Shane White and I gave a presentation to the Melbourne VMware User group this past Thursday evening.
For those who are interested, here is a copy of the presentation.
Josh Odgers & Shane White – MVMUG July 27th 2012 – VCDX Experience
I thought this example may be useful to show the benefits of Right sizing a virtual machine.
The VM is an SQL Database server with 4 vCPUs on a cluster which is highly overcommitted with lots of oversized VMs.
As we can see by the below graph, the CPU ready was more or less averaging 10% and on the 24th of July most vCPUs spiked to greater than 30% CPU ready each. ie: 30% of the time the server is waiting to be scheduled onto the pCPU cores.
The performance of applications using databases hosted on the server were suffering serious issues during this time.
On the 24th the VM was dropped from 4 vCPUs, down to 2 vCPUs and the results are obvious.
CPU ready dropped immediately (even in a heavily over-committed environment) to around 1% and CPU utilization remained at around the same levels. Performance also improved for applications (for example vCenter) using the database server.
TIP: Right Sizing not only helps the VM you right size, but it helps relieve the contention on the ESXi host (and cluster), which will improve performance for all VMs.
It is also important to point out this VM is the first VM to be right sized, so as more VMs are right sized in the cluster, Ready time will drop further and performance will continue to improve.
This also results in opportunities for greater consolidation within the environment without compromising performance or redundancy.
I would like to point out that I believe this server may benefit from 4 vCPUs, but definitely not in this highly CPU contended environment.
As more virtual machines are Right Sized, then this environment would likely have the opportunity to consider increasing vCPUs in suitable VMs after monitoring performance for a suitable period of time. Products like VMware vCenter Operations is excellent for reporting on Oversized and undersized VMs.
Do you believe in right sizing now?
One of the more common problems I see in virtual environments is over sized virtual machines which typically results in lower performance, and your guessed it, high CPU Ready.
What is CPU Ready?
CPU ready is basically the time it takes a VM to be scheduled onto physical core after it is placed in the CPU scheduling queue.
What is High CPU Ready?
In my opinion, during peak load, anything above 2% (or 400ms) is a concern and should be monitored. Above 5% will be impacting performance (resulting in lower CPU utilization) and 10% or more, should be considered a serious problem and remediated immediately.
The below is a screenshot showing CPU ready from a recent test I conducted in my home lab
To calculate the percentage of CPU Ready, we divide the VMs “Summation” value (in the screen shot above its the “W2K8 CPU TEST VM 1” line by 20000 (ms) which is the statistics collection interval, then divide the result by the number of vCPUs in the VM.
So if we use the value from the “latest” column, its 7337 divide 20000, equals : 0.36685, then we divide that by 2 as the VM has 2 vCPUs and we end up with 0.183425
That’s 18% CPU Ready, which basically means 18% of the time, the VM is not doing anything!
Note: CPU Ready % can be found using ESXTOP or RESXTOP via the vMA or on the ESXi host directly.
Now to try and diagnose the Performance/CPU ready issue, we need to work out if the VM is oversized and if so, Right Size the VM.
What is an Oversized VM?
Basically a VM which has more compute resources assigned than it requires, for example, a VM which uses no more than 20% of its CPU and has 4 vCPUs.
What is Right Sizing?
In the above example, the VM is oversized as it doesn’t use more than 1vCPU (or 25%) of the CPU resources and therefore could be reduced to to 1 vCPU and run at 80%.
So the VM is oversized, and has High CPU ready, what happens when we right size it from 4vCPUs to 1vCPU and why does this help performance?
Its pretty simple, the less vCPUs a VM has, the easier job the CPU scheduler has to find enough physical cores to schedule the VM onto. If a cluster has a lot of oversized VMs, all the VMs are all competing for the same physical cores, and making it more and more difficult for the scheduler.
But what about setting a CPU Reservation? Don’t reservations “guarantee” resources?
The answer is, Yes and No.
The reservation “reserves” CPU resources measured in Mhz, but this has nothing to do with the CPU scheduler.
So setting a reservation will help improve performance for the VM you set it on, but will not “solve” CPU ready issues caused by “oversized” VMs, or by too high an overcommitment ratio of CPU resources.
In my testing I set an 80% reservation of a VMs 2 vCPUs worth of Mhz and prior to setting the reservation the CPU ready was ~20% and then CPU Ready did drop to around 10%. Note: This test was performed with only 25% overcommitment – 5 vCPUs on 4 physical Cores using CPUBUSY to keep the CPUs running at 100% (measured within the guest by Windows Task Manager).
I then set a 100% reservation of the VMs 2 vCPUs worth of Mhz, prior to setting the reservation the CPU ready was ~10% and CPU Ready did not get below 2.5% even with 100% reservation.
The result would have been exponentially worse had I tested with 50% or 100% overcommitment which is generally easily achieved with VMware and a well architected cluster. (I have seen well above these overcommitment numbers with no CPU ready issues).
Reducing CPU Ready down to 2.5% may sound like a pretty good result, but when we look at the other 3 x 1vCPU VMs on the host (4 core test ESXi 5 host) they had CPU ready of 40%!! Not to mention 2.5% is still not good!
If you have poor performance, and you discover you have High CPU Ready the best solution is Right Size Your VMs!
I have recommended exactly that countless times and the customers never believe that performance can increase with less vCPUs, until after the Right Sizing exercise.
If after Right sizing, you still have CPU Ready, your overcommitment on CPU is simply to high for the workloads within your cluster.
You can address this by
1. Adding additional compute to the cluster. (Duh!)
2. Using Affinity rules to locate complimentary workloads together (Lots of small 1vCPU VMs which don’t have high CPU utilization will generally work well with a limited number of higher vCPU VMs)
3. Use Anti-Affinity rules to separate non complimentary workloads (eg: Don’t place all your 8vCPU VMs on one host with 300% overcommitment on CPU and expect them to work well).
4. Scaling out (not up) your VMs ie: Don’t have one 8 vCPU SQL DB server, use 4 smaller 2vCPU VMs
So now you know better than to use reservations to solve CPU contention.
Its time too go Right Sizing!
This simple task is about the best bang for buck you will get in your data center, since virtualizing on VMware in the first place.