[libvirt] staggered guest startup / startup order

[Daniel P. Berrange]

On Tue, May 26, 2009 at 12:42:47PM -0400, Gerry Reno wrote:
> Just a simple time-based solution would be sufficient.  Right now when 
> multiple guests all try to boot at the same time it takes about 4 times 
> as long for all of them to boot up as when you boot them individually.  
> Apparently too much context switching.  So if we could check off an 
> autoboot flag and then say +120 seconds, that guest would start booting 
> 120 seconds after the autoboot process started.  Even this simple 
> mechansim would help quite a bit.

Now this is a interesting problem :-) It doesn't so much suggest the
provision of guest startup ordering, but rather the serialization of
guest startup - only if there were no other viable solution elsewhere 
though.

I'd be surprised if context switching were the problem & if it was,
then I'd suggest beating up on the kernel guys to fix their schedular.
More likely is competition for disk access, and the parallel guest
startup causing storm of disk seeks, destroying overall I/O throughput.
I would still wonder if this could be solved in the kernel I/O schedular
either automatically, or with libvirt setting some tunable knob to tell
it to try and batch up I/O for the VM to avoid so much seeking. Of course
this is all conjecture without profiling / benchmarks to identify just
where the problem is.

Regards,
Daniel
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