> query / reply-available / control / reply-shrunk / (and repeat if
> necessary, every SHRINK_BATCH pages). Plus, it has to be done somewhat
> synchronously (all the four stages), and/or we have to make a "userspace
> shrinker" thread working in parallel with the normal shrinker, and here,
> I'm afraid, we'll see more strange interactions. :)
> 
> But there is a good news: for these kind of fine-grained control we have a
> better interface, where we don't have to communicate [very often] w/ the
> kernel. These are "volatile ranges", where userland itself marks chunks of
> data as "I might need it, but I won't cry if you recycle it; but when I
> access it next time, let me know if you actually recycled it". Yes,
> userland no longer able to decide which exact page it permits to recycle,
> but we don't have use-cases when we actually care that much. And if we do,
> we'd rather introduce volatile LRUs with different priorities, or
> something alike.
> 
> So, we really don't need the full-fledged userland shrinker, since we can
> just let the in-kernel shrinker do its job. If we work with the
> bytes/pages granularity it is just easier (and more efficient in terms of
> communication) to do the volatile ranges.
> 
> For the pressure notifications use-cases, we don't even know bytes/pages
> information: "activity managers" are separate processes looking after
> overall system performance.
> 
> So, we're not trying to make userland too smart, quite the contrary: we
> realized that for this interface we don't want to mess with the bytes and
> pages, and that's why we cut this stuff down to only three levels. Before
> this, we were actually trying to count bytes, we did not like it and we
> ran away screaming.
> 
> OTOH, your scheme makes volatile ranges unneeded, since a thread might
> register a shrinker hook and free stuff by itself. But again, I believe
> this involves more communication with the kernel.

Btw, I believe your idea is something completely new, and I surely cannot
fully evaluate it on my own -- I might be wrong here. So I invite folks to
express their opinions too.

Guys, it's about Andrew's idea of exposing shrinker-alike logic to the
userland (and I made it 'vs. volatile ranges'):

	http://lkml.org/lkml/2012/11/28/607

Thanks,
Anton.

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