[PATCHSET] Announce: High-res timers, tickless/dyntick and dynamic HZ

Thomas Gleixner
2006-06-18T15:09:13Z

We are pleased to announce the 2.6.17 based release of our high-res
timers kernel feature, upon which we based a tickless kernel (dyntick)
implementation and a 'dynamic HZ' feature as well:

http://www.tglx.de/projects/hrtimers/2.6.17/

The easiest way to try these features is to apply the combo patch to
vanilla 2.6.17. The patching order is:

http://www.kernel.org/pub/linux/kernel/v2.6/linux-2.6.17.tar.bz2
http://www.tglx.de/projects/hrtimers/2.6.17/patch-2.6.17-hrt-dyntick1.patch

A broken out patch series is available too:

http://www.tglx.de/projects/hrtimers/2.6.17/patch-2.6.17-hrt-dyntick1.patches.tar.bz2

The high-res timers feature (CONFIG_HIGH_RES_TIMERS) enables POSIX
timers and nanosleep() to be as accurate as the hardware allows (around
1usec on typical hardware). This feature is transparent - if enabled it
just makes these timers much more accurate than the current HZ
resolution. It is based on the Generic Time Of Day patchset from John
Stultz and it in essence finishes what we started with the
kernel/hrtimers.c code in 2.6.16.

The tickless kernel feature (CONFIG_NO_HZ) enables 'on-demand' timer
interrupts: if there is no timer to be expired for say 1.5 seconds when
the system goes idle, then the system will stay totally idle for 1.5
seconds. This should bring cooler CPUs and power savings: on our (x86)
testboxes we have measured the effective IRQ rate to go from HZ to 1-2
timer interrupts per second.

This feature is implemented by driving 'low res timer wheel' processing
via special per-CPU high-res timers, which timers are reprogrammed to
the next-low-res-timer-expires interval. This tickless-kernel design is
SMP-safe in a natural way and has been developed on SMP systems from
the
beginning.

Note: while our code should be similar in behavior to the existing
dynticks kernel patch from Con, it is a fundamentally different design
(being based on the high-res timers support and APIs) and is thus a
different implementation. We reused one area of dynticks: we integrated
and improved the 'timer top' profiling tool (CONFIG_TIMER_INFO).

When running the kernel then there's a 'timeout granularity'
runtime tunable parameter as well, under:

/proc/sys/kernel/timeout_granularity

it defaults to 1, meaning that CONFIG_HZ is the granularity of timers.

For example, if CONFIG_HZ is 1000 and timeout_granularity is set to 10,
then low-res timers will be expired every 10 jiffies (every 10 msecs),
thus the effective granularity of low-res timers is 100 HZ. Thus this
feature implements nonintrusive dynamic HZ in essence, without touching
the HZ macro itself.

Supported platforms: high-res timers and tickless works on x86 (x86_64,
PPC and ARM port are in the works). Other platforms should still work
fine with the usual HZ frequency timer tick.

Naturally, we'd like these features to be integrated into the upstream
kernel as well.

Bugreports and suggestions are welcome,

Thomas, Ingo