Convoy Effect in Scheduling Operating System

April 21, 2025

About Convoy Effect in OS

On this page, we will explore the Convoy Effect in Scheduling Operating System, a crucial concept that affects system performance, especially in First Come, First Served (FCFS) scheduling. The convoy effect, also referred to as starvation in FCFS, occurs when a single long process holds up the CPU, causing shorter processes to wait and leading to inefficient resource utilization.

As a result, the CPU and other critical resources remain idle for extended periods. Understanding this effect is essential for designing better scheduling algorithms that minimize delays and optimize throughput.

Convoy Effect in Scheduling OS

Convoy Effect in Scheduling Operating System

What is the Convoy Effect?

The Convoy Effect is a performance-degrading phenomenon observed in the First Come First Serve (FCFS) scheduling algorithm in operating systems. It occurs when a large or long-duration process occupies the CPU, causing delays for all the shorter processes queued behind it.

How Does it Happen?

  • In the FCFS scheduling algorithm:
    • Processes are scheduled in the order they arrive in the ready queue.
    • It is a non-preemptive algorithm, meaning once a process starts executing, it cannot be interrupted until it finishes.
  • If the first process in the queue is very time-consuming, it holds up the CPU for a long duration.
  • All subsequent processes, even if they are short and quick, must wait until the long process completes.

Analogy to Understand the Convoy Effect

To better understand this, consider the following analogy:

  • Imagine a very long train (convoy) crossing a railway crossing.
  • Smaller vehicles like cars, bikes, and buses are waiting behind it.
  • No matter how fast or small the vehicles are, they cannot proceed until the entire train (convoy) passes.
  • This leads to increased waiting time for all the vehicles.

Similarly, in FCFS:

  • A long-running process is like the train.
  • The shorter processes are like the smaller vehicles stuck behind it.

Consequences of the Convoy Effect

  • Increased waiting time for all subsequent processes.
  • Reduced CPU utilization if I/O-bound processes are held back.
  • Starvation of smaller or more interactive tasks.
  • Poor system responsiveness, especially in a multi user environment.

Mitigating the Convoy Effect

Some alternatives to avoid or reduce the convoy effect include:

  • Using Preemptive Scheduling Algorithms like:
    • Shortest Remaining Time First (SRTF)
    • Round Robin (RR)
    • Priority Scheduling
  • These algorithms allow the CPU to be reassigned before the current process finishes, thereby improving responsiveness and throughput.

Prepare for Interview with us:

Top 100 Codes
Top 150 Notout Qs
Learn via Videos
Convoy effect

Steps are as following below which leads to convoy effect:

  • The I/O bound processes are first allocated CPU time ,quickly get executed and goto I/O queues as they are less CPU intensive.
  • Now, the CPU intensive process is allocated CPU time. As its burst time is high, it takes time to complete.
  • While the CPU intensive process is being executed, the I/O bound processes complete their I/O operations and are moved back to ready queue.
  • However, the I/O bound processes are made to wait as the other CPU intensive process still hasn’t finished. This leads to I/O devices being idle and  hence performing inefficiently.
  • When the CPU intensive process gets executed, it is sent to the I/O queue so that it can access an I/O device.
  • Meanwhile, the I/O bound processes get their required CPU time and move back to I/O queue.
  • However, they are made to wait because the CPU intensive process is still accessing an I/O device. As a result, the CPU is sitting idle now.

Hence in Convoy Effect, one slow process affects and slows down the performance of the other processes, and leads to wastage of CPU time and other resources.

To avoid Convoy Effect, preemptive scheduling algorithms like Round Robin Scheduling can be used – as the smaller processes don’t have to wait much for CPU time – making their execution faster and leading to less resources sitting idle.

Final Thoughts

The Convoy Effect is a significant drawback of the First Come First Serve (FCFS) scheduling algorithm, where shorter processes are forced to wait behind a long-running process, leading to inefficient CPU utilization and increased waiting times. This effect highlights the limitations of non-preemptive scheduling in handling diverse workloads. To ensure better system performance and responsiveness, it is often more effective to use preemptive scheduling algorithms that allow for more flexible and fair CPU allocation.

FAQs

While most common in FCFS, similar delays can happen in any non-preemptive scheduling algorithm if not managed properly. However, its impact is less severe in algorithms designed with fairness and time-sharing in mind.

No, it affects the entire system’s efficiency, including I/O devices, as they remain idle while waiting for the CPU to free up. This leads to underutilization of multiple system resources.

Convoy Effect is a group delay caused by a long running process, while starvation refers to a specific process being perpetually denied CPU access. The Convoy Effect can contribute to starvation but is more of a system-wide bottleneck.

FCFS is simple to implement and works well in scenarios with uniform process lengths. It’s often used in batch systems where fairness by arrival time is prioritized over responsiveness.