Re: [PATCH 2/8] Fragmentation Avoidance V17: 002_usemap

[Mel Gorman <mel@csn.ul.ie>]

On Thu, 13 Oct 2005, Dave Hansen wrote:

> On Thu, 2005-10-13 at 15:10 +0100, Mel Gorman wrote:
> > On Thu, 13 Oct 2005, Dave Hansen wrote:
> > > > +static inline int pfn_to_bitidx(struct zone *zone, unsigned long pfn)
> > > > +{
> > > > +	pfn &= (PAGES_PER_SECTION-1);
> > > > +	return (int)((pfn >> (MAX_ORDER-1)) * BITS_PER_RCLM_TYPE);
> > > > +}
> > >
> > > Why does that return int?  Should it be "unsigned long", maybe?  Also,
> > > that cast is implicit in the return and shouldn't be needed.
> > >
> >
> > It returns int because the bit functions like assign_bit() expect an int
> > for the bit index, not an unsigned long or anything else.
>
> You don't need to explicitly cast between int and unsigned long.  It'll
> probably hide more bugs than it reveals.
>

Ok

> > > >  /*
> > > > + * RCLM_SHIFT is the number of bits that a gfp_mask has to be shifted right
> > > > + * to have just the __GFP_USER and __GFP_KERNRCLM bits. The static check is
> > > > + * made afterwards in case the GFP flags are not updated without updating
> > > > + * this number
> > > > + */
> > > > +#define RCLM_SHIFT 19
> > > > +#if (__GFP_USER >> RCLM_SHIFT) != RCLM_USER
> > > > +#error __GFP_USER not mapping to RCLM_USER
> > > > +#endif
> > > > +#if (__GFP_KERNRCLM >> RCLM_SHIFT) != RCLM_KERN
> > > > +#error __GFP_KERNRCLM not mapping to RCLM_KERN
> > > > +#endif
> > >
> > > Should this really be in page_alloc.c, or should it be close to the
> > > RCLM_* definitions?
> >
> > I can't test it right now, but I think the reason it is here is because
> > RCLM_* and __GFP_* are in different headers that are not aware of each
> > other. This is the place a static compile-time check can be made.
>
> Well, they're pretty intricately linked, so maybe they should go in the
> same header, no?
>

Will investigate. I can't at the moment.

> > It was pointed out that type used for use with the bit functions should
> > all be unsigned long, not int as they were previously. However, I found if
> > I used unsigned long throughout the code, including for array operations,
> > there was a 10-12% slowdown in AIM9. These casts were the compromise.
> > alloctype is unsigned long when used with the functions like assign_bit()
> > but int every other time.
>
> Why does it slow down?  Do you have any detailed profiles?
>

I have no idea, it made no sense to me at all. I did find that it was
only in the pcpu code that really suffered but I didn't figure out why.
Next time I am testing (probably Monday), I'll gather the profiles.

> > In this case, there is an implicit cast so the cast is redundent if that
> > is the problem you are pointing out. I can remove the explicit casts that
> > are dotted around the place.
>
> There needs to be a reason for the casts.  They certainly don't help
> readability or correctness, so there needs to be some justification.  If
> there are performance reasons somehow, they need to be analyzed as well.
>

I'll recheck it.

-- 
Mel Gorman
Part-time Phd Student                          Java Applications Developer
University of Limerick                         IBM Dublin Software Lab

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