Process Scheduling Algorithm in Operating Systems: A Detailed Explanation of Shortest Remaining Time First (SRTF)

1. Knowledge Point Description

Shortest Remaining Time First (SRTF) is a process scheduling algorithm. It is the preemptive version of the Shortest Job First (SJF) scheduling algorithm. Its core idea is: when a new process enters the ready queue, or when a currently running process finishes its current CPU burst, the scheduler always selects the process with the shortest estimated remaining execution time from the ready queue and the currently running process to allocate the CPU. If the remaining time of the newly arrived process is less than that of the currently running process, the current process is preempted, and the CPU is allocated to the new process.

SRTF is often considered "theoretically optimal" among preemptive scheduling algorithms because it minimizes the average waiting time and average turnaround time of processes. However, in real operating systems, it is difficult to implement because it relies on accurately predicting each process's "next CPU burst time," which is usually unknown.

2. Core Concepts and Background Knowledge

Before delving into SRTF, we need to clarify several concepts:

• CPU Burst Time: The length of CPU time a process requires to execute continuously after obtaining the CPU until it performs an I/O operation again or finishes. Scheduling algorithms need to estimate this time.
• Preemption: The operating system can forcibly deprive a running process of its CPU usage rights and allocate the CPU to another higher-priority process during its execution.
• Preemptive vs. Non-Preemptive: SJF is non-preemptive; it makes scheduling decisions only when a process voluntarily relinquishes the CPU (e.g., for an I/O operation or termination). SRTF is preemptive; the arrival of a new process triggers a scheduling comparison.

3. Algorithm Principle and Execution Steps

Let's understand the workflow of SRTF step by step through a specific example.

Assume we have four processes with arrival times and CPU burst times as follows:

The scheduler's goal is: At any decision moment, let the process with the shortest remaining time run.

Step 1: Time 0ms

• Only P1 is in the ready queue (arrival time 0, remaining time 8ms).
• Scheduling Decision: Select P1 to run.

Step 2: Time 1ms

• P1 has run for 1ms, its remaining time becomes 7ms.
• New process P2 arrives (arrival time 1, remaining time 4ms).
• Compare the currently running process (P1, remaining 7ms) with the new process (P2, remaining 4ms).
• P2's remaining time (4ms) < P1's remaining time (7ms).
• Scheduling Decision: Preempt P1, allocate the CPU to P2. P1 is placed back into the ready queue.

Step 3: Time 2ms

• P2 has run for 1ms (from 1ms to 2ms), its remaining time becomes 3ms.
• New process P3 arrives (arrival time 2, remaining time 9ms).
• Compare the currently running process (P2, remaining 3ms) with the new process (P3, remaining 9ms).
• P2's remaining time (3ms) < P3's remaining time (9ms).
• Scheduling Decision: No preemption. P2 continues running.

Step 4: Time 3ms

• P2 has run for 2ms (from 1ms to 3ms), its remaining time becomes 2ms.
• New process P4 arrives (arrival time 3, remaining time 5ms).
• Compare the currently running process (P2, remaining 2ms) with the new process (P4, remaining 5ms).
• P2's remaining time (2ms) < P4's remaining time (5ms).
• Scheduling Decision: No preemption. P2 continues running.

Step 5: Time 5ms

• P2 finishes running at time 5ms (ran from 1ms to 5ms, total run time 4ms).
• Scheduling Decision: Now need to select the process with the shortest remaining time from the ready queue {P1(remaining 7ms), P3(remaining 9ms), P4(remaining 5ms)}.
• P4 has the shortest remaining time (5ms).
• Allocate the CPU to P4.

Step 6: Time 10ms

• P4 finishes running at time 10ms (ran from 5ms to 10ms, total run time 5ms).
• Scheduling Decision: Select from the ready queue {P1(remaining 7ms), P3(remaining 9ms)}.
• P1's remaining time (7ms) < P3's remaining time (9ms).
• Allocate the CPU to P1.

Step 7: Time 17ms

• P1 finishes running at time 17ms (ran from 10ms to 17ms, total run time 8ms).
• Scheduling Decision: Only P3 (remaining 9ms) remains in the ready queue.
• Allocate the CPU to P3.

Step 8: Time 26ms

• P3 finishes running at time 26ms (ran from 17ms to 26ms, total run time 9ms).
• All processes have finished.

The final scheduling Gantt chart is as follows:

Timeline: 0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26
Process:  | P1 | P2 | P2 | P2 | P2 | P4 | P4 | P4 | P4 | P4 | P1 | P1 | P1 | P1 | P1 | P1 | P1 | P3 | P3 | P3 | P3 | P3 | P3 | P3 | P3 | P3 |
        (Pre)   (Arr) (Arr)
        empt    ive   ive

4. Performance Metrics Calculation

We can calculate several key performance metrics and briefly compare them with First-Come, First-Served (FCFS) to highlight the advantages of SRTF.

• Turnaround Time = Completion Time - Arrival Time
• Waiting Time = Turnaround Time - CPU Burst Time
• Response Time = Time First Gets CPU - Arrival Time (important for interactive systems)

Average Values:

• Average Turnaround Time = (17 + 4 + 24 + 7) / 4 = 13.0 ms
• Average Waiting Time = (9 + 0 + 15 + 2) / 4 = 6.5 ms
• Average Response Time = (0 + 0 + 15 + 2) / 4 = 4.25 ms

Comparative Thinking: If using FCFS scheduling (order P1->P2->P3->P4), P2, P3, and P4 would all need to wait for P1's lengthy 8ms. The average waiting time and turnaround time would be much higher than SRTF. By allowing "short tasks to cut in line," SRTF significantly improves the responsiveness of short processes and the overall average system performance.

5. Advantages, Disadvantages, and Implementation Difficulties of SRTF

Advantages:

1. Theoretically Optimal: Among all preemptive scheduling algorithms based on CPU burst time, SRTF yields the minimum average waiting time and average turnaround time.
2. Good Responsiveness: Short processes can quickly obtain the CPU, making it friendly to interactive tasks.

Disadvantages and Difficulties:

1. Inability to Predict the Future: The biggest challenge is how to accurately know a process's "next CPU burst time." The operating system cannot foretell the future. Typically, exponential average prediction is used, estimating future times based on the process's historical CPU and I/O burst times, but this is only a prediction and not precise.
2. Starvation of Long Processes: If short processes continuously arrive in the system, long processes may be indefinitely postponed, never getting CPU time, leading to "starvation." In the example above, P3 (a long process) had a very long response time.
3. High Context Switch Overhead: Due to frequent preemptions, context switches between processes become very frequent. If the average burst time of processes is short, this overhead can severely impact system performance.

6. Practical Applications and Variants

Because pure SRTF is difficult to implement, real operating system schedulers borrow its ideas but make compromises:

• Multilevel Feedback Queue (MLFQ): This is the most successful practical application of the SRTF concept. MLFQ approximates SRTF by dynamically adjusting process priorities: keeping short jobs (I/O-bound) in high-priority queues for quick completion, and gradually demoting long jobs (CPU-bound) to lower-priority queues, running them only when no short jobs are available. This both accommodates the responsiveness of short jobs and prevents long jobs from starving.
• Linux's Completely Fair Scheduler (CFS): Although based on virtual runtime (vruntime), its goal is fairness. When a new process (with a very small vruntime) joins, it is scheduled immediately, which to some extent simulates the effect of "shortest remaining time first."

Summary: Shortest Remaining Time First (SRTF) is an important theoretical model for scheduling algorithms, demonstrating the optimal form of the "shortest task first" concept in a preemptive environment. Understanding SRTF helps deeply grasp the design philosophy of modern operating system schedulers (like MLFQ, CFS)—how to balance fairness, throughput, and response time, and how to intelligently guess process behavior and make efficient scheduling decisions without knowing the future.