F.7. Kernel-Related Information

Red Hat Enterprise Linux includes a modification to the way the Linux kernel timer interrupt is handled. Normally, a hardware timer is set to generate periodic interrupts at a fixed rate (100 times a second for most architectures). These periodic timer interrupts are used by the kernel to schedule various internal housekeeping tasks, such as process scheduling, accounting, and maintaining system uptime.

While a timer-based approach works well for a system environment where only one copy of the kernel is running, it can cause additional overhead when many copies of the kernel are running on a single system (for example, as z/VM(R) guests). In these cases, having thousands of copies of the kernel each generating interrupts many times a second can result in excessive system overhead.

Therefore, Red Hat Enterprise Linux now includes the ability to turn off periodic timer interrupts. This is done through the /proc/ file system. To disable periodic timer interrupts, issue the following command:

 echo "0" > /proc/sys/kernel/hz_timer

To enable periodic timer interrupts, issue the following command:

 echo "1" > /proc/sys/kernel/hz_timer

By default, periodic timer interrupts are enabled.

Periodic timer interrupt states can also be set at boot-time; to do so, add the following line to /etc/sysctl.conf to disable periodic timer interrupts:

 kernel.hz_timer = 0


Disabling periodic timer interrupts can violate basic assumptions in system accounting tools. If you notice a malfunction related to system accounting, verify that the malfunction disappears if periodic timer interrupts are enabled, then submit a bug at http://bugzilla.redhat.com/bugzilla/ (for malfunctioning bundled tools), or inform the tool vendor (for malfunctioning third-party tools).