Tuesday, September 20, 2011

Understanding %Ready with Monster VMs

I was doing some benchmark testing with a "Monster VM" of 24 vCPUs and 85 GB of RAM and was concerned that the %Ready CPU time for the VM was very high at 30% to 40%.  The strange thing was that performance of the VM seemed to be pretty good, with no real issues (I was of course trying to find another few percent of performance for the benchmark - but is another story).

Our guidance with %Ready has been that it should be below 5%, but this needs to be reconsidered when using large VMs.  The reason is that the %Ready you see in esxtop is the sum of %Ready for all of the individual vCPUs for that VM.  In versions prior to vSphere 5, this meant that only up to 8 vCPUs could be involved in reaching the %Ready.  With vSphere 5 and "Monster VMs" this is now up to 32 vCPUs and even small %Ready times or each individual vCPU can easily push the overall %Ready over 5%.

In general esxtop uses this addition method when showing the usage level of CPUs and it makes sense that it should be carried over to %Ready as well.  For example, when esxtop shows the CPU usage of an eight vCPU VM the maximum is 800, and for a 32 vCPU VM is is 3200.

When looking at %Ready it is important to also understand how many vCPUs are in that VM and take that into consideration.  If you expand out the stats for the VM (press "e" and then enter the GID for the VM when in esxtop) you will be able to see the %Ready for all of the individual vCPUs which should help to put things into context very quickly.

Returning to my example, it turns out that each of my 24 vCPUs was around 1 for %Ready and it did not represent a performance problem with the %Ready for the entire VM to be at 30%.


1 comment:

Anonymous said...

Thanks for posting. Currently building out a dedicated cluster for our Development groups dedicated code compile VMs. They are extremely good at using every vCPU possible. Most of them are set to 4vCPUs on dual chip quad core Nehalem and Harpertown blades. New blades will be dual chip 8 core Sandy Bridge (16 physical core, 32 logical). Hoping to bump each build VM up to 8vCPUs. Would love to go higher, but need to be careful. Thoughts?