Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt

[Eric St-Laurent <ericstl34@sympatico.ca>]

On Thu, 2005-07-14 at 17:24 -0700, Linus Torvalds wrote:
> 
> On Thu, 14 Jul 2005, Lee Revell wrote:
> 
> Trust me. When I say that the right thing to do is to just have a fixed 
> (but high) HZ value, and just changing the timer rate, I'm -right-.
> 
> I'm always right. This time I'm just even more right than usual.

Of course you are, jiffies are simple and efficient.

But it may be worthwhile to provide better/simpler API for relative
timeouts and also better hide the implementation details of the tick
system.


If i sum-up the discussion from my POV:

- use a 32-bit tick counter on 32-bit platforms and use a 64-bit counter
on 64-bit platforms

- keep the constant HZ=1000 (mS resolution) on 32-bit platforms

- remove the assumption that timer interrupts and jiffies are 1:1 thing
(jiffies may be incremented by >1 ticks at timer interrupt)

- determine jiffies_increment at boot

- have a slow clock mode to help power management (adjust
jiffies_increment by the slowdown factor)

- it may be useful to bump up HZ to 1e6 (uS res.) or 1e9 (nS res.) on
64-bit platforms, if there are benefits such as better accuracy during
time units conversions or if a higher frequency timer hardware is
available/viable.

- it may be also useful to bump HZ on -RT (Real-time) kernels, or with
-HRT (High-resolution timers support). Users of those kernel are willing
to pay the cost of the overhead to have better resolution

- avoid direct usage of the jiffies variable, instead use jiffies()
(inline or MACRO), IMO monotonic_clock() would be a better name

- provide a relative timeout API (see my previous post, or Alan's
suggestions)

- remove most of the direct use of jiffies through the code and replace
them with msleep(), relative timer, etc

- use human units for those APIs


- Eric St-Laurent