[Xen-users] xentop CPU Util on idle dom0 question

[Stefan Parvu <sparvu@xxxxxxxxxxxxxxxxxxxxxx>]

Hi,

I have the following configuration:

 Box: 24GB RAM, 1 physical CPU, 8 cores, 2TB disk
 OS: Ubuntu 12.04.4 LTS
 GRUB_CMDLINE_XEN_DEFAULT="dom0_mem=6500M dom0_max_vcpus=2 dom0_vcpus_pin"

 Xen 4.2.2
 dom0 - 2VCPUs, 6GB RAM 
 domU - each domain gets 1 VCPU, 1GB RAM
 15 domains

Im confused about xentop reporting 8 - 10% CPU Utilization on dom0, idle
dom0, where mpstat or sysrec [1] shows a lower value of something between 
1-3%. 

For example:

1. xentop -b -d1 -i2, including last sample and I dismissed the rest 
   of columns, for simplicity:

      NAME  STATE   CPU(sec) CPU(%)     
     c5932 --b---          6    0.0    
     c5964 --b---          8    0.0    
     c6464 --b---          7    0.0    
   deb7464 --b---          6    0.0    
  Domain-0 -----r       1253   10.1    
     lobby --b---          8    0.0     
     r5732 --b---        238    1.5    
     r5764 --b---        460    2.8    
     s10u8 --b---         63    0.3    
    sdrcom --b---          5    0.0     
    sdrorg --b---          6    0.0     
   u100432 --b---          4    0.0    
   u100464 --b---          4    0.0    
   u120464 --b---          6    0.0    
    win764 --b---        340    2.3    


2. mpstat:
04:59:56 PM  CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  
%guest   %idle
04:59:57 PM  all    0.00    0.00    0.00    0.00    0.00    0.00    0.00    
0.00  100.00
04:59:57 PM    0    0.00    0.00    0.00    0.00    0.00    0.00    0.00    
0.00  100.00
04:59:57 PM    1    0.00    0.00    0.00    0.00    0.00    0.00    0.00    
0.00  100.00

04:59:57 PM  CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  
%guest   %idle
04:59:58 PM  all    0.00    0.00    0.52    0.00    0.00    0.00    0.00    
0.00   99.48
04:59:58 PM    0    1.02    0.00    1.02    0.00    0.00    0.00    0.00    
0.00   97.96
04:59:58 PM    1    0.00    0.00    0.00    0.00    0.00    0.00    0.00    
0.00  100.00


So the 8-10% Im seeing all the time on dom0 reported by xentop seems to be 
xentop itself.
Now, thats ok, but I dont understand why mpstat will not even close to xentop's 
CPU usage ?
So why and how xentop gets the 10% usage ... !?

If I increase the load on dom0 to 50% or 100% usage the measurements start to 
get closer.
If I measure idle systems I always see a 10% difference no matter what ... 

I did execute both utilities same time for different periods of time, making 
sure they both
run.

Thanks a lot,

-- 
Stefan Parvu <sparvu@xxxxxxxxxxxxxxxxxxxxxx>


[1] - http://www.systemdatarecorder.org/recording/sysrec.html

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