umount takes a very long time

[Daniel Lezcano]

On 07/16/2010 09:28 AM, Stefan Bader wrote:
> 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
>    

Ok, good to know, thanks for the info.

> You would be welcome to try [2] and report in [1].
>    

Will do :)

Thanks
   -- Daniel