Re: [PATCH v4] vmevent: Implement greater-than attribute state andone-shot mode

[Anton Vorontsov]

Hello Rik,

Thanks for looking into this!

On Tue, May 01, 2012 at 05:04:21PM -0400, Rik van Riel wrote:
> On 05/01/2012 09:18 AM, Anton Vorontsov wrote:
> >This patch implements a new event type, it will trigger whenever a
> >value becomes greater than user-specified threshold, it complements
> >the 'less-then' trigger type.
> >
> >Also, let's implement the one-shot mode for the events, when set,
> >userspace will only receive one notification per crossing the
> >boundaries.
> >
> >Now when both LT and GT are set on the same level, the event type
> >works as a cross event type: it triggers whenever a value crosses
> >the threshold from a lesser values side to a greater values side,
> >and vice versa.
> >
> >We use the event types in an userspace low-memory killer: we get a
> >notification when memory becomes low, so we start freeing memory by
> >killing unneeded processes, and we get notification when memory hits
> >the threshold from another side, so we know that we freed enough of
> >memory.
> 
> How are these vmevents supposed to work with cgroups?

Currently these are independent subsystems, if you have memcg enabled,
you can do almost anything* with the memory, as memg has all the needed
hooks in the mm/ subsystem (it is more like "memory management tracer"
nowadays :-).

But cgroups have its cost, both performance penalty and memory wastage.
For example, in the best case, memcg constantly consumes 0.5% of RAM to
track memory usage, this is 5 MB on a 1 GB "embedded" machine.  To some
people it feels just wrong to waste that memory for mere notifications.

Of course, this alone can be considered as a lame argument for making
another subsystem (instead of "fixing" the current one). But see below,
vmevent is just a convenient ABI.

> What do we do when a cgroup nears its limit, and there
> is no more swap space available?
> 
> What do we do when a cgroup nears its limit, and there
> is swap space available?

As of now, this is all orthogonal to vmevent. Vmevent doesn't know
about cgroups. If kernel has the memcg enabled, one should probably*
go with it (or better, with its ABI). At least for now.

> It would be nice to be able to share the same code for
> embedded, desktop and server workloads...

It would be great indeed, but so far I don't see much that
vmevent could share. Plus, sharing the code at this point is not
that interesting; it's mere 500 lines of code (comparing to
more than 10K lines for cgroups, and it's not including memcg_
hooks and logic that is spread all over mm/).

Today vmevent code is mostly an ABI implementation, there is
very little memory management logic (in contrast to the memcg).

Personally, I would rather consider sharing ABI at some point:
i.e. making a memcg backend for the vmevent. That would be pretty
cool. And once done, vmevent would be cgroups-aware (if memcg
enabled, of course; and if not, vmevent would still work, with
no memcg-related expenses).

* For low memory notifications, there are still some unresolved
  issues with memcg. Mainly, slab accounting for the root cgroup:
  currently developed slab accounting doesn't account kernel's
  internal memory consumption, plus it doesn't account slab memory
  for the root cgroup at all.

  A few days ago I asked[1] why memcg doesn't do all this, and
  whether it is a design decision or just an implementation detail
  (so that we have a chance to fix it).

  But so far there were no feedback. We'll see how things turn out.

  [1] http://lkml.org/lkml/2012/4/30/115


Thanks!

-- 
Anton Vorontsov
Email: cbouatmailru@xxxxxxxxx
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