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Richard Lewis's Home Page

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Hello, today is: 6 Dec 2022.

I am part of the IBM Washington System Center, Advanced Technical Support Organization supporting z/VM and Linux on System z.

I have been working with z/VM (and it's predecessors) for 25 years, and with Linux on System z since the IBM port was completed in late 1999.

Technical Tip

We all know about the "timer tick" (also called the "jiffies timer") and how IBM created a patch so that it could be turned of on s390. Eventually this change was accepted into the mainline kernel, but it (as far as I know) only applied to s390 systems. It was configured into the kernel if it was complied with "CONFIG_NO_IDLE_HZ=y" and, later on, it could be defaulted to "on" if "CONFIG_NO_IDLE_HZ_INIT=y" was set. That also gave us the file /proc/sys/kernel/hz_timer to turn it on and off dynamically, usually by adding the line "kernel.hz_timer=0" to /etc/sysctl.conf.

In kernel level 2.6.21, the "tickless kernel" was introduced. For s390, this duplicated what IBM had done, so they dropped their architecture specific changes. This kernel feature is enabled when you compile the kernel with "CONFIG_NO_HZ=y". There is no control to turn it on and off dynamically. SLES 11 is compiled with this enabled

The net is - for SLES 11, the timer tick is always disabled and you can't turn it on. And if you code "kernel.hz_timer = 0" in sysctl.conf you will see an error message when sysctl runs.

Now for some fun!

  • Try out the new VM coloring book.
  • An analogy of a System/390 sever. Thanks for stopping by.



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    VM Carpe Forum!