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Date: Sat, 20 Nov 2004 12:07:51 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Linus Torvalds <torvalds@osdl.org>
CC: Christoph Lameter <clameter@sgi.com>, akpm@osdl.org,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Hugh Dickins <hugh@veritas.com>, linux-mm@kvack.org,
       linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: page fault scalability patch V11 [0/7]: overview
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Linus Torvalds wrote:
> 
> On Fri, 19 Nov 2004, Christoph Lameter wrote:
> 
>>Note that I have posted two other approaches of dealing with the rss problem:
> 
> 
> You could also make "rss" be a _signed_ integer per-thread.
> 
> When unmapping a page, you decrement one of the threads that shares the mm 
> (doesn't matter which - which is why the per-thread rss may go negative), 
> and when mapping a page you increment it.
> 
> Then, anybody who actually wants a global rss can just iterate over
> threads and add it all up. If you do it under the mmap_sem, it's stable,
> and if you do it outside the mmap_sem it's imprecise but stable in the
> long term (ie errors never _accumulate_, like the non-atomic case will 
> do).
> 
> Does anybody care enough? Maybe, maybe not. It certainly sounds a hell of 
> a lot better than the periodic scan.
> 

I think this sounds like it might be a good idea. I prefer it to having
the unbounded error of sloppy rss (as improbable as it may be in practice).

The per thread rss may wrap (maybe not 64-bit counters), but even so,
the summation over all threads should still end up being correct I
think.