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    SubjectRe: Thread implementations...
    Linus Torvalds writes:
    > In article <19980625161310.B22513@caffeine.ix.net.nz>,
    > Chris Wedgwood <chris@cybernet.co.nz> wrote:
    > >On Wed, Jun 24, 1998 at 10:13:57PM +1000, Richard Gooch wrote:
    > >
    > >> If we get madvise(2) right, we don't need sendfile(2), correct?
    > >
    > >It would probably suffice. In fact, having a working implementation of
    > >madvise, etc. would make sendfile pretty trivial to do in libc. (Again, I
    > >assuming that whether or not we need it, if it can be implemented in
    > >userspace then why not...)
    >
    > However, the thing to notice is that a "sendfile()" system call can
    > potentially be a lot faster than anything else. In particular, it can
    > be as clever as it wants about sending stuff directly from kernel
    > buffers etc.
    >
    > I know there are a lot of people who think zero-copying is cool, and
    > that tricks with mmap() etc can be used to create zero-copy. But don't
    > forget that it's a major mistake to think that performance is about
    > whether the algorithm is O(1) or O(n) or O(n^2). People tend to forget
    > the constant factor, and look blindly at other things.
    >
    > In particular, doing a mmap() itself is fairly expensive. It implies a
    > lot of bookkeeping, and it also implies a fair amount of mucking around
    > with CPU VM issues (TLBs, page tables etc). In short, it can be rather
    > expensive.
    >
    > Due to that expense, things that use mmap() often have a "cache" of
    > mappings that they have active. Thet gets rid of one expense, but then
    > there is the new expense of maintaining that cache (and it can be a
    > fairly costly thing to maintain if you want to doa threaded webserver).
    >
    > In contrast, a "sendfile()" approach can be extremely light-weight, and
    > threads much better because it doesn't imply the same kinds of
    > maintenance.
    >
    > Now, I'm no NT person, but I suspect that we actually do want to have a
    > "sendfile()" kind of thing just because it should be fairly easy to
    > implement, and would offer some interesting performance advantages for
    > some cases. No, it's not truly generic, but it is useful enough in many
    > circustances.

    Well, that's fine. I just hope that we get a better madvise(2) at some
    point too.

    Regards,

    Richard....

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