umount takes a very long time

[Daniel Lezcano]

Hi All,

I am using the system containers with lxc. At startup time, lxc does 
pivot_root and umount all the mount points belonging to the old rootfs.

I noticed the unmounting time is very long on the 2.6.32-23-generic 
kernel compared with a 2.6.34 or older kernel version. On a 2.6.34 
kernel, the container starts in 100ms, on the 2.6.23-23-generic it takes 
more than 12 seconds. On my testing I am trying to benchmark 1024 ubuntu 
containers but I can not do that with the ubuntu 10.04 kernel.

Is it a bug ? Or is there any sysctl I can change to improve the umount 
time ?

Thanks in advance
   -- Daniel