Hyperthreading on Dell 6850

[NYSDHCR]

We currently have hyperthreading enabled on our Dell 6850s (Quad Core, Inter Xeon 3GHz, 32GB ram each). We are running various OSs on our vms (MS Windows 2003, Netware 6.5, etc). A Novell consultant mentioned we might want to disable hyper-threading on the Esx hosts because it increases performance if it is disabled. Is there any truth to that statement?

[Schorschi]

Yes and No. Testing is the way to determine. Hyperthreading is a trick, if you will, that really does work well in come cases. However, are you checking esxcfg-info to make sure you really have HT support? Just because you set the setting in the vmkernel does not mean it is enabled. Some Quads support HT, others don't. We tend to enable HT if it is supported, and most of the time we do not see any issues with it.

[weinstein5]

Not really - my view is by enabling HT you actually are providing more slots for the vmkernel to schedul vcpus - yes it is a trick and slight slower than the full core but in virtualization it is more important to provide a VM more cycles to run -

[mike_laspina]

Hi,

In my experiences I have found that more often than not disabling HyperThreading is a better option. I have seen no difference in the performance of a simple VM and have consistently seen significant increased performance for SQL 2005 servers. So I would say off is the prefered setting.

[RParker]

First of all a 6850 doesn't have hyperthreading, so its not an issue. Dual Core / Quad Core processors replace hyperthreading with real cores, not "idle" ones. So you can't turn hyperthreading on or off.

In my experience using hyperthreading on the host is enabled, and it does offer slightly better performance overall, but as the others have noted it tricks the ESX server into thinking it has more logical cores. The problem is if you use multi CPU VM's, they may schedule time on a "hyper threaded" core and thus their performance may not be what you expect.

You have to test it for yourself, but I find that more often than not, hyperthreading does offer some improvement, but you don't get this on a 6850.

[mike_laspina · Accepted solution]

The Dell 6850 product fully supports HyperThreading and it can be turned on or off in bios.

Intel processors do not replace HyperThreading functions with anything on a separate core. HyperThreading just exposes a segment of a core that looks like another processor on that same core so that an extra app thread can be piped onto that core not some other core. If a thread is scheduled on a separate core we call that multiprocessing and the app has no idea it that it occured and that is not HyperThreading.

Message was edited by: mike.laspina - added what HyperThreading is.

[NYSDHCR]

Thanks - Here's VMware's response (just received it) - Hyperthreading technology allows a single physical processor to behave like two logical processors. The processor can run two independent applications at the same time. While hyperthreading does not double the performance of a system, it can slightly increase performance by keeping the processor pipeline busier. If the hardware and BIOS support hyperthreading, ESX automatically makes use of it. To enable hyperthreading: Ensure that your system supports hyperthreading technology. All Intel Xeon MP processors and all Intel Xeon DP processors with 512 L2 cache support hyperthreading. However, not every Intel Xeon system ships with a BIOS that supports hyperthreading. Consult your system documentation to see if the BIOS includes support for hyperthreading. Enable hyperthreading in the system BIOS. Some manufacturers label this option Logical Processor while others call it Enable Hyperthreading. An ESX Server system enabled for hyperthreading should behave almost exactly like a standard system. Logical processors on the same core have adjacent CPU numbers, so that CPUs 0 and 1 are on the first core together, CPUs 2 and 3 are on the second core, and so on. VMware ESX Server systems manage processor time intelligently to guarantee that load is spread smoothly across all physical cores in the system. If there is no work for a logical processor, it is put into a special halted state, which frees its execution resources and allows the virtual machine running on the other logical processor on the same core to use the full execution resources of the core.