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    SubjectRe: remove zero_page (was Re: -mm merge plans for 2.6.24)
    On Tue, 9 Oct 2007, Nick Piggin wrote:
    >
    > The commit b5810039a54e5babf428e9a1e89fc1940fabff11 contains the note
    >
    > A last caveat: the ZERO_PAGE is now refcounted and managed with rmap
    > (and thus mapcounted and count towards shared rss). These writes to
    > the struct page could cause excessive cacheline bouncing on big
    > systems. There are a number of ways this could be addressed if it is
    > an issue.
    >
    > And indeed this cacheline bouncing has shown up on large SGI systems.
    > There was a situation where an Altix system was essentially livelocked
    > tearing down ZERO_PAGE pagetables when an HPC app aborted during startup.
    > This situation can be avoided in userspace, but it does highlight the
    > potential scalability problem with refcounting ZERO_PAGE, and corner
    > cases where it can really hurt (we don't want the system to livelock!).
    >
    > There are several broad ways to fix this problem:
    > 1. add back some special casing to avoid refcounting ZERO_PAGE
    > 2. per-node or per-cpu ZERO_PAGES
    > 3. remove the ZERO_PAGE completely
    >
    > I will argue for 3. The others should also fix the problem, but they
    > result in more complex code than does 3, with little or no real benefit
    > that I can see. Why?

    Sorry, I've no useful arguments to add (and my testing was too much
    like yours to add any value), but I do want to go on record as still
    a strong supporter of approach 3 and your patch.

    Hugh
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