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Hardware-Assisted CPU Virtualization
Certain processors provide hardware assistance for CPU virtualization.
When using this assistance, the guest can use a separate mode of execution called guest mode. The
guest code, whether application code or privileged code, runs in the guest mode. On certain events, the
processor exits out of guest mode and enters root mode. The hypervisor executes in the root mode,
determines the reason for the exit, takes any required actions, and restarts the guest in guest mode.
When you use hardware assistance for virtualization, there is no need to translate the code. As a result,
system calls or trap-intensive workloads run very close to native speed. Some workloads, such as those
involving updates to page tables, lead to a large number of exits from guest mode to root mode.
Depending on the number of such exits and total time spent in exits, hardware-assisted CPU virtualization
can speed up execution significantly.
Virtualization and Processor-Specific Behavior
Although VMware software virtualizes the CPU, the virtual machine detects the specific model of the
processor on which it is running.
Processor models might differ in the CPU features they offer, and applications running in the virtual
machine can make use of these features. Therefore, it is not possible to use vMotion
®
to migrate virtual
machines between systems running on processors with different feature sets. You can avoid this
restriction, in some cases, by using Enhanced vMotion Compatibility (EVC) with processors that support
this feature. See the vCenter Server and Host Management documentation for more information.
Performance Implications of CPU Virtualization
CPU virtualization adds varying amounts of overhead depending on the workload and the type of
virtualization used.
An application is CPU-bound if it spends most of its time executing instructions rather than waiting for
external events such as user interaction, device input, or data retrieval. For such applications, the CPU
virtualization overhead includes the additional instructions that must be executed. This overhead takes
CPU processing time that the application itself can use. CPU virtualization overhead usually translates
into a reduction in overall performance.
For applications that are not CPU-bound, CPU virtualization likely translates into an increase in CPU use.
If spare CPU capacity is available to absorb the overhead, it can still deliver comparable performance in
terms of overall throughput.
vSphere Resource Management
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