userland access to devices is moving!

From: Poul-Henning Kamp (phk_at_phk.freebsd.dk)
Date: 06/18/03

  • Next message: Julian Elischer: "Re: userland access to devices is moving!" 
    To: arch@freebsd.org
Date: Wed, 18 Jun 2003 17:35:19 +0200

    I sat down and hacked up a simple prototype to test the concept I
    have been rambling about for some years: Going directly from
    filedescriptor to device driver thus bypassing the vnode, devfs and
    specfs layer.

    I implemented this for /dev/null and /dev/zero, and ran the simple
    benchmark "dd if=/dev/zero of=/dev/null count=1000000"

    Before:
    N 3 Average: 44.900752667 Stddev: 0.049906338

    After:
    N 3 Average: 18.460190333 Stddev: 0.074019507

    That is 26.4 microseconds saved for each read(2)+write(2) operation,
    or 41% improvement on my Athlon 700MHz machine.

    A bit more locking will probably be needed, so this will erode some
    of this number, but there will be something left I'm sure :-)

    The largest impact of this is that VOP_OPEN(), vn_open() and
    vn_open_cred() grows an argument (the fdesc index) which existing
    callers need to pass a -1, the rest is relatively local hacking in
    devfs and some adjustments in the descriptor code.

    I have overall found that the implementation of this is not as hard
    as I imagined, and if I doubted it before, I am now certain that
    this is the right way to go.

    I should have tried this long time ago...

    patch at:
            http://phk.freebsd.dk/patch/fdesc.patch 

    -- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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  • Next message: Julian Elischer: "Re: userland access to devices is moving!"