I needed to ensure good CPU and memory performance in a VirtualBox virtual machine running on a 4-core desktop, and googling didn’t provide any clear guidance. After some benchmarking, the surprise came in the shape of consistently getting the best result when ignoring VirtualBox’ warning on oversubscribing processors:

As always, these results are only valid for my particular configuration and on my chosen benchmarks, including the assumptions that the physical host is idling while the single virtual machine is running flat out – do test with your own systems and tasks.

Physical Host

Desktop system where the CPU and motherboard were released in AD 2013:

CPU	Single Intel Core i7 4770K, 3.5GHz, 4 physical cores, 8 logical cores (when Hyper-Threading is enabled) Note: All cores run at 3.7GHz during multi-threaded benchmarks; for single-threaded benchmarks, the core runs between 3.7 and 3.9GHz
Memory	G Skill F3-2400C10D-16GTX, Trident X Series, 2x8GB, PC3-19200, DDR3 2400MHz Dual Channel, DRAM:1200MHz, CL:10 tRCD:12 tRP:12 tRAS:31 tRFC:363 CR:2T
Motherboard	ASRock Z87 Extreme9/ac, BIOS 2.30
Disk	Samsung 840 EVO 1TB SSD
OS	Windows 7 Ultimate SP1 64-bit

Note: The results in this article are likely not applicable to NUMA systems with physical processors in multiple sockets, since these have very different memory, cache, and thread scheduling characteristics.

VirtualBox

Version	4.3.26
Guest Processors	4 or 8
Guest Memory	8GB
Guest OS	Windows Server 2008 R2 Standard SP1 64-bit
Guest Settings	IO-APIC, 100% Execution Cap, PAE/NX, VT-X / AMD-V, Nested Paging all enabled

Benchmarks

For my particular requirements, I chose mainly multi-threaded CPU and memory bound benchmarks, with some disk benchmarks to guard against IO regressions – do follow the links for specifics on the individual benchmarks:

y-cruncher

Multi-threaded. Calculates Pi. Mainly CPU and thread communication limited. Requires and uses SSE.

PassMark

PerformanceTest 8.0 (Build 1025) 64-bit   “Preferences > Number of processes” set to number of logical processors on host (i.e. 4 or 8) All CPU and memory benchmarks have been included Disk benchmarks were also executed, but detailed results are not included due to the variability in IO systems: Enabling/disabling Hyper-Threading did as expected not have any impact on disk performance With Samsung RAPID mode disabled, disk performance in the VirtualBox guest ranged from 12% slower to 4% faster than the physical host Enabling RAPID mode (which uses main memory as cache for SSD) improved VirtualBox guest disk performance with about 40%, and improved physical host disk performance with a whopping 9.5x – real life mileage will of course vary wildly

Results

To aid digestion, I’m presenting the data as speed-up or slowdown of different configurations vs. the on average fastest configuration, which was to run on the physical host with Hyper-Threading enabled.

The “Overall Average” section at the top of the chart is the average slowdown of all the actual benchmarks further down. Comparing to physical host with Hyper-Threading enabled, we see that running on:

• VB 4 NoHT (VirtualBox with 4 processors, host has Hyper-Threading disabled) is on average the slowest at -22%, with individual benchmarks ranging from -2% to -55% slower
• VB 4 HT (VirtualBox with 4 processors, host has Hyper-Threading enabled) is on average -22% slower, with individual benchmarks ranging from 2% faster to -44% slower
• Phys 4 NoHT (Physical host, Hyper-Threading disabled) is on average only -10% slower, with individual benchmarks ranging from 1% faster to -50% slower
• VB 8 HT (VirtualBox with 8 processors, host has Hyper-Threading enabled) is on average only -9% slower, with individual benchmarks ranging from -2% to -27%

Conclusions

Given my set-up, requirements and assumptions, I find that:

• Disabling Hyper-Threading makes both the physical host and the virtual machine on average quite a bit slower – I’ll leave it enabled.
• Following VirtualBox’ recommendation of limiting virtual processors to number of physical cores brings a slowdown of -22% (VB 4 HT above). I’ll instead configure as many virtual VirtualBox processors as there are logical (Hyper-Threaded) cores (VB 8 HT above), giving only a -9% slowdown.

Finally, a small warning: if you configure VirtualBox to use more processors than there are logical (Hyper-Threaded) cores (e.g 16 virtual processors on my 4770K) , it can run an order of magnitude slower than normal – simply ensure that you have no more VirtualBox processors configured than there are logical (Hyper-Threaded) cores available.

Hope it helps!