linux-kernel - Re: [patch, v2.6.22-rc6] sys

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Date:	Mon, 25 Jun 2007 15:15:08 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Roman Zippel <zippel@...ux-m68k.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch, v2.6.22-rc6] sys_time() speedup

On Mon, 25 Jun 2007 23:09:46 +0200
Roman Zippel <zippel@...ux-m68k.org> wrote:

> Hi,
> 
> On Monday 25 June 2007, Ingo Molnar wrote:
> 
> > the patch improves the sysbench OLTP macrobenchmark significantly:
> 
> Has that any real practical relevance?

Interesting question.  The patch adds a new test-n-branch to gettimeofday()
so if gettimeofday() is used much more frequently than time(), we lose.

> > @@ -373,6 +376,20 @@ void do_gettimeofday (struct timeval *tv
> >
> >  	tv->tv_sec = sec;
> >  	tv->tv_usec = usec;
> > +
> > +	/*
> > +	 * Make sure xtime.tv_sec [returned by sys_time()] always
> > +	 * follows the gettimeofday() result precisely. This
> > +	 * condition is extremely unlikely, it can hit at most
> > +	 * once per second:
> > +	 */
> > +	if (unlikely(xtime.tv_sec != tv->tv_sec)) {
> > +		unsigned long flags;
> > +
> > +		write_seqlock_irqsave(&xtime_lock);
> > +		update_wall_time();
> > +		write_seqlock_irqrestore(&xtime_lock);
> > +	}
> >  }
> >
> >  EXPORT_SYMBOL(do_gettimeofday);
> 
> Is this the do_gettimeofday() inside CONFIG_TIME_INTERPOLATION?

Yes.

> What did you test?
> There can be many ways to read the clock, do you want to put this hook 
> everywhere?

Yeah, it isn't immediately obvious (to this little black duck) why similar
fixups weren't needed in timekeeping.c.


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