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Dom0 and VCPU and Hyper Threading

Jeremy Brook


In my server I have 2, Xeon E5-2620 v3 processors, these are 6 core CPUs that support Hyperthreading.  Am I right in thinking that I when allocating vCPUs to VMs, I should totally ignore hyperthreading and simply base allocation on Physical cores, so  essentially my server no VM should use more than 12 vpcus?


The server was never set up by myself, it was setup and installed by a supplier (who no longer exist) and I am take over managing the server and trying to get my head around the whole setup.


I have really sluggish performance which is what brought me to these forums.  Running xl vcpu-list I can see that Dom0 is using 16 VCPUs, am I right in thinking VCPUs shouldn't exceed physical CPUs, so this being 16 is affecting overall performance?


Domain-0                             0     0    5   r--    2458.1  all / all
Domain-0                             0     1    0   -b-    1850.3  all / all
Domain-0                             0     2    1   -b-    2999.7  all / all
Domain-0                             0     3   12   -b-    1807.2  all / all
Domain-0                             0     4   23   -b-    1778.6  all / all
Domain-0                             0     5    0   -b-    1847.6  all / all
Domain-0                             0     6    4   -b-    1918.4  all / all
Domain-0                             0     7   22   -b-    1788.6  all / all
Domain-0                             0     8    7   -b-    2020.4  all / all
Domain-0                             0     9    3   -b-    1791.2  all / all
Domain-0                             0    10   10   -b-    1818.4  all / all
Domain-0                             0    11   16   -b-    1830.1  all / all
Domain-0                             0    12    2   -b-    1863.0  all / all
Domain-0                             0    13   18   r--    1913.6  all / all
Domain-0                             0    14    6   -b-    1835.1  all / all
Domain-0                             0    15   17   -b-    1852.9  all / all


Then the next question, I do have 12 available VCPUS (Server shows 24 cause of HT), each time I create a VM should I deduct the number of used VCPUs from 12, so no Host can run more than 12 across all running guest? 

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Over-allocation of vCPUs is common, as long as the overall load doesn't saturate the processor load. I've over-allocated by factors of several, especially in cases where not all VMs were active concurrently. The allocation of vCPUs can be done on CPUs either with or without hyperthreading; hyperthreading can be a plus or minus factor, depending on various circumstances that get quite involved. In your case it may depend on the types and sizes and loads of VMs you are running, hnce it's not a simple answer. But above all, your overall CPU load should not be too extreme or performance will suffer. Run top to make sure your load isn't too excessive.





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CPU load is next to nothing in performance monitor, as is memory and disk.  Testing file transfers I get nearly full speed, but still the whole setup runs like a blind dog with no legs set in cement.

I have one VM, a server 2012 machine running SQL 2014.  I have known this machine to take almost an hour just to reboot!

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On 3/10/2023 at 5:54 PM, Tobias Kreidl said:

That's not normal. How many VCPUs and how much memory do you have allocated to dom0?


Note that if you use more cores than are on a single physical CPU for a VM, you break NUMA, which can leas to performance degradation, but still, it should not have an effect that drastic.


It has the default 5.8GB which has been auto assigned.  The server itself does have 128GB

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