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From: john stultz <johnstul@us.ibm.com>
To: Roman Zippel <zippel@linux-m68k.org>
Cc: lkml <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ingo Molnar <mingo@elte.hu>, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH] correct inconsistent ntp interval/tick_length usage
Date: Wed, 30 Jan 2008 18:16:16 -0800	[thread overview]
Message-ID: <1201745776.6195.14.camel@localhost.localdomain> (raw)
In-Reply-To: <Pine.LNX.4.64.0801310251090.17507@scrub.home>


On Thu, 2008-01-31 at 02:55 +0100, Roman Zippel wrote:
> Hi,
> 
> On Tue, 29 Jan 2008, john stultz wrote:
> 
> > +/* Because using NSEC_PER_SEC would be too easy */
> > +#define NTP_INTERVAL_LENGTH ((((s64)TICK_USEC*NSEC_PER_USEC*USER_HZ)+CLOCK_TICK_ADJUST)/NTP_INTERVAL_FREQ)
> 
> Why are you using USER_HZ? Did you test this with HZ!=100?

I'm using USER_HZ, because ntp_update_frequency() uses USER_HZ.

And my tests were run at HZ=1000.

> Anyway, please don't make more complicated than it already is.
> What I said previously about the update interval is still valid, so the 
> correct solution is to use the simpler NTP_INTERVAL_LENGTH calculation 
> from my last mail and to omit the correction for NO_HZ.

I'm not sure I follow how having two separate and totally different
definitions of NTP_INTERVAL_LENGTH is less complicated then my last
patch.

Maybe I'm missing something from your suggestion? Or maybe could you
contribute your suggestion as a patch?

My concern is we state the accumulation interval is X ns long. Then
current_tick_length() is to return (X + ntp_adjustment), so each
accumulation interval we can keep track of the error and adjust our
interval length.

So if ntp_update_frequency() sets tick_length_base to be:

	u64 second_length = (u64)(tick_usec * NSEC_PER_USEC * USER_HZ)
			<< TICK_LENGTH_SHIFT;
	second_length += (s64)CLOCK_TICK_ADJUST << TICK_LENGTH_SHIFT;
	second_length += (s64)time_freq
				<< (TICK_LENGTH_SHIFT - SHIFT_NSEC);

	tick_length_base = second_length;
	do_div(tick_length_base, NTP_INTERVAL_FREQ);


The above is basically (X + part of ntp_adjustment)

So I'm trying to calculate the X as: 

#define NTP_INTERVAL_LENGTH ((((s64)TICK_USEC*NSEC_PER_USEC*USER_HZ)+CLOCK_TICK_ADJUST)/NTP_INTERVAL_FREQ)


thanks
-john



  reply	other threads:[~2008-01-31  2:16 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-24  2:38 john stultz
2008-01-24  9:14 ` Valdis.Kletnieks
2008-01-25 14:07 ` Roman Zippel
2008-01-29  2:28   ` john stultz
2008-01-29  4:02     ` Roman Zippel
2008-01-30  2:14       ` john stultz
2008-01-31  1:55         ` Roman Zippel
2008-01-31  2:16           ` john stultz [this message]
2008-01-31  5:02             ` Roman Zippel
2008-02-02  1:02               ` John Stultz
2008-02-08 17:33                 ` Roman Zippel
2008-02-09  2:17                   ` john stultz
2008-02-09  4:47                     ` Roman Zippel
2008-02-09  5:04                       ` Andrew Morton
2008-02-09 14:13                         ` Roman Zippel
2008-02-10 18:45                     ` Roman Zippel
2008-02-12  0:09                       ` john stultz
2008-02-12 14:36                         ` Roman Zippel
2008-02-14  4:36                           ` john stultz
2008-02-16  4:24                             ` Roman Zippel
2008-02-19  1:02                               ` john stultz
2008-02-19  4:04                                 ` Roman Zippel
2008-02-20  1:50                                   ` john stultz
2008-02-20 17:08                                     ` Roman Zippel
2008-02-22  2:39                                       ` john stultz
2008-02-25 14:44                                         ` Roman Zippel
2008-02-29  4:49                                         ` [PATCH] Remove obsolete CLOCK_TICK_ADJUST Roman Zippel
2008-02-29  6:25                                           ` Ray Lee
2008-02-29 13:31                                             ` Roman Zippel
2008-02-29 22:08                                           ` Andrew Morton
2008-02-29 22:27                                             ` Roman Zippel
2008-02-29 22:38                                               ` Andrew Morton
2008-02-29 23:11                                               ` john stultz
2008-02-29 23:11                                           ` john stultz
2008-03-02  4:03                                             ` Roman Zippel
2008-02-29 18:54                                         ` [PATCH] correct inconsistent ntp interval/tick_length usage Jörg-Volker Peetz

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