Multiple Processor Scheduling

Multiple Processor Scheduling

When multiple processors are available, then the scheduling gets more complicated, because now there is more than one CPU which must be kept busy and in effective use at all times.

Load sharing revolves around balancing the load between multiple processors.

Multi-processor systems may be heterogeneous, ( different kinds of CPUs ), or homogenous, ( all the same kind of CPU ). 

1. Approaches to Multiple-Processor Scheduling
Asymmetric multiprocessing: One processor is the master, controlling all activities and running all kernel code, while the other runs only user code. 

Symmetric multiprocessing (SMP): Each processor schedules its own jobs, either from a common ready queue or from separate ready queues for each processor.

2. Processor Affinity
Soft affinity occurs when the system attempts to keep processes on the same processor but makes no guarantees. 
Linux and some other OSes support hard affinity, in which a process specifies that it is not to be moved between processors.

3. Load Balancing
Obviously an important goal in a multiprocessor system is to balance the load between processors, so that one processor won't be sitting idle while another is overloaded.

4. Multicore Processors
Traditional SMP required multiple CPU chips to run multiple kernel threads concurrently.
Recent trends are to put multiple CPUs ( cores ) onto a single chip, which appear to the system as multiple processors.

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