[libvirt-users] VM Performance using KVM Vs. VMware ESXi

[Jatin Davey]

Hi All

We are currently testing our product using KVM as the hypervisor. We are 
not using KVM as a bare-metal hypervisor. We use it on top of a RHEL 
installation. So basically RHEL acts as our host and using KVM we deploy 
guests on this system.

We have all along tested and shipped our application image for VMware 
ESXi installations , So this it the first time we are trying our 
application image on a KVM hypervisor.

On this front i have done some tests to find out how our application's 
response time is when deployed on KVM and then compare it with a VM 
deployed on VMware ESXi. We have a benchmark test that basically loads 
the application simulating a load of 100 parallel users logging into the 
system and downloading reports. These tests basically use a HTTP GET 
query to load the application VM. In addition to that i have taken care 
to use the same hardware for both the tests , one with RHEL(Host)+KVM 
and another with VMware ESXi. All the hardware specifications for both 
the servers remain the same. The load test also remains the same for 
testing with both the servers.

First observation is that the average response time on the VMware ESXi 
is : 500 milli-seconds while the application's average response time 
when deployed using RHEL(Host)+ KVM is : 1050 milli-seconds. The 
response time of the application when deployed on KVM is twice as much 
as when it is deployed using VMware ESXi.

I did few more tests to find which sub-system on these servers shows 
varying metrics.

First i started with IOZone to find out if there is any mismatch in the 
speed with which data is read / written to the local disk on the two VMs 
and found that "Read" speed in the VM that was deployed using 
RHEL(Host)+KVM was twice as slow as the VM which was deployed using 
VMware ESXi.

For more on IoZone , Please refer : http://www.iozone.org/

more specifically the following IoZone metrics were twice as less when 
compared to the server running with VMware ESXi:

Read
Re-read
Reverse-Read
Stride Read

Pread


Note: I had run the IoZone tests on the VMs on both the servers.

Second observation to be made was the output from the "top" command. I 
could see that the VM deployed on RHEL(Host)+KVM was showing high 
numbers for the following metrics when compared with the VM deployed on 
VMware ESXi:

load averages
%sy for all the logical processors
%si for all the logical processors

i debugged further to find out which device is causing more interrupts 
and found it to be "ide0" , See the output from the /proc/interrupts 
file below:
The other interrupts apart from ide0 are pretty much similar to the VM 
deployed using VMware ESXi.

************/proc/interrupts *******************
[root at localhost ~]# cat /proc/interrupts
            CPU0       CPU1       CPU2       CPU3       CPU4 CPU5       
CPU6       CPU7
   0:     795827          0          0          0 0          0          
0          0    IO-APIC-edge  timer
   1:         65          0          0          0 0          0          
0          0    IO-APIC-edge  i8042
   6:          2          0          0          0 0          0          
0          0    IO-APIC-edge  floppy
   8:          0          0          0          0 0          0          
0          0    IO-APIC-edge  rtc
   9:          0          0          0          0 0          0          
0          0   IO-APIC-level  acpi
  10:     425785          0          0          0 0          0          
0          0   IO-APIC-level  virtio0, eth0
  11:         47          0          0          0 0          0          
0          0   IO-APIC-level  uhci_hcd:usb1, HDA Intel
  12:        730          0          0          0 0          0          
0          0    IO-APIC-edge  i8042
  14:     188086          0          0          0 0          0          
0          0    IO-APIC-edge  ide0
NMI:          0          0          0          0 0          0          
0          0
LOC:     795813     795798     795783     795767     795752 795737     
795723     795709
ERR:          0
MIS:          0
*********************************************

Any pointers to improving the response time for the VM for 
RHEL(Host)+KVM installation would be greatly appreciated.

Thanks
Jatin

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