linux-kernel - Re: [GIT PULL] x86/fpu for v2.6.32

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Date:	Sat, 12 Sep 2009 23:53:31 +0200 (CEST)
From:	Jesper Juhl <jj@...osbits.net>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [GIT PULL] x86/fpu for v2.6.32

On Sat, 12 Sep 2009, Arjan van de Ven wrote:

> On Fri, 11 Sep 2009 23:30:42 +0200 (CEST)
> Jesper Juhl <jj@...osbits.net> wrote:
> 
> > 
> > Hi Ingo,
> > 
> > First of all, I want to say that I have no real objections to the
> > patch what-so-ever. The following is merely to satisfy my own
> > personal curiosity - and I'm sure you have better things to do than
> > satisfy my curiosity, so if you want to ignore me; feel free :-)
> > 
> > On Fri, 11 Sep 2009, Ingo Molnar wrote:
> > 
> > [...]
> > > +	bool preload_fpu;
> > >  
> > >  	/* never put a printk in __switch_to... printk() calls
> > > wake_up*() indirectly */ 
> > > -	__unlazy_fpu(prev_p);
> > > +	/*
> > > +	 * If the task has used fpu the last 5 timeslices, just do
> > > a full
> > > +	 * restore of the math state immediately to avoid the
> > > trap; the
> > > +	 * chances of needing FPU soon are obviously high now
> > > +	 */
> > > +	preload_fpu = tsk_used_math(next_p) && next_p->fpu_counter
> > > > 5; 
> > > +	__unlazy_fpu(prev_p);
> > >  
> > [...]
> > > +	 * If the task has used fpu the last 5 timeslices, just do
> > > a full
> > > +	 * restore of the math state immediately to avoid the
> > > trap; the
> > > +	 * chances of needing FPU soon are obviously high now
> > > +	 */
> > > +	preload_fpu = tsk_used_math(next_p) && next_p->fpu_counter
> > > > 5; 
> > 
> > I'm wondering about two things:
> > 
> 
> (btw this is not new code)
> 
> > 1) Where did that magic constant "5" come from?
> 
> from me running a bunch of experiments noticing that tasks that do 5
> do on average many more. Could it be 4? Perhaps. 
> 
> > Is there some fundamental thing about CPU's, cache layout,
> > scheduling, benchmarks or something else that I just don't know that
> > makes 5 the magic "right number"?  Why not 2, 3, 9 or 42?
> 
> At some point *a* number must be taken. That is currently 5.
> Show us that any other number is better and we'll switch ;-)
> 
Thank you for taking the time to answer and explain.
I'm sure 5 is fine, I was merely currious :-)

-- 
Jesper Juhl <jj@...osbits.net>             http://www.chaosbits.net/
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