Anything that
doesn’t relate to time, such as disc occupancy should be ok to use.
Ready Time
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.
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:
Threads – being
processed and allocated to a thread.
Ready – in a ready
state where they wish to process but aren’t able to.
Idle – where they
exist but don’t need to be doing anything at this time.
In the diagram below
you can see that work has moved over the threads to be processed and there is
some available headroom. Work that is waiting to be processed requires 2 CPU’s
so is unable to fit and creates wasted space that we are unable to use at this
time.
We need to remove a VM before we can put a 2 CPU VM on to a thread and
remain 100% busy.
In the meantime other VM’s are coming along and we now have a 4vCPU VM
accumulating Ready Time.
2 VM’s moves off but the 4vCPU VM waiting cannot move on as there are
not enough vCPU’s available.
It has to wait and other work moves ahead of it to process.
Even when 3vCPU’s are available it is still unable to process and will
be ‘queue jumped’ by other VM’s who require less vCPU’s.
Hopefully that is a clear illustration of why it makes sense to reduce
contention by having as few vCPUs as possible in each VM.
Ready Time impacts on performance and needs to be monitored.
On Wednesday I'll be looking at Memory performance, in the meantime don't forget to register for our 'Taking a trip down vSphere Memory Lane' webinar taking place on June 24th
