Xen devel - Sep 2007 - Starting many VMs concurrently causes vif connect errors

Until now, we have been starting a large number of VMs sequentially
using a simple
    "for i in vmcfg* ; do xm create $i; done"
and this has the effect of starting VMs sequentially but with some
concurrency. The domain structure is created and unpaused, and the "xm
create completes". While the next iteration of the loop is running "xm
create" again, xen is busy scheduling and running any started domains,
which each boot their OS.
 
Since the total elapsed time to start a large number of VMs in this way
is significant, we looked for another way to start them. If we change
the script to:
    "for i in vmcfg* ; do xm create $i & done"
then many background tasks are spawned, each running "xm create", and
this has an interesting effect. When we monitor with "xm top", we
notice
that all the domain structures are created and completed first, then xen
begins scheduling the domains and they boot. This works well for 50 VMs.
 
However, when I try to start 100 VMs with this script, all the domain
structures get created, but then all of them are dismantled and removed,
and each one reports "Error: Device 0 (vif) could not be connected.
Hotplug scripts not working".
 
I''ve verified that, in fact, networking is still working, because after
all the VMs are destroyed, I can then start one or two without error.
 
Any idea what''s breaking? Should I try changing the value of
DEVICE_CREATE_TIMEOUT in tools/python/xen/xend/server/DevController.py?
If so, what would I have to rebuild to use the changed file? Or is there
some networking component that gets overloaded? I have no problem
starting the 100 VMs sequentially.

brian carb
unisys corporation - malvern, pa



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