linux-kernel - Re: [RFC v11][PATCH 05/13] Dump memory address space

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Date:	Thu, 18 Dec 2008 07:54:36 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	Mike Waychison <mikew@...gle.com>, jeremy@...p.org, arnd@...db.de,
	linux-api@...r.kernel.org, containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Linux Torvalds <torvalds@...l.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC v11][PATCH 05/13] Dump memory address space

On Thu, 2008-12-18 at 06:10 -0500, Oren Laadan wrote:
> >> +    for (i = pgarr->nr_used; i--; /**/)
> >> +        page_cache_release(pgarr->pages[i]);
> > 
> > This is sorta hard to read (and non-intuitive).  Is it easier to do:
> > 
> > for (i = 0; i < pgarr->nr_used; i++)
> >     page_cache_release(pgarr->pages[i]);
> > 
> > It shouldn't matter what order you release the pages in..
> 
> Was meant to avoid a dereference to 'pgarr->nr_used' in the comparison.
> (though I doubt if the performance impact is at all visible)

That's a bit to aggressive an optimization.  You two piqued my
curiosity, so I tried a little experiment with this .c file:

extern void bar(int i);

struct s {
        int *array;
        int size;
};

extern struct s *s;
void foo(void)
{
        int i;
#ifdef OREN
        for (i = s->size; i--; )
#else
        for (i = 0; i < s->size; i++)
#endif
                bar(s->array[i]);
}

for O in "" -O -O1 -O2 -O3 -Os; do
	gcc -DOREN $O -c f1.c -o oren.o;
	gcc $O -c f1.c -o mike.o;
	echo -n Oren:; objdump -d oren.o | grep ret;
	echo -n Mike:; objdump -d mike.o | grep ret;
done

Smaller numbers are better, and indicate the size of that function,
basically:

Oren:  38:	c3                   	ret    
Mike:  3b:	c3                   	ret    
Oren:  44:	c3                   	ret    
Mike:  36:	c3                   	ret    
Oren:  44:	c3                   	ret    
Mike:  36:	c3                   	ret    
Oren:  43:	c3                   	ret    
Mike:  34:	c3                   	ret    
Oren:  43:	c3                   	ret    
Mike:  34:	c3                   	ret    
Oren:  3a:	c3                   	ret    
Mike:  2a:	c3                   	ret    

gcc version 4.2.4 (Ubuntu 4.2.4-1ubuntu3).  In all but the unoptimized
case, Mike's version wins.  Readability, and icache footprint all in one
package!

-- Dave

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