Thread Management and Context Switching

Threads and Their Utility on a Single CPU

Introduction to Threads on CPUs

  • The question arises whether threads are beneficial when the number of threads exceeds the number of CPUs.

Disk Request Scenario

  • Illustration of threads with a scenario involving disk requests:

    • Thread T1 makes a disk request.

    • The disk requires a certain amount of time to position the spindle to access the requested data until it replies.

    • This period of waiting is denoted as T idle.

Thread and CPU Idle Time

  • Idle Time for Thread T1:

    • During the T idle, Thread T1 is idle and unable to perform any operations, which leads to the CPU also being idle.

    • When the T idle is significantly longer than the time taken to perform a context switch from T1 to another thread, it becomes practical to switch.

Context Switching Mechanism

  • Definition of Context Switch: Transition from executing one thread to another, which involves saving the state of the current thread and loading the state of the new thread.

  • The efficiency of context switching depends on the comparison between:

    • T idle: The idle waiting period of the thread.

    • T context switch: The time taken to perform the context switch.

  • Context switching becomes beneficial when:

    • T<em>idle>2imesT</em>context switchT<em>{idle} > 2 imes T</em>{context \, switch}

    • In this scenario, it is justifiable to switch to Thread T2, allowing it to perform productive work while Thread T1 is waiting.

Advantages of Threading vs. Processes

  • Context switching is applicable to both processes and threads, but:

    • With processes, a costly step during context switching is the creation of new virtual to physical address mappings for the scheduled process.

    • With threads, since they share the same address space, there is no requirement to create new virtual to physical address mappings, which saves time.

  • This leads to:

    • Time Efficiency in Context Switching Among Threads:

    • The time taken to switch among threads is shorter than switching among processes due to the elimination of the physical address mapping step.

Implications of Efficient Context Switching

  • By decreasing the time of context switches:

    • More instances of T idle situations can be utilized effectively by switching to other threads.

    • This improves the overall performance in environments where idle time occurs due to IO operations.

  • Conclusion:

    • Multi-threading is particularly advantageous even in single CPU systems, as it assists in concealing latency related to IO operations and optimizes CPU utilization during idle periods.