Today I’ll explain
the effect of what is happening inside the host to schedule the physical
CPUs/cores to the vCPUs of the VMs.
Clearly most hosts have more than 4 consecutive threads that can be
processed but let’s keep this simple to follow.
·
VMs
that are “ready” are moved onto the Threads.
·
There
is not enough space for all the vCPUs in all the VMs so some are left
behind. (CPU Utilization = 75%, capacity
used = 100%)
·
If
a single vCPU VM finishes processing, the spare Threads can now be used to process
a 2 vCPU VM. (CPU Utilization = 100%)
·
A
4 vCPU VM needs to process.
·
Even
if the 2 single vCPU VMs finish processing, the 4 vCPU VM cannot use the CPU
available and while it’s accumulating Ready Time, other single vCPU VMs are
able to take advantage of the available Threads
·
Even
if we end up in a situation where only a single vCPU is being used, the 4 vCPU
VM cannot do any processing. (CPU Utilization = 25%)
As
mentioned when we discussed time slicing, improvements have been made in the
area of co-scheduling with each release of VMware. Among other things the time between
individual CPUs being scheduled onto the physical CPUs has increased, allowing
for greater flexibility in scheduling VMs with large number of vCPUs. Acceptable performance is seen from larger
VMs.
Along
with Ready Time, there is also a Co-Stop metric. Ready Time can be accumulated against any
VM. Co-Stop is specific to VMs with 2 or
more vCPUs and relates to the time “stopped” due to Co-Scheduling
contention. E.g. One or more vCPUs has
been allocated a physical CPU, but we are stopped waiting on other vCPUs to be
scheduled.
Imagine
the bottom of a “ready” VM displayed, sliding across to a thread and the top
sliding across as other VMs move off the Threads, so the VM is no longer rigid
it’s more of an elastic band.
VMs and Resource Pools can be allocated Reservations, Shares and Limits and I'll be taking a look at these on Monday.
If you haven't already done so don't forget to sign up to get free access to our Resources, there are some great VMware white papers and on-demand webinars on there.
Phil Bell
Consultant
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