Re: [RFC v3 0/3] vmpressure_fd: Linux VM pressure notifications

[David Rientjes <rientjes@google.com>]

On Fri, 16 Nov 2012, Glauber Costa wrote:

> My personal take:
> 
> Most people hate memcg due to the cost it imposes. I've already
> demonstrated that with some effort, it doesn't necessarily have to be
> so. (http://lwn.net/Articles/517634/)
> 
> The one thing I missed on that work, was precisely notifications. If you
> can come up with a good notifications scheme that *lives* in memcg, but
> does not *depend* in the memcg infrastructure, I personally think it
> could be a big win.
> 

This doesn't allow users of cpusets without memcg to have an API for 
memory pressure, that's why I thought it should be a new cgroup that can 
be mounted alongside any existing cgroup, any cgroup in the future, or 
just by itself.

> Doing this in memcg has the advantage that the "per-group" vs "global"
> is automatically solved, since the root memcg is just another name for
> "global".
> 

That's true of any cgroup.

> I honestly like your low/high/oom scheme better than memcg's
> "threshold-in-bytes". I would also point out that those thresholds are
> *far* from exact, due to the stock charging mechanism, and can be wrong
> by as much as O(#cpus). So far, nobody complained. So in theory it
> should be possible to convert memcg to low/high/oom, while still
> accepting writes in bytes, that would be thrown in the closest bucket.
> 

I'm wondering if we should have more than three different levels.

> Another thing from one of your e-mails, that may shift you in the memcg
> direction:
> 
> "2. The last time I checked, cgroups memory controller did not (and I
> guess still does not) not account kernel-owned slabs. I asked several
> times why so, but nobody answered."
> 
> It should, now, in the latest -mm, although it won't do per-group
> reclaim (yet).
> 

Not sure where that was written, but I certainly didn't write it and it's 
not really relevant in this discussion: memory pressure notifications 
would be triggered by reclaim when trying to allocate memory; why we need 
to reclaim or how we got into that state is tangential.  It certainly may 
be because a lot of slab was allocated, but that's not the only case.

> I am also failing to see how cpusets would be involved in here. I
> understand that you may have free memory in terms of size, but still be
> further restricted by cpuset. But I also think that having multiple
> entry points for this buy us nothing at all. So the choices I see are:
> 

Umm, why do users of cpusets not want to be able to trigger memory 
pressure notifications?

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