When vSphere was still in beta one of the requirements for using FT was to have hyperthreading disabled. For most people this wasn’t an issue as traditional hyperthreading usually did not improve performance and thus was disabled by default. However with the Nehalem all this changed. Of course I can’t guarantee a specific percentage of performance increase but increases of up to 20% have been reported which is the primary reason for having HT enabled on any Nehalem system.
As you can imagine the HT requirement for FT has been floating around ever since and is a myth which have never been debunked. I’ve spoken with product management about it and they confirmed it’s an obsolete requirement. Hyperthreading does not have to be disabled for FT to work. Or to put it even more strongly: FT is supported on systems which have hyperthreading enabled. Product Management promised me that a KB article will be created to debunk this myth or an entry will be added to the FT FAQ KB article soon.
UPDATE: The FT FAQ KB Article has been updated and includes the following statement.
Does Fault Tolerance support Intel Hyper-Threading Technology?
Yes, Fault Tolerance does support Intel Hyper-Technology on systems that have it enabled. Enabling or disabling Hyper-Threading has no impact on Fault Tolerance.
Alan Renouf says
Thanks for clearing that up.
Duco Jaspars says
Duncan, the “vSpere FAQ.doc” in the vSphere 4 Services Kit R3 states as a requirement for FT:
o Turn off hyper threading (HT) in the BIOS. This ensures that a secondary VM does not have to share CPU cycles and can run fast enough to process the tasks already run on the primary VM.
That seems to be in contradiction to what you say.
In an other VMware document (don’t remember which one) I readt that if you use HT, the chances are a secondary VM might get scheduled on a hyperthreaded core where the the other thread gets hammered, and so has a negative impact on the perfromance of the protected VM
So if what you say is right, the consfusion is created by statements in the official documentation, and should be changed accordingly, agreed?
Duncan Epping says
Like I said product management told me that it is supported and the info out there is outdated.
Brian Knudtson says
I have gotten the same message: HT and FT are compatible and they are still working on updating documents that state otherwise.
buckmaster says
To help validate Duncan’s statement while at PTAB last week the Product Manager for FT also stated hyper-threading does NOT need to be turned off to support FT. The services delivery kit will be updated to reflect this statement.