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

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

ballooning on other systems then I think we'll have to wait for
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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