linux-kernel - Re: Use of high-res timers

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Date:	Thu, 31 May 2012 09:36:56 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Alan Stern <stern@...land.harvard.edu>
cc:	Ingo Molnar <mingo@...hat.com>,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: Use of high-res timers

On Wed, 30 May 2012, Alan Stern wrote:

> Thomas, Ingo, or anyone else:
> 
> My driver needs to time a bunch of events, with roughly millisecond
> precision.  Up to now it has used old-fashioned timer_lists and the
> jiffies counter, but I'm switching over to high-resolution timers and
> ktime_get.
> 
> This leads to a few questions (these issues don't seem to be addressed 
> anywhere in Documentation/timers):
> 
> 	Should I be concerned about efficiency?  I may well end up
> 	calling ktime_get several times per millisecond; is it fast
> 	enough for this to be okay?

It better is.
 
> 	I need timed intervals with reliable lower bounds.  Let's say
> 	I call ktime_get twice, maybe once in an interrupt handler and
> 	once in an hrtimer callback (not necessarily on the same CPU).
> 	Some action has to be taken no earlier than 1 ms after the 
> 	first call.  If the second call returns a value that is at 
> 	least 1 ms larger than the first call, is that enough of a 
> 	guarantee?  If not, how much larger does it have to be?

ktime_get() is precise. Can you explain what you are trying to solve ?
 
> 	Which has more overhead: adding and cancelling an hrtimer
> 	several times, or simply letting it expire and returning
> 	immediately from the callback?  (I wouldn't be surprised if
> 	there was no good answer.)

There is no really good answer.

Thanks,

	tglx
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