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    SubjectRe: [patch 0/5] refault distance-based file cache sizing

    On 2012/05/16 14:51, Johannes Weiner wrote:
    > Hi Nai,
    > On Wed, May 16, 2012 at 01:25:34PM +0800, nai.xia wrote:
    >> Hi Johannes,
    >> Just out of curiosity(since I didn't study deep into the
    >> reclaiming algorithms), I can recall from here that around 2005,
    >> there was an(or some?) implementation of the "Clock-pro" algorithm
    >> which also have the idea of "reuse distance", but it seems that algo
    >> did not work well enough to get merged? Does this patch series finally
    >> solve the problem(s) with "Clock-pro" or totally doesn't have to worry
    >> about the similar problems?
    > As far as I understood, clock-pro set out to solve more problems than
    > my patch set and it failed to satisfy everybody.
    > The main error case was that it could not partially cache data of a
    > set that was bigger than memory. Instead, looping over the file
    > repeatedly always has to read every single page because the most
    > recent page allocations would push out the pages needed in the nearest
    > future. I never promised to solve this problem in the first place.
    > But giving more memory to the big looping load is not useful in our
    > current situation, and at least my code protects smaller sets of
    > active cache from these loops. So it's not optimal, but it sucks only
    > half as much :)

    Yep, I see ;)

    > There may have been improvements from clock-pro, but it's hard to get
    > code merged that does not behave as expected in theory with nobody
    > understanding what's going on.
    > My code is fairly simple, works for the tests I've done and the
    > behaviour observed so far is understood (at least by me).

    OK, I assume that you do aware that the system you constructed with
    this simple and understandable idea looks like a so called "feedback
    system"? Or in other words, I think theoretically the refault-distance
    of a page before and after your algorithm is applied is not the same.
    And this changed refault-distance pattern is then feed as input into
    your algorithm. A feedback system may be hard(and may be simple) to
    analyze but may also work well magically.

    Well, again I confess I've not done enough course in this area. Just hope
    that my words can help you think more comprehensively. :)



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