[Xen-devel] Re: [PATCH][BALLOON] Fix minimum target

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 28 May 2006, at 03:01, Puthiyaparambil, Aravindh wrote:

> > ballooning on other systems then I think we'll have to wait for
> someone
> > to implement a more dynamic scheme (e.g., by hooking off the
> > OOM/low-memory paths, or by slowly allocating memory only when we can
> > see a reasonable amount of pages available on the free lists).
> From the little code reading I have done, your second option seems
> better. Once the code goes down the OOM path the system is already in a
> shaky unstable state. The balloon driver has to be made to back off way
> before that. I have been meaning to try and look into this. I will try
> and do so.
The two strategies I suggested could broadly be described as 
event-driven quench (kick the balloon driver off some existing or 
modified path in the mm code) or polling based (balloon driver 
periodically evaluates the status of the memory system for itself). 
Either way, the key to getting this to work properly will be to find a 
simple but effective metric for evaluating memory pressure. Something 
that does not stop the balloon driver, or slow it down too much, until 
it gets close to the crunch, but is basically 100% guaranteed to kick 
in before oom.
We do need to implement this without slowing down the balloon driver 
too much. Slowing it down by more than a factor of two, say, might well 
be unacceptable. Even then there will likely be timeouts in xend and 
xm-test that will need adjusting. Still, I think this is quite a 
necessary feature so I suppose we'll see how it turns out when it is 
implemented. I agree that trying out the 'measurement from within the 
balloon driver' approach first makes sense.
 -- Keir


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