Linux与Windows 7（VM）C ++执行速度

Question

I have a simple machine learning program I wrote in C++ with dlib. On the machine I wrote and tested it on, an ASUS Eee PC with a 1.6 GHz Atom processor running Kali Linux and compiling with g++ via cmake, it runs from start to finish in 2 minutes. However, in a Windows 7 VM (in VMWare Fusion) running on a Macbook Pro with a 2.3 GHz Core i7, the exact same code, compiled with VS 2013, takes 2 hours to run. I'm working on a way to implement multi-threading in the code to take advantage of the 4 vCPUs I allocated to the VM but still, the difference in performance is extreme and I haven't been able to figure out the cause. Even accounting for the performance overhead of virtualization I expected the VM to be faster... I feel like I might be missing something obvious. How can I get the execution speed in the VM to at least be on par with the Linux machine?

Answer 1

Without seeing the source code and profiling it is hard to say (if not impossible), there are many possible reasons.

1) windows on the VM detect in some way it is running slow (because of VM) so to keep responsive it reduce execution of your code (a way to speed it up in theory is adding a loop that periodically sample user input, the scheduler tend to give more execution time to processes that need interaction)

2) VS produced debug/profile symbols, if I remember correct that's the default, and they add a huge overhead (try build release then).

3) The overhead is already correct. If I remember correctly starting Eclipse on my machine takes somewhat 10 seconds, but 3/4 minutes on the VM (same PC) (approximately I think it has similiar ratio for 2 minutes to several hours.)

4) there may be other bottlenecks (like bandwith usage)

5) you don't know how exactly the VM works internally (but you could do benchmarks for various cases on both machines and see where do you find anomalies)

6)Curiosity there exists cases where VM is faster (where I remember somone noted a code runned faster on WinE than on Windows, because WinE was using a faster fmod implementation)