[Xen-users] network CPU consumption question

[Tim Freeman <tfreeman@xxxxxxxxxxx>]

Running into a mysterious situation here... 

I have a Xen3 setup running SEDF measuring bandwidth offnode on a
non-VMM machine. Running one guest domain bridged through domain
0, transfers from memory (/dev/zero) to the non-VMM machine.  Another
domain runs pure CPU bound activity, as much as it can (this avoids the
non-interrupt-batching context-switching-heavy situation that exists
otherwise).

On these computers, getting roughly 12.3 MB/s with this distribution
of CPU consumption (measured with XenMon): 

dom0: 17.79%, dom1 (net): 11.41%, dom2 (cpu): 69.48% 

(these are means from many runs)

I get an extremely similar bandwidth reading in another scenario with
two net transferring domains (their bandwidths are halved, talking
about the *sum* of the two).  However, the CPU consumption did not
scale very well:

dom0: 19.38%, dom1 (net): 9.61%, dom2 (net): 9.55%, dom3 (cpu): 60.18% 

In the previous scenario dom1 used around 11.5 percent of the CPU to
accomplish what here takes roughly 19 percent.  

Any ideas what this extra CPU work is?  There is extra context
switching in the second situation (which could account for the dom0
increase), but the network bound guest disparity seems a little extreme
to only be that?  

Thanks,
Tim 


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