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From: Ingo Molnar <mingo@elte.hu>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Simon Kirby <sim@hostway.ca>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Dave Jones <davej@redhat.com>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: Linux 3.1-rc9
Date: Tue, 18 Oct 2011 00:03:21 +0200	[thread overview]
Message-ID: <20111017220321.GA18523@elte.hu> (raw)
In-Reply-To: <20111017213958.GA27126@elte.hu>


* Ingo Molnar <mingo@elte.hu> wrote:

> > > In particular we could try something like:
> > > 
> > > 	(high*2^32 + low)/1e9 ~==  ( high * (2^64/1e9) ) / 2^32
> > > 
> > > ... which reduces it all to a 64-bit multiplication (or two 
> > > 32-bit multiplications) with a known constant, at the cost of 1 
> > > nsec imprecision of the result - but that's an OK approximation 
> > > in my opinion.
> > > 
> > 
> > We can do much better than that with reciprocal multiplication.
> 
> Yes, 2^64/1e9 is the reciprocal.

So basically, to extend on the pseudocode above, we could do the 
equivalent of:

/* 2^64/1e9: */
#define MAGIC 18446744073ULL

        secs_fast = ((nsecs >> 32) * MAGIC) >> 32;
        secs_fast += (nsecs & 0xFFFFFFFF)/1000000000;

to get to the precise 'timeval.secs' field - these are all 32-bit 
operations: a 32-bit multiplication and a 32-bit division if i 
counted it right.

(Likewise we can get the remainder as well, for timeval.nsecs.)

So I think if we add 32-bit optimized reciprocal multiplication based 
timeval and timespec routines, we can change ktime_t to a simple 
scalar type on 64-bit and 32-bit architectures alike.

It would likely be faster as well: the 32-bit ktime operations are 
more complex than straightforward u64 operations.

Thomas, what do you think?

Thanks,

	Ingo

  reply	other threads:[~2011-10-17 22:04 UTC|newest]

Thread overview: 98+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-10-05  1:40 Linux 3.1-rc9 Linus Torvalds
2011-10-07  7:08 ` Simon Kirby
2011-10-07 17:48   ` Simon Kirby
2011-10-07 18:01     ` Peter Zijlstra
2011-10-08  0:33       ` Simon Kirby
2011-10-08  0:50       ` Simon Kirby
2011-10-08  7:55         ` Peter Zijlstra
2011-10-12 21:35           ` Simon Kirby
2011-10-13 23:25             ` Simon Kirby
2011-10-17  1:39               ` Linus Torvalds
2011-10-17  4:58                 ` Ingo Molnar
2011-10-17  9:03                   ` Thomas Gleixner
2011-10-17 10:40                     ` Peter Zijlstra
2011-10-17 11:40                       ` Alan Cox
2011-10-17 18:49                     ` Ingo Molnar
2011-10-17 20:35                       ` H. Peter Anvin
2011-10-17 21:19                         ` Ingo Molnar
2011-10-17 21:22                           ` H. Peter Anvin
2011-10-17 21:39                             ` Ingo Molnar
2011-10-17 22:03                               ` Ingo Molnar [this message]
2011-10-17 22:04                                 ` Ingo Molnar
2011-10-17 22:08                               ` H. Peter Anvin
2011-10-18  6:01                                 ` Ingo Molnar
2011-10-18  7:12                                 ` Geert Uytterhoeven
2011-10-18 18:50                                   ` H. Peter Anvin
2011-10-17 21:31                           ` Ingo Molnar
2011-10-17  7:55                 ` Martin Schwidefsky
2011-10-17  9:12                   ` Peter Zijlstra
2011-10-17  9:18                     ` Martin Schwidefsky
2011-10-17 20:48                   ` H. Peter Anvin
2011-10-18  7:20                     ` Martin Schwidefsky
2011-10-17 10:34                 ` Peter Zijlstra
2011-10-17 14:07                   ` Martin Schwidefsky
2011-10-17 14:57                   ` Linus Torvalds
2011-10-17 17:54                     ` Peter Zijlstra
2011-10-17 18:31                       ` Linus Torvalds
2011-10-17 19:23                         ` Peter Zijlstra
2011-10-17 21:00                           ` Thomas Gleixner
2011-10-18  8:39                             ` Thomas Gleixner
2011-10-18  9:05                               ` Peter Zijlstra
2011-10-18 14:59                                 ` Linus Torvalds
2011-10-18 15:26                                   ` Thomas Gleixner
2011-10-18 18:07                                   ` Ingo Molnar
2011-10-18 18:14                                   ` [GIT PULL] timer fix Ingo Molnar
2011-10-18 16:13                                 ` Linux 3.1-rc9 Dave Jones
2011-10-18 18:20                                 ` Simon Kirby
2011-10-18 19:48                                   ` Thomas Gleixner
2011-10-18 20:12                                     ` Linus Torvalds
2011-10-25 15:26                                       ` Simon Kirby
2011-10-26  1:47                                         ` Yong Zhang
2011-10-24 19:02                                     ` Simon Kirby
2011-10-25  7:13                                       ` Linus Torvalds
2011-10-25  9:01                                         ` David Miller
2011-10-25 12:30                                           ` Thomas Gleixner
2011-10-25 23:18                                             ` David Miller
2011-10-25 20:20                                       ` Simon Kirby
2011-10-31 17:32                                         ` Simon Kirby
2011-11-02 16:40                                           ` Thomas Gleixner
2011-11-02 17:27                                             ` Eric Dumazet
2011-11-02 17:46                                               ` Linus Torvalds
2011-11-02 17:53                                                 ` Eric Dumazet
2011-11-02 18:00                                                   ` Linus Torvalds
2011-11-02 18:05                                                     ` Eric Dumazet
2011-11-02 18:10                                                       ` Linus Torvalds
2011-11-02 17:49                                               ` Eric Dumazet
2011-11-02 17:58                                                 ` Eric Dumazet
2011-11-02 19:16                                                   ` Simon Kirby
2011-11-02 22:42                                                     ` Eric Dumazet
2011-11-03  0:24                                                       ` Thomas Gleixner
2011-11-03  0:52                                                       ` Simon Kirby
2011-11-03 22:07                                                         ` David Miller
2011-11-03  6:06                                                       ` Jörg-Volker Peetz
2011-11-03  6:26                                                         ` Eric Dumazet
2011-11-03  6:43                                                           ` David Miller
2011-11-02 17:54                                               ` Thomas Gleixner
2011-11-02 18:04                                                 ` Eric Dumazet
2011-11-02 18:28                                             ` Simon Kirby
2011-11-02 18:30                                               ` Thomas Gleixner
2011-11-02 22:10                                           ` Steven Rostedt
2011-11-02 23:00                                             ` Steven Rostedt
2011-11-03  0:09                                               ` Simon Kirby
2011-11-03  0:15                                                 ` Steven Rostedt
2011-11-03  0:17                                                   ` Simon Kirby
2011-11-18 23:11                                         ` [tip:perf/core] lockdep: Show subclass in pretty print of lockdep output tip-bot for Steven Rostedt
2011-10-20 14:36                 ` Linux 3.1-rc9 Martin Schwidefsky
2011-10-23 11:34                   ` Ingo Molnar
2011-10-24  7:48                     ` Martin Schwidefsky
2011-10-24  7:51                       ` Linus Torvalds
2011-10-24  8:08                         ` Martin Schwidefsky
2011-10-18  5:40             ` Simon Kirby
2011-10-09 20:51 ` Arkadiusz Miśkiewicz
2011-10-10  2:29   ` [tpmdd-devel] " Stefan Berger
2011-10-10 16:23     ` Rajiv Andrade
2011-10-10 17:05       ` Arkadiusz Miśkiewicz
2011-10-10 17:22         ` Stefan Berger
2011-10-10 17:57           ` Arkadiusz Miśkiewicz
2011-10-10 21:08             ` Arkadiusz Miśkiewicz
2011-10-11  7:09             ` [tpmdd-devel] " Peter.Huewe

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