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    SubjectRe: all syscalls initially taking 4usec on a P4? Re: nonblocking UDPv4 recvfrom() taking 4usec @ 3GHz?
    On Sun, Feb 25, 2007 at 11:41:54AM +0100, Pavel Machek (pavel@ucw.cz) wrote:
    > > > I've done so, with some interesting results. Source on
    > > > http://ds9a.nl/tmp/recvtimings.c - be careful to adjust the '3000' divider
    > > > to your CPU frequency if you care about absolute numbers!
    > > >
    > > > These are two groups, each consisting of 10 consecutive nonblocking UDP
    > > > recvfroms, with 10 packets preloaded. Reported is the number of microseconds
    > > > per recvfrom call which yielded a packet:
    > > >
    > > > $ ./recvtimings
    > > > 4.142333
    > >
    > > It can be recvfrom only problem - syscall overhead on my p4 (core duo,
    > > debian testing) is bout 300 usec - to test I ran
    >
    > core duo is _not_ p4 class cpu; rsulets there will be very different.

    Results nevertheless are the same.
    Each syscall takes some time first (noticebly more than subsequent
    calls), and that was a main problem for Bert.
    Given the high load, recvfrom() can even take tens of microseconds
    (although I can not provide a profile output yet, but I showed a data).

    So, syscall overhead itself is very small no matter which type of the
    CPU is used - athlon is about 300 nsec, via epia about 1.4 usec),
    but the whole function can take quite a lot of time.

    > Pavel
    >
    > --
    > (english) http://www.livejournal.com/~pavelmachek
    > (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

    --
    Evgeniy Polyakov
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