Re: [patch 2/2] htlb forget rss with pt sharing

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Thu, 2006-10-19 at 12:12 -0700, Chen, Kenneth W wrote:
> Imprecise RSS accounting is an irritating ill effect with pt sharing. 
> After consulted with several VM experts, I have tried various methods to
> solve that problem: (1) iterate through all mm_structs that share the PT
> and increment count; (2) keep RSS count in page table structure and then
> sum them up at reporting time.  None of the above methods yield any
> satisfactory implementation.
> 
> Since process RSS accounting is pure information only, I propose we don't
> count them at all for hugetlb page. rlimit has such field, though there is
> absolutely no enforcement on limiting that resource.  One other method is
> to account all RSS at hugetlb mmap time regardless they are faulted or not.
> I opt for the simplicity of no accounting at all.

I do feel I must object to this. Especially with hugetlb getting real
accessible with libhugetlbfs etc., I suspect administrators will shortly
be confused where all their memory went.

Also, like stated earlier, I don't like breaking RSS accounting now, and
when we do have thought up a valid meaning for the field, again. You
state correctly that RLIMIT_RSS is currently not enforced, but its an
active area int that we do want to enforce it in the near future.

I do grant its a very hard problem, comming up with a
valid/meaningfull/workable definition of RSS, but I dislike this opt out
of just not counting it at all - and thereby making the effort of
enforcing RSS harder.

Just my 0.02 eurocent ;-)



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