Re: cgroups: Assigning tasks to a cgroup slow and does not scale

[Mike Galbraith <efault@gmx.de>]

On Sat, 2011-06-04 at 10:23 +0200, Martin Baum wrote:
> Hello,
> 
> assigning a task to new cgroup takes an unexpected high amount of time  
> on my system - about 0.05s. Strangely it does it does not scale with  
> more processes: When I run 10 processes that do nothing but change  
> their cgroups each cgroup change takes about 0.5 seconds while CPU  
> usage remains very low. So my system maxes out at 20 cgroup changes  
> per second while cpu and disk usage remain minimal (according to top).  
> Is this expected behaviour?

Attaching a task invokes synchronize_rcu() along the way, so yup, it's
the expected behavior.

> It is a problem for me: I run a daemon that forks and calls suid for  
> each request. I want to assign each of these forks to a different  
> cgroup. However this currently kills performance going down from >100  
> req/sec to about 20 req/sec.

You can work around it via static cgroups with an agent waiting inside
each one.  Tell your agent what to do, he forks off a worker.  You can
inherit a cgroup quickly, but moving between cgroups is very expensive,
as you noticed.

(right way is eventually stop exporting rcu grace periods to userland)

	-Mike