Vsphere 5.1: Virtual Machine not sharing Host RAM

Question

Vsphere 5.1: Virtual Machine not sharing Host RAM

Hi,

I am using Licensed Vsphere 5.1 (Upgraded from Vsphere 4)

I am facing a problem after upgrade, Virtual machines are not sharing RAM

Like if i have two machine with 2 GB RAM then total RAM Usage on HOST becomes 4 GB but actually active memory usage is about 1 GB only.

This was not a problem in Vsphere 4 it shows active memory as HOST memory usage but in Vsphere 5 Virtual machine are not sharing RAM and reserve full RAM when powered on.

Maybe there is some setting to enable memory sharing or to turn off this full reservation of RAM.

Answer 1

got the answer

Changes are the VMkernel is using large pages to back the virtual memory pages. For more information, this article from Duncan will provide enough information and links to other articles about this topic:

http://www.yellow-bricks.com/2011/01/26/re-large-pages-gabvirtualworld-frankdenneman-forbesguthrie/

In short VMkernel will place the pages issued by the virtual machine in large pages on physical memory to speed up memory access. Large pages are used when the physical CPU that has EPT(Intel) or RVI(AMD) capabilities.

Answer 2

As far as I understand, basically, when ESXi host is under memory pressure large pages are being broken into 4k pages to allow TPS to work.

Now my doubt is: using EPT (or RVI) technology "breaking up large page in small page" means actually that the (EPT) address translation scheme is changed and ESXi VMkernel has to build up on the fly the page tables needed to support the last level of indirection required by the new (EPT) translation scheme ?

thanks.