Software & AppsOperating SystemLinux

Why is ksoftirqd/0 process using all of my CPU?

Ubuntu 17

Understanding and troubleshooting high CPU usage in Linux systems can be a daunting task, especially when it involves kernel-level processes. One such process is ksoftirqd/0, which can sometimes consume a significant amount of CPU resources. In this article, we will delve into why this happens and how to mitigate this issue.

Quick Answer

The ksoftirqd/0 process is using all of your CPU because your machine is under heavy interrupt load. This can happen when a high-speed network card receives a large number of packets in a short time frame. To reduce the CPU usage of ksoftirqd/0, you can assign specific CPUs to handle certain interrupts by modifying the smp_affinity value in the /proc/irq/$interrupt_number/smp_affinity file.

What is ksoftirqd/0?

The ksoftirqd/0 process is a per-CPU kernel thread that becomes active when the machine is under heavy soft-interrupt load. Soft interrupts are interrupt requests that come from devices attached to your computer. When these interrupts arrive too quickly for the operating system to handle, they are queued for later processing by ksoftirqd.

Why is ksoftirqd/0 Using High CPU?

If ksoftirqd/0 is consuming a large amount of CPU, it’s an indication that your machine is under heavy interrupt load. This often happens when a high-speed network card receives a large number of packets in a short time frame. The excessive interrupt load can cause your system to feel sluggish or unresponsive.

How to Identify the Cause?

To identify the cause of high CPU usage, we can use the command cat /proc/interrupts. This command provides a list of interrupts and their corresponding devices. The output will look something like this:

 CPU0 CPU1 
 0: 10 0 IO-APIC-edge timer
 1: 2 0 IO-APIC-edge i8042
 8: 1 0 IO-APIC-edge rtc0
 9: 0 0 IO-APIC-fasteoi acpi
 12: 4 0 IO-APIC-edge i8042
 14: 0 0 IO-APIC-edge ata_piix
NMI: 0 0 Non-maskable interrupts
LOC: 17595 12367 Local timer interrupts
SPU: 0 0 Spurious interrupts
PMI: 0 0 Performance monitoring interrupts
IWI: 0 0 IRQ work interrupts
RTR: 0 0 APIC ICR read retries
RES: 1053 418 Rescheduling interrupts
CAL: 206 207 Function call interrupts
TLB: 304 316 TLB shootdowns
TRM: 0 0 Thermal event interrupts
THR: 0 0 Threshold APIC interrupts
DFR: 0 0 Deferred Error APIC interrupts
MCE: 0 0 Machine check exceptions
MCP: 4 4 Machine check polls
ERR: 0
MIS: 0

In this output, each row represents an interrupt, and the columns under CPU0 and CPU1 represent the number of interrupts handled by each CPU.

How to Reduce the CPU Usage of ksoftirqd/0?

To reduce the CPU usage of ksoftirqd/0, we can assign specific CPUs to handle certain interrupts. This can be done by changing the contents of the /proc/irq/$interrupt_number/smp_affinity file.

The smp_affinity value is a bitmap of CPUs represented in hex code. By setting the smp_affinity to a specific value, you can assign certain CPUs to handle specific interrupts.

For example, if you have an 8-core system and you want to assign cores 1, 3, and 4 to handle interrupt 29 (eth0), you can set the smp_affinity to 1a (0001 1010 in binary). This can be done with the command echo 1a | sudo tee /proc/irq/29/smp_affinity.

Here, echo 1a prints the value 1a, | pipes this value to the next command, sudo tee writes the value to the file with root privileges, and /proc/irq/29/smp_affinity is the file where we want to write the value.

By assigning specific CPUs to handle interrupts, you can distribute the interrupt load more evenly and potentially reduce the CPU usage of ksoftirqd/0.

Conclusion

While ksoftirqd/0 can consume a significant amount of CPU under heavy interrupt load, it’s important to note that it is not necessarily causing the sluggishness of your system. It is a symptom of heavy interrupt load, which could be caused by other factors as well. If you are experiencing performance issues, it’s recommended to investigate other potential causes such as high CPU usage by other processes or insufficient system resources.

In summary, ksoftirqd/0 is a kernel thread that processes soft interrupts when the machine is under heavy interrupt load. To reduce its CPU usage, you can assign specific CPUs to handle certain interrupts by modifying the smp_affinity value in the /proc/irq/$interrupt_number/smp_affinity file. However, it’s important to investigate other potential causes if you are experiencing system sluggishness.

What is the purpose of the `ksoftirqd/0` process?

The ksoftirqd/0 process is a per-CPU kernel thread that handles soft interrupts when the system is under heavy interrupt load.

What are soft interrupts?

Soft interrupts are interrupt requests that come from devices attached to your computer. When these interrupts arrive too quickly for the operating system to handle, they are queued for later processing by the ksoftirqd process.

Why is the `ksoftirqd/0` process using high CPU?

If the ksoftirqd/0 process is consuming a large amount of CPU, it indicates that your system is under heavy interrupt load, usually caused by a high-speed network card receiving a large number of packets in a short time frame.

How can I identify the cause of high CPU usage by `ksoftirqd/0`?

You can use the command cat /proc/interrupts to obtain a list of interrupts and their corresponding devices. This can help identify which device is generating the high interrupt load.

How can I reduce the CPU usage of `ksoftirqd/0`?

To reduce the CPU usage of ksoftirqd/0, you can assign specific CPUs to handle certain interrupts by modifying the smp_affinity value in the /proc/irq/$interrupt_number/smp_affinity file. This helps distribute the interrupt load more evenly.

Is high CPU usage by `ksoftirqd/0` the only cause of system sluggishness?

No, high CPU usage by ksoftirqd/0 is a symptom of heavy interrupt load but not necessarily the sole cause of system sluggishness. Other factors such as high CPU usage by other processes or insufficient system resources should also be investigated if experiencing performance issues.

What should I do if I am experiencing system sluggishness?

If you are experiencing system sluggishness, it is recommended to investigate other potential causes such as high CPU usage by other processes, insufficient system resources, or other factors that may be impacting system performance.

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