[Devel] Re: Linux-VServer example results for sharing vs. separate mappings ...

[Andrew Morton]

On Sun, 25 Mar 2007 15:20:35 +0530 Balbir Singh <balbir at in.ibm.com> wrote:

> Andrew Morton wrote:
> <snip>
> > The problem is memory reclaim.  A number of schemes which have been
> > proposed require a per-container page reclaim mechanism - basically a
> > separate scanner.
> > 
> > This is a huge, huge, huge problem.  The present scanner has been under
> > development for over a decade and has had tremendous amounts of work and
> > testing put into it.  And it still has problems.  But those problems will
> > be gradually addressed.
> > 
> > A per-container recaim scheme really really really wants to reuse all that
> > stuff rather than creating a separate, parallel, new scanner which has the
> > same robustness requirements, only has a decade less test and development
> > done on it.  And which permanently doubles our maintenance costs.
> > 
> 
> The current per-container reclaim scheme does reuse a lot of code. As far
> as code maintenance is concerned, I think it should be easy to merge
> some of the common functionality by abstracting them out as different
> functions. The container smartness comes in only in the
> container_isolate_pages(). This is an easy to understand function.

err, I think I'd forgotten about container_isolate_pages().  Yes, that
addresses my main concern.

> > So how do we reuse our existing scanner?  With physical containers.  One
> > can envisage several schemes:
> > 
> > a) slice the machine into 128 fake NUMA nodes, use each node as the
> >    basic block of memory allocation, manage the binding between these
> >    memory hunks and process groups with cpusets.
> > 
> >    This is what google are testing, and it works.
> 
> Don't we break the global LRU with this scheme?

Sure, but that's deliberate!

(And we don't have a global LRU - the LRUs are per-zone).

> > 
> > b) Create a new memory abstraction, call it the "software zone", which
> >    is mostly decoupled from the present "hardware zones".  Most of the MM
> >    is reworked to use "software zones".  The "software zones" are
> >    runtime-resizeable, and obtain their pages via some means from the
> >    hardware zones.  A container uses a software zone.
> > 
> 
> I think the problem would be figuring out where to allocate memory from?
> What happens if a software zone spans across many hardware zones?

Yes, that would be the tricky part.  But we generally don't care what
physical zone user pages come from, apart from NUMA optimisation.

> The reclaim mechanism proposed *does not impact the non-container users*.

Yup.  Let's keep plugging away with Pavel's approach, see where it gets us.

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