Virtualization Oversubscription - What’s so scary? CPU Oversubscription ( 13 of 20)

CPU Oversubscription

Memory is fairly easy to describe but there are a lot of things going on.  CPU Oversubscription and the technologies involved can be a little more complex to visualize, but there are less tools that the hypervisor has to work with.

·        Time slicing

·        Co-Scheduling

·        Reservations

·        Shares

·        Limits

For a start, time is no longer a constant.  The hypervisor has the ability to run time at whatever speed it likes, just so long as it averages out in the end.

Co-Scheduling is where we have to have all the vCPUs for a single VM, mapped to logical CPUs from the hardware.

Reservations and Shares apply here also and we’ll have more of a look at how they work later.

Limits (also exist for memory), but these can be applied to restrict some VMs down to a smaller amount of CPU than their vCPU allocation would otherwise allow them to have.

Let’s start with Time Slicing.

Time Slicing

In a typical VMware host we have more vCPUs assigned to VMs than we do physical cores. The processing time of the physical cores (or logical CPUs if hyper threading is in play), has to be shared among the vCPUs in the VMs.  The more vCPUs we have, the less time each can be on the core, and therefore the slower time passes for that VM.  To keep the VM in time, extra time interrupts are sent in quick succession when the VM is processing, so time passes slowly and then very fast.

Significant improvements have been made in this area over the releases of VMware. vCPUs can be scheduled onto the hardware a few milliseconds apart but the basic concept remains in place.

Join me again on Friday when I'll look at VMWare vCPU Co-Scheduling & Ready Time.

Phil Bell

Consultant