Re: [patch 00/43] ktimer reworked

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 1 Dec 2005, Roman Zippel wrote:

> Hi,

Thank you Roman, this has cleared up a little where you are coming from.
The name issue should never have been brought up, and what you state here
should have been mentioned first. (If you did mention this earlier, I
guess it was just lost in the noise).

>
> On Thu, 1 Dec 2005, Steven Rostedt wrote:
>
> > It should just be documented simply as: If you need to set some timed
> > event to happen that you don't expect to occur then use a ktimeout.
> > Where this timed event is an event that lets you know that another event
> > hasn't happened in a given time. (I want to know if x didn't happen by
> > this time).
> >
> > If a timed event is expected to occur then use ktimer.  Where it is
> > mostly used for the event itself (I want x to happen at this time).
>
> If that is really _the_ defining difference, then we are _seriously_
> screwed.

I'm not convinced that we are.

>
> Here are a few items I would consider in choosing a timer:
>
> - reading time: to program a timer you have to read the time first,
> reading jiffies is practically free, whereas reading the precise time can
> be very expensive. With the right hardware it can be optimized to be quite
> cheap, but if portability is important you may want to avoid the extra
> cost.

If you are only concerned with ms resolution, then the added expense of
precise time is pretty negligible.  And these times are still for usage
with timers that are expected to expire.  Remember that timeouts are still
using jiffies.

As for portablility, I believe John Stultz has some nice plugins coming
to what timer source you want to use, so if there's a better way to get a
time, these should make things easy to add.

>
> - calculations: jiffies is a long integer whereas ktime_t is 64bit, so if
> you need a lot of complex time calculation, you should take the cost for
> 32bit archs into account.

Again, the cost is negligible for ms resolution.  And we're getting close
to 2038 so we need to be thinking in 64 bits anyway ;)

I'm not running huge multiuser systems, so I don't know what the average
amount of timers that are used (ones that are expected to expire), and how
much of an overhead it would be for these timers to be using 64 bit
calculations and precise timers.

>
> - resolution: how precise must the timer be? jiffies can't represent time
> values less than 1ms, but if time is e.g. measured in 10th of a second,
> jiffies may be enough.

And they would be if that is all you need. But coming from an embedded
point of view, that is not nearly enough.  I really see HighRes making it
into the kernel soon, and any new code in this area really needs to take
that into account.

>
> - timer life time: if only a short interval is needed (e.g. a fraction of
> a second) timer_list is often a lot cheaper.

And again, you are only limited to 1000 choices to go off in that fraction
of a second if jiffies is the resolution (with jiffies an 1000HZ).

As for the timer_list, you mean a linear list?  If you have a bunch of
timers to go off within a fraction of a second, couldn't a linear list
actually take longer?

-- Steve

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/