unionfs makes -CURRENT very sluggish

Hi,

I tried to compile lib/ncurses through unionfs.  It is utterly slow, no
matter I mount with noatime or not.

Relevant mounts:
% /usr/src on /space/chroot/usr/src (nullfs, local, read-only)
% <above>:/space/chroot/usr/src.union on /space/chroot/usr/src (unionfs, local)

% root:~# chroot /space/chroot
% root:/# cd /usr/src/
% root:/# make buildenv
% # cd lib/ncurses
% # time make

Compile time without noatime:
     5089.47 real      1670.72 user      3364.89 sys

Compile time with noatime:
     5083.22 real      1664.10 user      3369.22 sys

Now if I remove the unionfs mount and just use nullfs:
      255.47 real       150.51 user        83.82 sys


As you can see, most of the time is spent in kernel.  I'm recompiling my
kernel with hwpmc right now in order to profile it.  Any other debug
hint would be welcome.

FWIW, "while sleep 1 ; do date ; done" runs jerkily when unionfs is run:
Tue Apr 22 21:20:06 CEST 2008
Tue Apr 22 21:20:13 CEST 2008
Tue Apr 22 21:20:17 CEST 2008
Tue Apr 22 21:20:22 CEST 2008
Tue Apr 22 21:20:29 CEST 2008

Whereas it runs smoothly without unionfs.

I'll give the result of hwpmc ASAP.

Best regards,
-- 
Jeremie Le Hen
< jeremie at le-hen dot org >< ttz at chchile dot org >