I've only read about this topic a handful of times. I found an article at www.vmguy.com here that sums up the consensus on this HT issue (direct from the article):
There are pros and cons to using HT in ESX.
Better co-scheduling of SMP VM’s
Hyperthreading provides more CPU
contexts and because of this, SMP VM’s
can be scheduled to run in scenarios
which would not have enough CPU
contexts without Hyperthreading.
- Typical applications see performance improvement in the 0-20%
range (the same as non-virtualized
- Processor resources are shared with Hyperthreading enabled
Processor resources are shared such as
the L2 and L3 caches. This means that
the two threads running on the same
processor compete for the same
resources if they both have high
demand for them. This can, in turn,
All things considered, it is difficult
to generalize the performance impact
of Hyperthreading. It is highly
dependant on the workload of the VM.
One additional point is that you can
always utilize the CPU min and max
values on a per-VM or Resource Pool
basis to reserve certain amounts of
CPU for your most critical workloads.
As with the majority of performance
items I enounter, test, test, test.
Try out the workloads and see what
works the best on the hardware you
Again, this is directly from the article. I'm not certain that HT is worth it as I too use AMD Opterons so I can't speak from experience.