CompTIA Linux+ XK0-005 - 4.3 - High Run Queues

In a Linux environment, the run queue is a crucial metric for understanding the CPU's workload. A high run queue indicates that there are more processes waiting to be executed than the CPU can handle immediately. This guide aims to provide a detailed explanation of the run queue, its impact on system performance, and how to analyze it using the sar command.

What is a Run Queue?

The run queue, also known as the task queue or the run queue length, represents the number of processes waiting to be executed by the CPU. When a process is ready to run but cannot be immediately scheduled, it joins the run queue and waits for its turn. The run queue is typically maintained by the kernel's scheduler.

Impact of High Run Queue

A high run queue indicates that the CPU is struggling to keep up with the workload. When the run queue length exceeds the available CPU resources, processes may experience delays in execution, leading to decreased system responsiveness and performance. This can result in sluggishness, increased response times, and a decrease in overall system efficiency.

Analyzing Run Queue with sar

The sar command is a powerful tool for collecting and reporting system activity, including CPU-related metrics. To analyze the run queue using sar, follow these steps:

Install the sysstat package if it is not already installed on your system.

Use the following command to display the run queue length:

sar -q

Example

$ sar -q

  Linux 5.4.0-72-generic (hostname)  09/06/23  _x86_64_  (2 CPU)

  12:00:01 AM       runq-sz  plist-sz   ldavg-1   ldavg-5  ldavg-15   blocked
  12:10:01 AM            1       292      0.00      0.00      0.00         0
  12:20:01 AM            2       293      0.02      0.02      0.03         0
  12:30:01 AM            3       295      0.01      0.02      0.02         0
...

The sar -q command displays the system's run queue length over a specific time interval. It provides information such as the average run queue length (runq-sz column), which represents the number of tasks waiting for run time.

Conclusion

Analyzing and troubleshooting high run queues in a Linux environment is crucial for maintaining optimal CPU and memory performance. By understanding the concept of the run queue and utilizing the sar command with the runq-sz column, you can identify instances of high run queues and take appropriate measures to improve system responsiveness and efficiency.

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