eCloudManager Memory Usage - Knowledge Base - Forum

The eCloudManager uses a maximum heap size of 1.5GB. So the actual memory used can be lower but will never exceed this upper limit. However, if you install the eCloudManager into a virtual machine, you may encounter a higher memory usage of such a VM than the maximum memory limit of the eCM.

Depending on the hypervisor used, different memory allocation techniques are implemented to assign memory to a virtual machine which can result in a higher used memory size than the actually active memory size.

For example, VMware has three metrics that describe the memory footprint of a VM: granted memory, allocated memory and guest memory. The granted memory is the amount of memory the user assigns to a virtual machine. Guest memory shows how much memory the operating system inside the VM is actually using. The amount of allocated memory describes how much memory pages have been used by the guest operating system since the VM was started. Since the hypervisor does not have any possibility to decide which memory page is still used by a guest operating system and which one already has been freed, the allocated memory will always be larger than the active guest memory.

Here's a small example: The VM starts and initially a large amount of memory is used as file cache to speed up the boot process. After all services, including the eCM, have been started, the system cache will be reduced. But the hypervisor does not know that the guest OS does no longer need such a huge amount of memory for file caching and keeps the used memory pages allocated to the VM. As a result the active memory usage of a VM can differ dramatically from the allocated memory set.

Here is a very good white paper about memory management in a VMware environment:

http://www.vmware.com/files/pd…..gement.pdf

If you do not want the hypervisor to allocated much more memory for a VM than the guest OS is actually using, keep the amount of granted memory very close to the average amount of guest memory used. Only if your guest OS constantly keeps getting very close to the maximum amount of memory, it makes sense to increase the amount of granted memory in order to prevent paging inside the guest OS.

For the eCloudManager using the maximum memory footprint as a basis for the guest OS, we can estimate that a VM that only hosts the eCM does not require more than 2GB of RAM.