umount takes a very long time

[Stefan Bader]

On 07/15/2010 11:38 PM, Daniel Lezcano wrote:
> 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
> 

That sounds a bit like it could be related to the writeback regression that has
been around for a while[1]. Upstream fixed and realized to have broken other
things. It seemed to have reached point of stableness now and I have put
together some test kernels which should get some testing to see whether
a) they fix the slow umount times and
b) they don't break something
You would be welcome to try [2] and report in [1].

-Stefan

[1] https://bugs.launchpad.net/ubuntu/+source/linux/+bug/585092
[2] http://people.canonical.com/~smb/lp585092/