I think everybody who upgraded to vSphere on Intel Nehalem processors noticed that there is no more Page Sharing. I was expecting this to happen after reading about it but I didn’t imagine it would be that bad, it turned out we were very dependent on Page Sharing.

So I configured Mem.AllocGuestLargePage to 0 on some hosts, rebooted and voila, we have Transparent Page Sharing again (Memory Granted-Memory Consumed=20-25GB, cool :) ).

After this I tried to measure the performance loss using 7-Zip, a simple benchmark that stresses memory and CPU but I could not find any measurable difference between hosts using this setting and hosts using default settings (I did find some minor differences but it was actually faster when Mem.AllocGuestLargePage was disabled, probably because of differences in number of DIMM slots occupied – one host had 12x4GB DIMMS, another one 6x8GB DIMMS or Power Management settings or other activities on the hosts). I tried both 32 bit and 64 bit operating systems, even Windows 2008 but nothing emerged. The performance gain because of lower memory usage and no more ballooning and swaping on the other hand is definitely measurable :) .

So go ahead and test and activate this setting, it will save you tons of RAM!

For more information about this setting, check out http://www.vmware.com/files/pdf/large_pg_performance.pdf

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