VirtualBox: Is it a bad idea to assign more virtual CPU cores than number of physical CPU cores

cpuhyperthreadingperformancevirtual machinevirtualbox

As I have a Hyper-Threading capable CPU, I wonder, is it a bad idea to assign more virtual CPU cores than number of physical CPU cores as the following warning suggests:

VirtualBox Warning

Transcript:

More virtual CPUs are assigned to the virtual machine than the number of physical CPUs on the host system. This is likely to degrade performance of your virtual machine. Please consider reducing the number of virtual CPUs.

Can someone put a reasoning to this topic?

EDIT1:

The CPU in question is Intel Core i7-4700HQ, Ark Intel, CPU Benchmark

EDIT2:

Supposing, there is no obsolete HW, like HDD (instead of an SSD), and / or Low RAM (16GB here, minimum vm.swappiness, 4GB for this VM), and so on.

Best Answer

Hardware / OS / Software

Host: Linux Mint 18 Cinnamon 64-bit (fully updated); Kernel version 4.4.0-47-generic

Guest: Windows 8.1 Pro 64-bit (fully updated)

Processor: Intel Core i7-4700HQ, (6MB cache, 4 physical cores, or 8 using Hyper-Threading), CPU Benchmark

VirtualBox: Version 5.1.10 r112026 (Qt5.5.1)

Guest Additions: Installed and up-to-date

Benchmark Tool #1: WinRAR version 5.40 final 64-bit

Benchmark Tool #2: VeraCrypt version 1.19 final 64-bit

Preparation

In both cases I waited after boot until the CPU, RAM, disk drive are at stable near zero-point hits.

Method

  1. Cloning the original virtual machine to have two identical ones.
  2. I have, for the second pass, since the reboot disabled Antivirus pointed out at the bottom of this answer and updated WinRAR in both cases from a Beta to the Final version.
  3. I have done the same Preparation as pointed out earlier.
  4. The virtual machine ran in foreground, without any other CPU time hungry application running, I have disabled what I could for the purpose of the test not being influenced.
  5. To include potential caching inside or outside the system, I ran the same test twice consequently. The benefit being almost none.

Results

WinRAR

  1. 4 cores => 7.5 minutes (shorter time is better)

    WinRAR with 4 cores enabled

    WinRAR with 4 cores enabled, 1.5GiB processed in 7.5 minutes.

  2. 8 cores => 4.5 minutes (shorter time is better)

    WinRAR with 8 cores enabled

    WinRAR with 8 cores enabled, 1.5GiB processed in 4.5 minutes.

VeraCrypt

  1. 4 cores => speed 2.6 GiB/s (higher speed is better)

    VeraCrypt with 4 cores enabled

    VeraCrypt with 4 cores enabled, HW-accelerated AES (AES-NI) speed 2.6 GiB/s.

  2. 8 cores => speed 3.9 GiB/s (higher speed is better)

    VeraCrypt with 8 cores enabled

    VeraCrypt with 8 cores enabled, HW-accelerated AES (AES-NI) speed 3.9 GiB/s.

Conclusion

I could run as many tests as necessary. But I figure, if these two, one of which is rather complex compression test, the second being a set of rather complex encryption tests, what would be the point.

Both of the benchmarks show a marked difference. I see no reason to believe, that their results are inaccurate, as I followed a rather rigorous preparation and method, moreover these tests have taken place in RAM to rule out I/O bottleneck. From my standpoint, the warning mentioned in the question may apply to some conditions, but certainly not all of them. Having shared with you these pretty remarkable results, I am certain for you to agree with me, that this warning probably should not be taken so seriously on modern CPUs featuring Hyper-Threading with the latest VirtualBox version. One thing for sure: Don't take me for the word and test it under your own conditions, before you decide to apply this setting permanently.

New benchmark

Host + Guest: Linux Mint 19.2 "Tina" - Cinnamon (64-bit); both with kernel: 5.3.0-24-generic.

Processor: Intel® Core™ i7-7700HQ; 6 MB Cache, up to 3.80 GHz, 4 physical cores, or 8 using Hyper-Threading, CPU Benchmark comparison

VirtualBox: Version 6.1.0 r135406 (Qt5.9.5)

Guest Additions: Installed and up-to-date

Benchmark Tool: VeraCrypt version 1.24 Hotfix1 64-bit final (web page, direct deb download link)

Preparation and Method

Same as previous benchmark.

Results

VeraCrypt AES encryption with 4 cores

⟶ speed 4.8 GiB/s (higher speed is better)

VeraCrypt AES encryption 4 cores = speed 4.8 GiB/s

VeraCrypt AES encryption with 8 cores (Hyper-Threading warning issued)

⟶ speed 7.2 GiB/s (higher speed is better)

VeraCrypt AES encryption 8 cores = speed 7.2 GiB/s

Conclusion

Wonderful 50% performance increase with Hyper-Threading enabled, but only with the AES sadly, I will have to run some more comprehensive test. Will be back in a few days with results.

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