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SubjectRe: Thread implementations...
Date
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.

Linus

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