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Re: [libvirt] [PATCH 0 of 2] [RFC] Add cgroup manipulation and LXC driver support



On Wed, Oct 01, 2008 at 08:41:19AM +0530, Balbir Singh wrote:
> Dan Smith wrote:
> > 
> > DB> At the same time having the controllers mounted is mandatory for
> > DB> libvirt to work and asking the admin to set things up manually
> > DB> also sucks. So perhaps we'll need to mount them automatically, but
> > DB> make this behaviuour configurable in some way, so admin can
> > DB> override it
> > 
> > Perhaps we can:
> > 
> >  - Have a list of controllers we use (memory and devices so far)
> >  - Create each group in all mounts required to satisfy our necessary
> >    controllers
> >  - Select the appropriate mount when setting a cont.key value
> > 
> 
> I am not sure how libvirt provides thread safety, but I did not see any explicit
> coding for that?

The thread safety model for libvirt has two levels

 - A single virConnectPtr object must only be used by one thread. 
   If you have multiple threads, you must provide each with its
   own conenct object

 - Within a stateless driver (Xen, OpenVZ, Test), there is no shared
   state between virConnectPtr objects, so there are no thread issues
   in this respect

 - With a stateful driver, the libvirtd daemon ensures that only a
   single thread is active at once, so against there are no thread
   issues there either.

Now, in a short while I will be making the daemon fully-multithreaded. When
this happens, the stateful drivers will be required to maintain mutexes for
locking. The locking model wil have 2 levels, one lock over the driver as
a whole. This is held only while acquiring a lock against the object being
modified (eg the virtual domain object).

Each virtual domain, lives in one cgroup, so there is a single virCGroup
object associated with each domain. the virCGroup object state is seflf
contained, so independant virCGroup objects can be accessed concurrently
from multiple threads, without any threads safety issues.

Daniel
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