I ran across this the other day while onsite at a customer. After performing an upgrade from ESX 3.5 to vSphere 4, the console would show the message:
"Significant Imbalance between NUMA nodes detected. Performance may be impacted."
A quick Google search came up with this post on the VMware community forums:
http://communities.vmware.com/thread/217244;jsessionid=62FBC1EAEDAFA087E683113D69680090
There is also an official VMware KB article on this as well:
http://kb.vmware.com/kb/1018754
Another great resource on this error is over on Duncan's site at YellowBricks.com, here:
http://www.yellow-bricks.com/2007/12/28/memory-incorrectly-balanced/
You can see the imbalance if you run "esxtop" and then press the "m" button, which will show additional details for Memory. As mentioned at the forum link above, in esxtop there is a NUMA row that displays how much memory each NUMA node contains and how much of that memory is in use.
If you have a HP server and need help with balancing memory across physical CPU's, then check out the HP DDR3 Memory Configuration Tool:
http://h18000.www1.hp.com/products/servers/options/tool/hp_memtool.html
If you have a Dell server, here's the link to their DDR3 Memory Advisor Tool:
https://roianalyst.alinean.com/DellDDR3MemoryAdvisor/launch.html
Both tools will help you place the memory in the correct slots and in the correct order for optimal performance, as well as fixing any "NUMA errors" reported by vSphere.
