First, the "virtualization technology" bit in the computer's BIOS is probably not on. See
I have a 64bit host, but can't install 64bit guests, post 1.
You should see a green portion after turning this on. If not, check if Hyper-V is on, and turn it off. See posts 2 & 3 in the 64-bits link.
If you still see less than the maximum processors available on the slider, this would probably be a hard-coded limitation in the GUI. You would need to use 'vboxmanage' on the command line to add more processors to a VM.
Adding more processors only helps when the OS in the VM has more data to process than it can with the processors it has now. Extra processors make a VM run slower, due to extra scheduling overhead in the host OS. If the OS in the VM does not have enough data to use up all its processors, the data will execute more slowly. A typical Windows or Linux OS likes two processors for its typical usage. Only add processors if the VM is fully stabilized after install, and you add your own programs, and the VM is still running 100% and your added programs have lots of multi-processing parallel threads.
qiping wrote: 56 cores (112 processors).
This is probably 56 hyperthreaded cores. For Virtualbox purposes, only the cores count. Hyperthreads don't help a Virtualbox guest. So you have a maximum of 56 cores (55, actually, the host needs one) to assign to guests. And as mentioned above, only assign all 55 to one guest if there is 55 cores' worth of parallel threads to process, as well as memory and disk bandwidth, or the VM will execute as slow as an 8086....
