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Memory Use: Host Operating System and Virtual Machines

VMware Workstation assigns memory to virtual machines in the following way:

In addition, when you power-on a virtual machine, VMware Workstation checks to be sure there is enough memory available to run that virtual machine. See Memory Limits (Checks) below.

Reserved Memory

To provide good overall system performance, VMware Workstation limits how many virtual machines can run at once based on the amount of available reserved memory. In doing so, VMware Workstation prevents virtual machines from causing each other to perform poorly. VMware Workstation permits an acceptable number of virtual machines to run simultaneously, ensuring that they work together to manage the reserved memory.

If all the reserved memory is in use by one or more virtual machines, the host operating system cannot use this memory for any other purpose. VMware Workstation uses the reserved memory only if it determines that a virtual machine needs the memory to have reasonable performance. Even if multiple virtual machines are running at the same time, VMware Workstation may be using only a fraction of the reserved memory, thus allowing any unused reserved memory to be used by other applications running on the host.

The default amount of reserved memory is calculated when you install VMware Workstation, but you can change the amount of reserved memory in the Reserved Memory configuration screen.

Warning: Changing the amount of reserved memory is recommended only for advanced users because it can have an adverse impact on the host or virtual machine performance.

VMware Workstation calculates reserved memory by

The table below shows the default host requirements that VMware Workstation uses in calculating reserved memory.

Host

Host Memory

Host Default Requirements

Linux

Less than 384MB

32MB

Linux

Less than 512MB

64MB

Linux

512MB or more

96MB

Windows

Less than 384MB

64MB

Windows

Less than 512MB

96MB

Windows

512MB - 1024MB

128MB

Windows

1024MB or more

0MB

For example, if your host operating system is a version of Microsoft Windows and your host machine has 256MB of memory, VMware Workstation reserves 192MB for all of your virtual machines.

When a virtual machine is booted, VMware Workstation checks to be sure that there is enough reserved memory available to start the virtual machine. This is described in Memory Limits (Checks), below.

Memory Per Virtual Machine

VMware Workstation allocates a specific amount of the available reserved memory for a given virtual machine. When you create a new virtual machine, VMware Workstation chooses a default amount of memory to allocate based on:

The amount of overhead memory used by a virtual machine depends on the RAM of the host machine. For hosts with 256MB of RAM or more, the memory overhead is 16MB per virtual machine. For hosts with less than 256MB of RAM, the overhead value is 0MB per virtual machine. The amount of reserved memory actually used by a particular virtual machine varies dynamically as a virtual machine runs.

Either when you create a new virtual machine, or later on, you can change the default amount of memory allocated to that virtual machine. If you have memory limits (checks) enabled, the virtual machine cannot be allocated more than the reserved memory minus the overhead. If memory limits are disabled, you can assign any amount of memory to the virtual machine, although you may not be able to boot and run the virtual machine (or it may adversely affect the performance of the host and other virtual machines.)

Memory Limits (Checks)

By default, a virtual machine checks to ensure that enough free memory is left to the host machine to continue to operate efficiently:

If these checks fail, the virtual machine terminates. You can disable memory checks, but remember that disabling either of these checks can result in poor performance and even system crashes if you try to run a virtual machine with too much (or too little) memory for your host system.

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