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    SubjectRe: Getting FS access events
    Linus Torvalds writes:
    >
    > On Mon, 14 May 2001, Richard Gooch wrote:
    > >
    > > Is there some fundamental reason why a buffer cache can't ever be
    > > fast?
    >
    > Yes.
    >
    > Or rather, there is a fundamental reason why we must NEVER EVER look at
    > the buffer cache: it is not coherent with the page cache.
    >
    > And keeping it coherent would be _extremely_ expensive. How do we
    > know? Because we used to do that. Remember the small mindcraft
    > benchmark? Yup. Double copies all over the place, double lookups, double
    > everything.
    >
    > You could think: "oh, we only need to look up the buffer cache when we
    > create a new page cache mapping, so..".
    >
    > You'd be wrong. We'd need to go the other way too: every time we create a
    > new buffer cache entry, we'd need to make sure that it isn't mapped
    > somewhere in the page cache (impossible), or otherwise we'd do the wrong
    > thing sometimes (ie we might have two dirty copies, and we wouldn't know
    > _which_ one is valid etc).
    >
    > Aliasing is bad. Don't do it.

    OK, this (combined with the other message) explains why we want to
    keep away from the buffer cache. Thanks.

    > You know, the mark of intelligence is realizing when you're making
    > the same mistake over and over and over again, and not hitting your
    > head in the wall five hundred times before you understand that it's
    > not a clever thing to do.

    But you didn't have to add this. Please note that I asked why not use
    the buffer cache. I didn't proclaim that it was the ideal solution. I
    did say what benefits it had, but I didn't assert that the benefits
    outweighed the disadvantages.

    > Please show some intelligence.

    Well, frankly, I think I have. Things are obvious when you know them
    already. Even if I'm ignorant, I'm not stupid!

    Regards,

    Richard....
    Permanent: rgooch@atnf.csiro.au
    Current: rgooch@ras.ucalgary.ca
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