Imagine you are driving a car,
and you are stationary, there could be several reasons for this. You may be waiting to pick someone up, you
may have stopped to take a phone call, or it might be that you have stopped at
a red light. The first two of these
(pick up, phone) you have decided to stop the car to perform a task. In the third instance the red light is
stopping you doing something you want to do.
In fact you spend the whole time at the red light ready to move away as
soon as the light turns to green. That
time is ready time.
For every 1 second of processing the VM is waiting around 2 seconds to process, so it’s spending almost twice as long to process than it is processing. This is going to impact on the performance being experienced by the end user who is reliant on this VM.
Now let’s compare that to the proportion of time spent processing on a 2 vCPU VM. The graph below shows a 2 vCPU VM doing the same amount of work, around 500
When a VM wants to use the
processor, but is stopped from doing so.
It accumulates ready time and this has a direct impact on performance.
For any processing to happen all
the vCPUs assigned to the VM must be running at the same time. This means if you have a 4 vCPU all 4 need
available cores or hyperthreads to run.
So the fewer vCPUs a VM has, the more likely it is to be able to get
onto the processors.
To avoid Ready Time
You can reduce contention by
having as few vCPUs as possible in each VM.
If you monitor CPU Threads, vCPUs and Ready Time you’ll be able to see
if there is a correlation between increasing vCPU numbers and Ready Time in
your systems.
Proportion of Time: 4 vCPU VM
Below is an example of a 4vCPU
VM, each doing about 500 seconds worth of real CPU time and about a 1000’s
worth of Ready Time.
For every 1 second of processing the VM is waiting around 2 seconds to process, so it’s spending almost twice as long to process than it is processing. This is going to impact on the performance being experienced by the end user who is reliant on this VM.
Now let’s compare that to the proportion of time spent processing on a 2 vCPU VM. The graph below shows a 2 vCPU VM doing the same amount of work, around 500
seconds worth of real CPU time
and as you can see the Ready Time is significantly less.
There are 3 states which the VM
can be in and we'll take a look at these on Friday.
Don't forget to book on to our VMware vSphere Capacity & Performance Essentials workshop starting on Dec 6 http://www.metron-athene.com/services/online-workshops/index.html
Phil Bell
Consultant
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