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    SubjectRe: Linux 2.6.29
    On Thu, 26 Mar 2009 17:27:43 -0700 (PDT) Linus Torvalds <torvalds@linux-foundation.org> wrote:

    >
    >
    > On Thu, 26 Mar 2009, Andrew Morton wrote:
    > >
    > > userspace can do it quite easily. Run a self-tuning script after
    > > installation and when the disk hardware changes significantly.
    >
    > Uhhuh.
    >
    > "user space can do it".
    >
    > That's the global cop-out.

    userspace can get closer than the kernel can.

    > The fact is, user-space isn't doing it, and never has done anything even
    > _remotely_ like it.
    >
    > In fact, I claim that it's impossible to do. If you give me a number for
    > the throughput of your harddisk, I will laugh in your face and call you a
    > moron.
    >
    > Why? Because no such number exists. It depends on the access patterns.

    Those access patterns are observable!

    > If
    > you write one large file, the number will be very different (and not just
    > by a few percent) from the numbers of you writing thousands of small
    > files, or re-writing a large database in random order.
    >
    > So no. User space CAN NOT DO IT, and the fact that you even claim
    > something like that shows a distinct lack of thought.

    userspace can get closer. Even if it's asking the user "what sort of
    applications will this machine be running" and then use a set of canned
    tunables based on that.

    Better would be to observe system behaviour, perhaps in real time and
    make adjustments.

    > > Maybe we should set the tunables to 99.9% to make it suck enough to
    > > motivate someone.
    >
    > The only times tunables have worked for us is when they auto-tune.
    >
    > IOW, we don't have "use 35% of memory for buffer cache" tunables, we just
    > dynamically auto-tune memory use. And no, we don't expect user space to
    > run some "tuning program for their load" either.
    >

    This particular case is exceptional - it's just too hard for the kernel
    to be able to predict the future for this one.

    It wouldn't be terribly hard for a userspace daemon to produce better
    results than we can achieve in-kernel. That might of course require
    additional kernel work to support it well.


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