Kernel Timer Systems

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This page has links to information about various proposals for new timer systems for the Linux kernel. The current linux kernel (2.6.11 as of this writing) has a few problems, particularly for realtime systems or for virtualized systems.


Current Linux Timer System

To understand the proposals, you need to understand the current timer system in Linux. The current timer system is based on incrementing a kernel-internal value (jiffies) every timer interrupt. The timer interrupt becomes the default scheduling quamtum, and all other timers are based on jiffies. The timer interrupt rate (and jiffy increment rate) is defined by a compile-time constant called HZ. Different platforms use different values for HZ. Historically, the kernel used 100 as the value for HZ, yielding a jiffy interval of 10 ms. With 2.4, the HZ value for i386 was changed to 1000, yeilding a jiffy interval of 1 ms. Recently (2.6.13) the kernel changed HZ for i386 to 250. (1000 was deemed too high).

[should continue this description]

- timer structures
- timer API
- do_gettimeofday
  - why is it 

High Resolution Timers

George Anzinger authored High Resolution Timers, which provide sub-jiffy timers. This patch has not been mainlined.

Jun Sun's "tock" proposal

See http://linux.junsun.net/HRT/index.html

This systems replaces jiffies and xtime with tocks (arch-dependent), mtime (monotonic time) and wtime (wall time), and proposes a strategy for migrating to that.

John Stultz

John Stultz has proposed changes to the timers to use a 64-bit nanosecond value as the base. He did a presentation and BOF at OLS 2005. (It should be available online)

Timer Tick Thread - LKML July 2005

There was a very long thread about timers, jiffies, and related subjects in July of 2005 on the kernel mailing list.

The title was: "Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt"

Linus said jiffies is not going away

- still need 32-bit counter, shouldn't be real-time value (too much overhead to calculate)
- high-res timers shouldn't be sub-HZ, but instead, HZ should be high and timer tick should not be 1:1 with HZ
  - in other words, have HZ be high (like 2K), have the timer interrupt fire off at some lower frequency,
  and increment jiffies by more than one on each interrupt.
  - rationale for this is to keep a single sub-system

Arjan had good points about coalescing low-res timers

- 3 use cases:
- low res timeouts
- high res timer for periodic absolute wakeup (wake up every 10 ms, whether last one was late or nt
- high res timer for periodic relative wakeup (wake up 10 ms from now)

Ingo Molnar's explanation of timer wheel performance

Ingo Molnar did an in-depth explanation about the performance of the current "timer wheel" implementation of timers. This was part of a series of messages trying to justify the addition of ktimers (which have different characteristics).

It is possibly the best explanation of the timer wheel avaiable: See http://lkml.org/lkml/2005/10/19/46 and http://lwn.net/Articles/156329/