Robert Love Explains Variable HZ

Posted by timothy on Tuesday October 15, 2002 @01:44PM from the powers-of-ten dept.

An anonymous reader writes "Robert Love, author of the kernel preemption patch for Linux, has backported a new performancing boosting patch from the 2.5 development kernel to the 2.4 stable kernel. This patch allows one to tune the frequency of the timer interrupt, defined in 2.4 as "HZ=100". Robert explains 'The timer interrupt is at the heart of the system. Everything lives and dies based on it. Its period is basically the granularity of the system: timers hit on 10ms intervals, timeslices come due at 10ms intervals, etc.' The 2.5 kernel has bumped the HZ value up to 1000, boosting performance."

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Re:It doesn't improve performance. by Piquan · 2002-10-15 17:41 · Score: 2, Redundant

I disagree with your analysis.
If a process isn't doing processing, that's because it's blocked in the kernel. (Q: What does a HLT do in userland?) As soon as the kernel puts a process on a wait queue, it reschedules. So you don't have any loss 'waiting for the next context switch'; that's just time that another process is running, or if nothing has anything to do, that the kernel halts the processor.
Note: I haven't studied how process scheduling is handled under Linux, but I can't imagine any OS that wouldn't do what I said here... or at least, I can't imagine one that would halt the processor after a process blocks, while it waits for a timer interrupt to schedule the next process.
Okay, maybe one.