perfSONAR User Q&A and Other Discussion

[Purna Mididuddi <>]

Hi,
As mentioned in the documentation, clock stability and accuracy are very 
important for tests like latency measurement.
We are looking to run a perfsonar node on a VM, with dedicated NIC, dedicated 
CPU cores and disabling VM swapping. Also we are running NTP on the VM. Given 
these, can we assume clock on VM will be accurate? If not, what other factors 
will play a role in VM environment that aren't in bare-metal case.

The "ntpq -p" output on VM shows the offset column is changing slightly over 
a period. But same behavior is seen on bare-metal output too. So, how can we 
say baremetal has more clock accuracy than VM?

On VM:
---------
root@VM:~# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
+chronos.asda.gr .GPS.            1 u   55   64  177  231.542  -13.305   7.205
-tick.ucla.edu   .GPS.            1 u   57   64  177   28.798   -1.070   4.894
+ntp.itl.waw.pl  .ATOM.           1 u   50   64  177  203.149   -7.759   2.650
-saturn.es.net   .CDMA.           1 u   58   64  177   24.740    1.389   2.615
*tempus1.gum.gov .PPS.            1 u   57   64  177  204.282  -11.470   5.082
root@VM:~#
root@VM:~#
root@VM:~# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
-chronos.asda.gr .GPS.            1 u  136  256  377  231.800  -15.517   3.405
+tick.ucla.edu   .GPS.            1 u   85  256  377   29.182   -0.619   1.417
+ntp.itl.waw.pl  .ATOM.           1 u  166  256  377  203.748   -9.379   0.435
*saturn.es.net   .CDMA.           1 u  221  256  377   25.082   -0.908   1.344
-tempus1.gum.gov .PPS.            1 u  142  256  377  204.425  -10.678   0.749



On bare-metal:
-----------------
root@HOST:~# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 eth-0.nms-rlat. .INIT.          16 u    -   64    0    0.000    0.000   0.000
 nms-rlat.hous.n .INIT.          16 u    -   64    0    0.000    0.000   0.000
 nms-rlat.losa.n .INIT.          16 u    -   64    0    0.000    0.000   0.000
 eth-1.nms-rlat. .INIT.          16 u    -   64    0    0.000    0.000   0.000
+chronos.es.net  .CDMA.           1 u   59   64  177   89.979   10.674   5.734
*saturn.es.net   .CDMA.           1 u   57   64  177   24.547   10.046   5.885
root@HOST:~#
root@HOST:~#
root@HOST:~# ntpq -p
     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 nms-rlat.chic.n .INIT.          16 u    -  512    0    0.000    0.000   0.000
 eth-1.nms-rlat. .INIT.          16 u    -  512    0    0.000    0.000   0.000
 eth-1.nms-rlat. .INIT.          16 u    -  512    0    0.000    0.000   0.000
 nms-rlat.newy32 .INIT.          16 u    -  512    0    0.000    0.000   0.000
+chronos.es.net  .CDMA.           1 u   59   64  377   90.446    4.540   6.255
*saturn.es.net   .CDMA.           1 u   59   64  377   24.889   -4.279   5.590