Installation guide

C H A P T E R 1 2 VMware ESX Server Resource Management
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CPU shares entitle a virtual machine to a relative fraction of CPU resources. For
example, a virtual machine that has twice as many shares as another is generally
entitled to consume twice as much CPU time, subject to their respective
minimum and maximum percentages.
You may specify shares by specifying a numerical value, or specifying high,
normal, or low. By default, the setting for normal shares is twice that of low.
Similarly, high shares are twice that of normal (or four times that of low).
You have the option of specifying a minimum percentage, a maximum percentage,
CPU shares, or a combination of these. The system automatically allocates CPU time to
each virtual machine somewhere between its minimum and maximum percentages,
refined by the number of shares.
Admission Control Policy
ESX Server uses an admission control policy. While CPU reservations are used for
admission control, actual CPU time allocations vary dynamically, and unused
reservations are not wasted.
Note: If ESX Server is unable to guarantee a virtual machine’s specified minimum
percentage, it will not allow you to power on that virtual machine.
Over the next few sections, we discuss managing CPU resources using CPU
percentages, CPU shares, and scheduling affinity by assigning virtual machines to run
on specific processors.
• Specifying Minimum and Maximum CPU Percentages on page 389
• Assigning Virtual Machines to Run on Specific Processors on page 390
• Using Proportional-share Scheduling by Allocating Shares on page 391
• Managing CPU Time with Percentages and Shares on page 392
Specifying Minimum and Maximum CPU Percentages
Starting with ESX Server 2.0, you have the option to specify a minimum and maximum
percentage of CPU for each virtual machine. The minimum percentage represents an
absolute, fixed lower limit while the maximum percentage represents an absolute,
fixed upper limit. A virtual machine will always be able to use at least as much CPU
time as specified by the minimum percentage, and never use more CPU time than the
specified maximum percentage.
For a single virtual CPU virtual machine, the percentage ranges from 0% to 100%. For a
dual-virtual CPU machine, the percentage ranges from 0% to 200%.
Note: Set a virtual machine’s minimum for the minimal acceptable performance.