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    SubjectRe: mmap() versus read()


    On Mon, 9 Mar 1998, Richard Gooch wrote:

    > The syscall overhead is peanuts. It's more like 10 000 times faster
    > than a disc seek. The syscall overhead is simply not worth
    > considering.

    It's 10k times faster sure if you *have to go to disk*. What happens when
    I build a 512Mb web server with only 256Mb of active content? Suddenly
    I'm paying for per-request syscalls I don't need. I know, it's all about
    trade-offs :)

    I could show you a beautiful trace of apache 1.3 hacked up a bit in which
    it served 75 requests in something like 20 system calls. This means there
    are no per-request syscalls required currently... There's a few
    connection setup and teardown calls, and the rest were writev()s or
    write()s -- the client was http/1.1 pipelining, so I'm "cheating"... but
    pipelining clients are here already and 6 months to a year from now they
    should be common.

    The reason I started this thread is that I've got a 128Mb server with
    about 150Mb of active data... so on it yup extra syscalls would be no
    problem at all. As an application programmer though, I'm not sure how I
    can discern the two cases efficiently.

    Incidentally, benchmarks, and most high volume sites I've dealt with fall
    into the first category -- working sets are smaller than RAM. I'd crank
    up the memory on my 128Mb box... except it's a Tyan Tomcat III and it's
    snobbish about the RAM it will talk with.

    Dean



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