Usage time is tallied only for use of compute machines accessed through the Velocity Scheduler (vsched). Time on the login machines or the collaboratory machines is not debited from your allocation.

All machines types are debited the same flat wall-clock rate., even though the number of processors on a machine varies between one and two. If you can parallelize your application, you can get two processors for the price of one.

Wall-clock hours, not compute time, are used to calculate charging. This means your allocation will be depleted by the amount of time you have compute machines reserved for your use, not for the time you are actually using them. If a batch job finishes two hours early but is not returned to the pool by using vsched -c you will be debited for the time. This topic is covered in detail in the Velocity Scheduler module, specifically the section on ending your job.