[libvirt] anyone implementing host device enumeration?

[David Lively]

On Thu, 2008-10-09 at 18:01 +0100, Daniel P. Berrange wrote:
> On Thu, Oct 09, 2008 at 12:25:18PM -0400, David Lively wrote:
> > Hi Daniel -
> >   I'm implementing virNodeDeviceDef, as suggested, but I just noticed
> > something that really bothers me.  In a nutshell, it's that the current
> > implementation of this scheme unnecessarily increases the (time)
> > complexity of O(1) operations to O(n).
> >   For example, take virDomainGetXMLDesc().  One would think dumping the
> > XML descriptor for a domain object we have in hand would be O(1), but in
> > fact, it's O(n) (where n is the number of domains with the same driver),
> > because qemudDomainDumpXML must find the DomainObjPtr (from which it can
> > get the def).  I suppose this lookup could be made O(log n) if the
> > domains were sorted by UUID on the list, but the fact remains that
> > lookup could be eliminated entirely.
> 
> Sure the lookup algorithm is O(n), but have you actually got real
> world benchmarks showing this is the serious bottleneck? In the XML

Well ... uhh ... no.  :-o

> dump example, the time to lookup the object is likely dwarfed by the
> time to generate the XML doc, or to talk to the QEMU monitor.
> IMHO, optimizing this is overkill. Worst case we can just hash on
> the UUID if it ever provdes a problem.

Okay, I'll buy that.  And (now) I see what you mean about the differing
object lifetime.

... In fact (perhaps I shouldn't be admitting this publicly), your whole
public-obj/Obj scheme now finally makes sense to me!  The public objs
are really just a kind of handle to the private Objs, and they exist
independently of one another.  public objs belong to the apps, private
Objs to the appropriate driver.  And (now that I look at the Xen
driver), I finally understand that there are stateless drivers.

In fact, both the devkit and HAL node-dev drivers need to be stateful
(since the device name isn't a reasonable handle for either devkit or
HAL devices), so I'm creating a virNodeDeviceObj as well as a
virNodeDeviceDef.

Dave