Multiple processor Scheduling

•  In a multiprocessor systems that are multiple CPUs.
• The processing load is distributed among these available processors.
• Multiple processor have a shared memory.
• There are two types of Multi processor System.
• They are 

                          1. Asymmetric Multiprocessing

                          2. Symmetric Multiprocessing

 Asymmetric Multiprocessing

One processor is made the master processor handles all the scheduling policies, I/O operations resource allocation & other systems actions other processors works like a slave execution only user code.

    Single Schedular

         It had a dedicated CPU to run the schedular.

           

             Advantages

•  Simple to implement.
• Schedular have local queue in which processes are ready to run.

         Disadvantages              

     

•    CPU utilization is not proper so, performance is degrading.

Symmetric Multiprocessing System

• We will have a copy of OS is maintained by all processors.
• All processors share I/O bus & memory.
• The processes are waiting for CPU yo get execute are maintained in global ready queue.
• Each processor runs a schedular independently to select the process to execute.
• They are two methods to execute scheduling.
• They are 1.Using global queues.

                         2. Using Separate queue for each CPU.

1.Symmetrical Scheduling with global queue

Here we using locking System to not get selected same process by different CPUs.

Advantage 

• Good CPU utilization.
• Fair to all processes.

Disadvantage

• Not scalable because contention for the global queue.
• Schedulers need to highly efficient.
• Processor affinity not easily achievable.

Symmetrical Scheduling with separate queue for each CPU

• Each CPU execute processes from its own local ready queue of processes.
• It uses static partition of processes across CPUs at the start of  the execution of process either the user , So decides in which CPU process will be placed on corresponding CPU queue.

Advantage

• Easy to implement.
• No locking mechanism required 
• Scalable

Disadvantage

    Load imbalance

      If  some CPUs have lot of processes in ready queue & some have fare less processes in ready queue.

Hybrid Approach

• It uses both global & local queues.
• Local queues are associated with each CPUs.
• Global queues is shared among CPUs.
• Global queue is used to maintain load balancing .
• Used in Linux 2.6.

        Advantage

           Load balancing across queues feasible.