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SubjectRe: [PATCH 24/27] Convert gfp_zone() to use a table of precalculated values
On Mon, Mar 16, 2009 at 03:12:50PM -0400, Christoph Lameter wrote:
> On Mon, 16 Mar 2009, Mel Gorman wrote:
>
> > +int gfp_zone_table[GFP_ZONEMASK] __read_mostly;
>
> The gfp_zone_table is compile time determinable. There is no need to
> calculate it.
>

The cost of calculating it is negligible and the code is then freed later
in boot. Does having a const table make any difference?

> const int gfp_zone_table[GFP_ZONEMASK] = {
> ZONE_NORMAL, /* 00 No flags set */
> ZONE_DMA, /* 01 Only GFP_DMA set */
> ZONE_HIGHMEM, /* 02 Only GFP_HIGHMEM set */
> ZONE_DMA, /* 03 GFP_HIGHMEM and GFP_DMA set */
> ZONE_DMA32, /* 04 Only GFP_DMA32 set */
> ZONE_DMA, /* 05 GFP_DMA and GFP_DMA32 set */
> ZONE_DMA32, /* 06 GFP_DMA32 and GFP_HIGHMEM set */
> ZONE_DMA, /* 07 GFP_DMA, GFP_DMA32 and GFP_DMA32 set */
> ZONE_MOVABLE, /* 08 Only ZONE_MOVABLE set */
> ZONE_DMA, /* 09 MOVABLE + DMA */
> ZONE_MOVABLE, /* 0A MOVABLE + HIGHMEM */
> ZONE_DMA, /* 0B MOVABLE + DMA + HIGHMEM */
> ZONE_DMA32, /* 0C MOVABLE + DMA32 */
> ZONE_DMA, /* 0D MOVABLE + DMA + DMA32 */
> ZONE_DMA32, /* 0E MOVABLE + DMA32 + HIGHMEM */
> ZONE_DMA /* 0F MOVABLE + DMA32 + HIGHMEM + DMA
> };
>
> Hmmmm... Guess one would need to add some #ifdeffery here to setup
> ZONE_NORMAL in cases there is no DMA, DMA32 and HIGHMEM.
>

Indeed, as I said, this is somewhat error prone which is why the patch
calculates the table at run-time instead of compile-time trickery.


--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab


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