Calculating CPU frequency in C with RDTSC always returns 0

Question

The following piece of code was given to us from our instructor so we could measure some algorithms performance:

#include 
#include 

        

Answer 1

As for VMWare, take a look at the time keeping spec (PDF Link), as well as this thread. TSC instructions are (depending on the guest OS):

• Passed directly to the real hardware (PV guest)
• Count cycles while the VM is executing on the host processor (Windows / etc)

Note, in #2 the while the VM is executing on the host processor. The same phenomenon would go for Xen, as well, if I recall correctly. In essence, you can expect that the code should work as expected on a paravirtualized guest. If emulated, its entirely unreasonable to expect hardware like consistency.