Multiprocessor Scheduling

 # Multiple Processors Scheduling in Operating System

## Introduction

In the world of operating systems, multiple processors scheduling, also known as multiprocessor scheduling, plays a crucial role in efficiently managing the workload in systems that have more than one processor. The goal of multiprocessor scheduling is to ensure that various processes run simultaneously on multiple central processing units (CPUs). While single processor scheduling can be complex in itself, the challenges multiply when dealing with multiple processors, making it a fascinating area of study. In this article, we will explore the intricacies of multiple processor scheduling, the two main approaches to it, the concept of processor affinity, load balancing, multi-core processors, and various multiprocessor models.

## Table of Contents

1. [Understanding Multiprocessor Scheduling](#understanding-multiprocessor-scheduling)

    1. [The Complexity of Multiprocessor Scheduling](#the-complexity-of-multiprocessor-scheduling)

    2. [Homogeneous and Heterogeneous Multiprocessor Systems](#homogeneous-and-heterogeneous-multiprocessor-systems)

2. [Approaches to Multiple Processor Scheduling](#approaches-to-multiple-processor-scheduling)

    1. [Symmetric Multiprocessing (SMP)](#symmetric-multiprocessing-smp)

    2. [Asymmetric Multiprocessing](#asymmetric-multiprocessing)

3. [Processor Affinity](#processor-affinity)

    1. [Soft Affinity](#soft-affinity)

    2. [Hard Affinity](#hard-affinity)

4. [Load Balancing](#load-balancing)

    1. [Push Migration](#push-migration)

    2. [Pull Migration](#pull-migration)

5. [Multi-core Processors](#multi-core-processors)

    1. [The Challenge of Memory Stalls](#the-challenge-of-memory-stalls)

    2. [Coarse-Grained Multithreading](#coarse-grained-multithreading)

    3. [Fine-Grained Multithreading](#fine-grained-multithreading)

6. [Symmetric Multiprocessor (SMP)](#symmetric-multiprocessor-smp)

    1. [Contending Sources in SMP Systems](#contending-sources-in-smp-systems)

7. [Master-Slave Multiprocessor](#master-slave-multiprocessor)

    1. [Asymmetric Multiprocessing in Action](#asymmetric-multiprocessing-in-action)

8. [Virtualization and Threading](#virtualization-and-threading)

    1. [Challenges in Virtualized Environments](#challenges-in-virtualized-environments)

## 1. Understanding Multiprocessor Scheduling

When dealing with systems that have multiple processors, the scheduling function becomes more intricate. The primary goal is to ensure that numerous processes can run concurrently on the available CPUs. Multiprocessor systems are commonly used in applications such as satellite operations and weather forecasting, where large amounts of data need to be processed efficiently.

### 1.1 The Complexity of Multiprocessor Scheduling

Compared to single processor scheduling, managing multiple processors can be quite challenging. With multiple CPUs working together, there is a need for close communication between them, sharing resources like memory, peripheral devices, and a common bus. This tightly coupled nature of the system adds to the complexity of multiprocessor scheduling.

### 1.2 Homogeneous and Heterogeneous Multiprocessor Systems

In the world of multiprocessor systems, there are two main types: homogeneous and heterogeneous. In homogeneous systems, all processors are identical in terms of their functionality. Any process can be executed on any available processor, providing a certain level of flexibility. On the other hand, heterogeneous systems consist of different kinds of CPUs. In such cases, there may be special scheduling constraints, such as devices connected via a private bus to only one CPU.

## 2. Approaches to Multiple Processor Scheduling

There are two main approaches to multiple processor scheduling in operating systems. These approaches define how the scheduling decisions are made and who handles I/O processing.

### 2.1 Symmetric Multiprocessing (SMP)

Symmetric Multiprocessing, often referred to as SMP, is an approach where each processor is self-scheduling. All processes may be in a common ready queue, or each processor may have its private queue for ready processes. The scheduling process involves each processor's scheduler examining the ready queue and selecting a process to execute.

### 2.2 Asymmetric Multiprocessing

Asymmetric Multiprocessing, on the other hand, relies on a single processor, known as the Master Server, to handle all scheduling decisions and I/O processing. The other processors in the system are primarily responsible for executing user code. This approach simplifies the system and reduces the need for extensive data sharing.

## 3. Processor Affinity

Processor affinity refers to a process having a preference or affinity for the processor on which it is currently running. This preference is due to the cache memory, where data recently accessed by the process is stored. If the process migrates to another processor, the cache contents must be invalidated, incurring high costs. There are two types of processor affinity:

### 3.1 Soft Affinity

Soft affinity is when an operating system has a policy of keeping a process running on the same processor but doesn't guarantee it will always do so. This flexibility allows for some level of load balancing among processors.

### 3.2 Hard Affinity

In contrast, hard affinity enables a process to specify a subset of processors on which it may run. This approach can be helpful in scenarios where specific tasks require dedicated processor resources.

## 4. Load Balancing

Load balancing is the process of distributing the workload evenly among all processors in an SMP system. This is particularly crucial in systems where each processor maintains its private queue of eligible processes for execution.

### 4.1 Push Migration

In push migration, a task routinely checks the load on each processor and redistributes the load to balance it across the processors. Overloaded processes are moved to idle or less busy processors.

### 4.2 Pull Migration

Pull migration occurs when an idle processor pulls a waiting task from a busy processor for execution, further enhancing load balancing.

## 5. Multi-core Processors

Multi-core processors have multiple processor cores placed on the same physical chip. Each core maintains its architectural state and appears as a separate physical processor to the operating system.

### 5.1 The Challenge of Memory Stalls

When a processor accesses memory and experiences a delay, called a memory stall, it spends a significant amount of time waiting for data to become available. To address this, hardware designs have implemented multithreaded processor cores.

### 5.2 Coarse-Grained Multithreading

Coarse-grained multithreading involves switching between threads when a long latency event occurs, such as a memory stall. However, this approach incurs high costs due to terminating and filling instruction pipelines.

### 5.3 Fine-Grained Multithreading

Fine-grained multithreading switches between threads at a much finer level, minimizing the cost of thread switching and improving overall processor efficiency.

## 6. Symmetric Multiprocessor (SMP)

Symmetric Multiprocessors (SMP) is a model where there is one copy of the OS in memory, and any CPU can run it. Scheduling is

 performed independently by each processor, and processes are selected from the ready queue for execution.

### 6.1 Contending Sources in SMP Systems

SMP systems face contention in three main areas: locking, shared data, and cache coherence. Locking is essential to protect shared resources from multiple processors' simultaneous access. Shared data access requires protocols or locking schemes to avoid data inconsistencies. Cache coherence ensures that shared resource data stored in multiple local caches is kept consistent.

## 7. Master-Slave Multiprocessor

The master-slave multiprocessor model employs a single data structure to track ready processes. It consists of one central processing unit acting as the master and other processors as slaves. The master server runs the operating system process, while the slave servers execute user processes.

### 7.1 Asymmetric Multiprocessing in Action

Master-slave multiprocessor systems reduce data sharing and are an example of asymmetric multiprocessing, where a single processor handles scheduling decisions and I/O processing.

## 8. Virtualization and Threading

Virtualization can transform a single CPU system into a multi-processor system. Virtual machines (VMs) run on the host operating system and are managed by it. Each VM has its guest operating system, and applications run within it.

### 8.1 Challenges in Virtualized Environments

Virtualization can lead to challenges in scheduling, especially for time-sharing operating systems. The allocation of virtual CPU time may not align with the actual time spent on the physical CPUs, impacting response times for users.

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## Conclusion

In conclusion, multiple processor scheduling in operating systems presents unique challenges and approaches. Symmetric and asymmetric multiprocessing offer different solutions for managing processes and resources efficiently. Processor affinity and load balancing help optimize performance, while multi-core processors aim to address memory stalls and enhance efficiency. Different models, such as SMP and master-slave multiprocessors, offer various strategies for handling contention and data sharing. Virtualization introduces new complexities in scheduling, making it crucial to consider the interactions between virtual and physical CPUs.

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## FAQs (Frequently Asked Questions)

1. **What is the primary goal of multiprocessor scheduling in operating systems?**

   The primary goal of multiprocessor scheduling is to ensure that various processes run simultaneously on multiple central processing units (CPUs) in an efficient manner.

2. **What are the two main approaches to multiple processor scheduling?**

   The two main approaches are Symmetric Multiprocessing (SMP), where each processor is self-scheduling, and Asymmetric Multiprocessing, where scheduling decisions and I/O processing are handled by a single processor called the Master Server.

3. **What is processor affinity, and what are its types?**

   Processor affinity refers to a process having a preference for the processor on which it is currently running. The two types of processor affinity are Soft Affinity (no guarantee of running on the same processor) and Hard Affinity (specifying a subset of processors for execution).

4. **What is the purpose of load balancing in multiprocessor systems?**

   Load balancing distributes the workload evenly among all processors in an SMP system, ensuring optimal utilization of all available processors.

5. **How does virtualization impact scheduling in multiple processor systems?**

   Virtualization introduces challenges in scheduling, as virtual machines may not receive the expected CPU time, affecting response times for users and VMs.

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Remember that multiple processor scheduling is a critical aspect of operating systems, especially in modern systems with multi-core processors and virtualization. By effectively managing the workload and balancing tasks, multiprocessor scheduling enhances overall system performance and responsiveness.