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    SubjectRe: [PATCH] Cleanup and kernelify shrinker registration (rc5-mm2)
    On Tue, 03 Apr 2007 15:47:05 +1000 Rusty Russell <rusty@rustcorp.com.au> wrote:

    > On Mon, 2007-04-02 at 21:57 -0700, Andrew Morton wrote:
    > > On Tue, 03 Apr 2007 14:45:02 +1000 Rusty Russell <rusty@rustcorp.com.au> wrote:
    > > > Does that mean the to function correctly every user needs some internal
    > > > cursor so it doesn't end up scanning the first N entries over and over?
    > > >
    > >
    > > If it wants to be well-behaved, and to behave as the VM expects, yes.
    > >
    > > There's an expectation that the callback will be performing some scan-based
    > > aging operation and of course to do LRU (or whatever) aging, the callback
    > > will need to remember where it was up to last time it was called.
    > >
    > > But it's just a guideline - callbacks could do something different but
    > > in-the-spirit, I guess.
    >
    > Hmm, actually the callers I looked at (nfs, dcache, mbcache) seem to use
    > an LRU list and just walk the first "nr_to_scan" entries, and nr_to_scan
    > is always 128.

    That's just because of the batching logic up in shrink_slab(). And iirc we
    only break the scanning into lumps of 128 items so we can add a
    cond_resched() into it.

    > Someone who keeps a cursor will be disadvantaged: the other shrinkers
    > could well get less effective on repeated calls, but we won't. Someone
    > who picks entries at random might have the same issue.

    To examine the balancing one would need to examine the value of total_scan
    in shrink_slab(), rather than looking at the value which shrink_slab()
    passes into the callback.

    > I think it is clearest to describe how we expect everyone to work, and
    > let whoever is getting creative worry about it themselves.
    >
    > How's this:
    > ==
    > Cleanup and kernelify shrinker registration.

    hm, well, six-of-one, VI of the other. We save maybe four kmallocs across
    the entire uptime at the cost of exposing stuff kernel-side which doesn't
    need to be exposed.

    But I think we need to weed that crappiness out of XFS first.

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