Since upgrading our 2 host lab environment from 3.5 to 4.0 we are seeing poor Transparent Page Sharing performance on our new Nehalem based HP ML350 G6 host.

Host A : ML350 G6 - 1 x Intel E5504, 18GB RAM
Host B : Whitebox - 2 x Intel 5130, 8GB RAM

Under ESX 3.5 TPS worked correctly on both hosts, but on ESX 4.0 only the older Intel 5130 based host appears to be able to scavenge inactive memory from the VMs.

To test this out I created a new VM from an existing Win2k3 system disk. (Just to ensure it wasn't an old option in the .vmx file that was causing the issue.) The VM was configured as hardware type 7 and was installed with the latest tools from the 4.0 release.

During the test the VM was idle and reporting only 156MB of the 768MB as in use. The VM was vmotioned between the two hosts and as can be seen from the attached performance graph there is a very big difference in active memory usage.

I've also come across an article by Duncan Epping at yellow-bricks.com that may point the cause as being vMMU...
MMU article

If vMMU is turned off in the VM settings and the VM restarted then TPS operates as expected on both hosts. (See second image)

So if it comes down to chosing between the two, would you choose TPU over vMMU or vice versa?