18.9.1 Process Scheduling

Within an OpenVMS environment, processes share access to the CPU based on the OpenVMS process-scheduling algorithm. The most important aspect of this algorithm is the scheduling priority of a process. Scheduling priority for interactive users is initially established based on the priority specified in the users AUTHORIZE record. Batch-job priority is set to the default base priority established with the INITIALIZE/QUEUE/ BASE_PRIORITY command. In general, various processes are assigned the following priorities, where 1 is lowest:

• 1 3 Batch jobs

• 4 7 Interactive users (usually 4)

• 8 6 VMS system processes (SWAPPER, etc.)

• 16 32 Real-time processes

Internally, priorities on Alpha/VMS range from 0 to 63 in compliance with POSIX threads. OpenVMS always gives precedence to the computable process with the highest priority. Processes with the same priority are serviced in a round-robin manner. It is important to understand that a CPU-intensive process at priority 5 completely blocks the CPU from all priority 4 processes. As a result, all interactive users should be set to the same priority (generally priority 4). Batch jobs can be assigned priorities between 1 and 3, depending on which are more time critical. However, priorities are not static, as is explained in section 18.9.2.