Re: [patch 1/7 -mm] oom: filter tasks not sharing the same cpuset

[David Rientjes]

On Tue, 16 Feb 2010, Nick Piggin wrote:

> Yes we do need to explain the downside of the patch. It is a
> heuristic and we can't call either approach perfect.
> 
> The fact is that even if 2 tasks are on completely disjoint
> memory policies and never _allocate_ from one another's nodes,
> you can still have one task pinning memory of the other task's
> node.
> 
> Most shared and userspace-pinnable resources (pagecache, vfs
> caches and fds files sockes etc) are allocated by first-touch
> basically.
> 
> I don't see much usage of cpusets and oom killer first hand in
> my experience, so I am happy to defer to others when it comes
> to heuristics. Just so long as we are all aware of the full
> story :)
> 

Unless you can present a heuristic that will determine how much memory 
usage a given task has allocated on nodes in current's zonelist, we must 
exclude tasks from cpusets with a disjoint set of nodes, otherwise we 
cannot determine the optimal task to kill.  There's a strong possibility 
that killing a task on a disjoint set of mems will never free memory for 
current, making it a needless kill.  That's a much more serious 
consequence than not having the patch, in my opinion, than rather simply 
killing current.
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