Subject: Re: Timer idea
From: john stultz <johnstul@us.ibm.com>
To: george@mvista.com
Cc: evan@coolrunningconcepts.com, linux-kernel@vger.kernel.org
In-Reply-To: <437A2FDA.6090204@mvista.com>
References: <20051115102425.0iln2874xjoc4g84@coolrunningconcepts.com>
	 <437A2FDA.6090204@mvista.com>
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Date: Tue, 15 Nov 2005 11:12:20 -0800
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On Tue, 2005-11-15 at 10:58 -0800, George Anzinger wrote:
> evan@coolrunningconcepts.com wrote:
> > I was thinking about benchmarking, profiling, and various other applications
> > that might need frequent access to the current time.  Polling timers or
> > frequent timer signal delivery both seem like there would be a lot of overhead.
> >  I was thinking it would be nice if you could just read the time information
> > without making an OS call.
> > 
> > I figure the kernel keeps accurate records of current time information and the
> > values of various timers.  I then had the idea that one could have a /dev or
> > maybe a /proc entry that would allow you to mmap() the kernel records (read
> > only) and then you could read this information right from the kernel without
> > any overhead.
> > 
> 
> Your are describing the vsyscall.  John Stultz and company are actively working on this as we speak. 
>   If memory serves, it is already available on some platforms.

Yep. x86-64 already supports this as does ppc64 via vsyscall/VDSOs. ia64
has a different variant on it as well (fast syscalls).

thanks
-john

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