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I have (as some are probably tired of seeing on SF) a Dell 2950, dual 4 core 3Ghz processors running ESXi. I noticed on the VSphere management console that the server says it doesn't have hyperthreading enabled ("inactive")

Apparently it is by default turned off in the BIOS.

I know I could shut down all the VM's and reboot and edit that setting, but should I? Is there a real benefit to having hyperthreading enabled on this type of setup, or is the performance gain marginal, if any??

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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.

Pros

  • 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 workloads).

Cons

  • 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, degrade performance.

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 have available.

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.

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    It has been our experience that hyperthreading does boost performance around 15-25%. We are running 16 VM's on a 4 CPU (HT enabled) IBM xSeries server, with a variety of workloads. VMware has an article on hyperthreading, somewhere on their site. Commented Jul 24, 2009 at 18:47
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Are you sure your CPUs even have HyperThreading on them? I think most quad core CPUs don't. My ESX Servers also report that HyperThreading is "inactive" and I've got AMD CPUs which have never had HT on them.

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  • Hmm...maybe not. I thought all the newer ones had it now and perhaps I was assuming too much! Commented Jul 24, 2009 at 18:08
  • Check with the CPU vendor to see if the CPU model even supports HT. You may save yourself a reboot right there. Commented Jul 24, 2009 at 18:21
  • It's an X5450 Xeon, so from Intel they say it doesn't support HT. The message in the summary is misleading! Commented Jul 24, 2009 at 18:58
  • @Bar Silverstrim: Only the new nehalem(sp?) based procs have HT every other multi core proc doesn't have it. Commented Jul 24, 2009 at 19:20
  • @Bart Silverstrim sounds like you've got your answer then. Commented Jul 24, 2009 at 19:22
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The only Intel CPU's that support Hyperthreading are the Netburst (Pre Core\Core Duo) architecture and the most recent Nehalem based CPU's (ie for servers only the Xeon 5500's at this point in time). The 2950 range does not support the Xeon 5500 but it does support the earlier 5000, 5100,5200, 5300 & 5400 ranges (the precise CPU's supported varies a bit between the various 2950 models). Of those only the oldest, the 5000 series (Dempsey), supports Hyperthreading as it was based on the Netburst architecture.

All Quad Core Xeon 5xxx's are based on the Core or Nehalem architecture so in your case your CPU's do not support Hyperthreading.

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