Guest RAM performance

Hello,

I am experiencing a problem with a slow guest and have been completing some performance tests and notice that the RAM performance seems to be poor. The Host server is a DELL R715 with 2 AMD 6212 processors and has 64 G of DDR3 RAM. I currently only have a single guest machine running W2k8 configued with 1 socket and 2 cores and 4G of reserved memory so we are not overloading the system. The hard disks on the guest  a using the hosts disks (we have disk array connected but I did not want to add this as a factor in the tests) and they have lots of free space.

I have run memtest on the server and no problems were reported.

I ran  PerformanceTest on the guest and it's memory score is very low (~400).  I created a test W7 guest and the results were also very low.

I also checked the BIOS settings on the server and all seems to be OK.

http://en.community.dell.com/techcenter/virtualization/w/wiki/dell-vmware-vsphere-performance-best-practices.aspx

Does anyone have some advice as how to investigate this further ?

Thanks in advance,

Ashley