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    SubjectRe: [PATCH UPDATED] percpu: use dynamic percpu allocator as the default percpu allocator

    On Wed, 1 Apr 2009, Ingo Molnar wrote:
    > The rest (unannotated variables) is to be assumed "access-rarely" or
    > "we-dont-care", by default. This is actually 95% of the global
    > variables.

    I'm not at all convinced that this is a good option.

    The thing is, things like "read_mostly" or "access_rarely" may talk about
    how we access those individual variables, but you're missing a _huge_
    chunk of the puzzle if you ignore the _correlations_ of those accesses
    with accesses to other variables.

    The thign is, if you have variables 'a' and 'b', and they are always
    accessed together, then it's probably worth it to put them in the same

    And that is true EVEN IF 'b' is basically read-only, but 'a' is always
    written to. Marking 'a' as read-only or read-mostly is actually a bad
    idea, since the only thing it can ever result in is a bigger cache

    The fact that the read-only cacheline has "nicer" behavior is totally
    irrelevant - it's still an extra cacheline that we're accessing. And they
    aren't free, even if read-only cachelines that can be shared across CPU's
    are certainly a lot cheaper than ones that bounce.


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      Last update: 2009-04-01 22:03    [W:0.035 / U:2.336 seconds]
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