linux-kernel - Re: itimer again (Re: [PATCH] RTC: Add mmap method to rtc character driver)

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Date:	Sat, 29 Jul 2006 17:24:22 -0700
From:	Bill Huey (hui) <billh@...ppy.monkey.org>
To:	Edgar Toernig <froese@....de>
Cc:	Neil Horman <nhorman@...driver.com>, Jim Gettys <jg@...top.org>,
	"H. Peter Anvin" <hpa@...or.com>, Dave Airlie <airlied@...il.com>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, a.zummo@...ertech.it,
	jg@...edesktop.org, Keith Packard <keithp@...thp.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Bill Huey (hui)" <billh@...ppy.monkey.org>
Subject: Re: itimer again (Re: [PATCH] RTC: Add mmap method to rtc character driver)

On Sun, Jul 30, 2006 at 02:16:59AM +0200, Edgar Toernig wrote:
> Bill Huey (hui) wrote:
> > It's a different topic than what Keith needs,
> 
> Hmm, actually, people with problems like Keith's are the target
> audience, or at least were meant to be.  See the mmap example
> I posted in the original thread.
> 
> > but this is useful for another set of purposes. It's something that's
> > really useful in the RT patch since there isn't a decent API to get at
> > high resolution timers in userspace.
> 
> The /dev/itimer wasn't meant for high resolution, only accurate and
> reliable within the limits of the jiffy counter and easy to use. That
> doesn't mean that it can't be improved to provide high resolution; only,
> that this wasn't the design goal.  But I think, that the API is good
> enough to provide high resolution at any time without changing user
> space code.
> 
> (IMHO most people consider a resolution of 1 ms to be "high enough".)

Have you thought about making it an 'rtc' replacement and getting it to
conform to the API of it to what ever degree makes sense ? then it would
be a general replacement for 'rtc' if it could be opened multipule times
(as with generic event interfaces) with different timing scenarios per
thread.

> Hm... I'm not sure what you mean.  Sure, a blocking read may be a nice hint
> to the scheduler because we know exactly how long we're gonna sleep.  But
> I think that a blocking read is used very seldom.  Normally, the apps would
> block via select/poll.  And then the hints become looser - you only know
> the latest time when the process definitely wants to run again.
> 
> Another scheduling hint could be the set interval.  One could assume that
> an app that sets an interval of 1/50th second does want to run regularly
> every 1/50th second.  But that may be hard to use for scheduling decisions,
> especially when an app starts to use more than one timer.

Don't worry about what I just said, really. The fact that this driver exists
make it possible for heavy modification of just about any sort.

bill

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