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I've just received an upgraded Host machine, and am looking to push some of those advances to my workstations Guest OS(s). In particular, I used to have a single processor, with 2 cores, so my Guest OS only had 1/1.

Now, I've got a single processor with 8 cores, so I'm curious about what would be recommended for my Guest OS now?

  • 1 processor/4 cores?
  • 2 processors/2 cores?
  • 4 processors/1 core?

My instinct says to stick with the number of physical processors (or less), but, is that based on reality? I spent a good while looking for an answer to this, but perhaps my google-karma isn't in my favor today.

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Is your guest using up the processing capabilities it has as it is now? –  OldWolf Sep 8 '11 at 20:54
    
This link may be helpful, suggesting that you match your configuration to the host's NUMA configuration. –  Glenn Nov 9 '13 at 0:26

3 Answers 3

In my own testing, with VMWare Workstation, using latest GeekBench 3, 64-bit tests, on a host machine with 1 cpu, 2 cores (with HT turned on, so 4 cores):

Host System:

  • 2866 Single Core Score, 5939 Multi-Core Score

Virtualized:

  • 1 cpu, 1 core: 2783 sc, 2705 mc
  • 1 cpu, 2 cores: 2758 sc, 4271 mc
  • 1 cpu, 3 cores: 2783 sc, 5234 mc
  • 1 cpu, 4 cores: 2769 sc, 5793 mc

So, at least in my testing, it looks like there's a benefit to setting your virtual number of cores to match your physical cores. Maybe I'm wrong. I'd love to discuss.

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Excellent research! Thanks. Could you extend this research to cover some of the other options I highlighted in the original post? Like a virtual 2/2 and 4/1 system (having already identified that a 1/4 out performs 1/4- setups)? That's the real question. Should you go 1/4, 2/2, or 4/1 (assuming you're going with 4 virtual cores overall). Also, I'll admit that I don't understand the difference in "single core score" and "multi-core score" in each of your examples. How can a 1cpu/1core machine get a multi-core score at all? Same for the 1cpu/4core situation, in reverse.... –  reidLinden Jan 9 at 14:01

At most I give my guest OS 2 cores. It doesn't matter if you do 1 processor 2 cores or 2 processor 1 core each. The procesor setting is for compatibility reasons. Processor/cores don't scale so well for guest OS because of core/processor process scheduling. 2 cores seems to be the sweet spot.

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You want to avoid having one processor and one core for a VM because if any operation that the host OS doesn't expect to block blocks unexpectedly in the host, the entire VM is blocked. –  David Schwartz Nov 9 '13 at 0:45

I don't know if this information is still valid, but in the not-too-long-ago past, additional guest CPUs didn't scale nearly as well as host CPUs did. In fact, best recommendation was to stay with single CPUs in your guest configurations unless you were specifically testing/debugging multithreaded software in your guests and required an SMP environment.

Edit: This answer is particular to desktop virtualization, not server virtualization.

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so, you're saying 1 processor, with a single core? –  reidLinden Feb 14 '11 at 16:26
    
Yes. Does VMware workstation allow for setting those values independently now? I haven't used it since ver 6, and it was just a # of CPUs. If you're going to do multi-core/multi-CPUs in your guests, I doubt it much matters which way you scale things unless licensing considerations (# of "sockets" vs. # of "cores") come into play. e.g. XP Home only allows 1 socket, but up to 4 cores, while XP Pro allows for 2 sockets. –  afrazier Feb 14 '11 at 16:50
    
yea, I appear to have the option to pick "# processors" and "# cores" independently... My Host machine has 1 processor with 8 cores, on a windows7 Professional, and my (main) guestOS is a windows server 2003... –  reidLinden Feb 14 '11 at 17:07

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