userland access to devices is moving!

[Poul-Henning Kamp]

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 at 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.