Re: [PATCH 04/20] Convert gfp_zone() to use a table of precalculated value

KAMEZAWA Hiroyuki
2009-02-24T01:33:59Z

On Mon, 23 Feb 2009 16:40:47 +0000
Mel Gorman <mel@csn.ul.ie> wrote:

> On Mon, Feb 23, 2009 at 10:43:20AM -0500, Christoph Lameter wrote:
> > On Tue, 24 Feb 2009, Nick Piggin wrote:
> >
> > > > Are you sure that this is a benefit? Jumps are forward and pretty short
> > > > and the compiler is optimizing a branch away in the current code.
> > >
> > > Pretty easy to mispredict there, though, especially as you can tend
> > > to get allocations interleaved between kernel and movable (or simply
> > > if the branch predictor is cold there are a lot of branches on x86-64).
> > >
> > > I would be interested to know if there is a measured improvement.
>
> Not in kernbench at least, but that is no surprise. It's a small
> percentage of the overall cost. It'll appear in the noise for anything
> other than micro-benchmarks.
>
> > > It
> > > adds an extra dcache line to the footprint, but OTOH the instructions
> > > you quote is more than one icache line, and presumably Mel's code will
> > > be a lot shorter.
> >
>
> Yes, it's an index lookup of a shared read-only cache line versus a lot
> of code with branches to mispredict. I wasn't happy with the cache line
> consumption but it was the first obvious alternative.
>
> > Maybe we can come up with a version of gfp_zone that has no branches and
> > no lookup?
> >
>
> Ideally, yes, but I didn't spot any obvious way of figuring it out at
> compile time then or now. Suggestions?
>

Assume
ZONE_DMA=0
ZONE_DMA32=1
ZONE_NORMAL=2
ZONE_HIGHMEM=3
ZONE_MOVABLE=4

#define __GFP_DMA ((__force gfp_t)0x01u)
#define __GFP_DMA32 ((__force gfp_t)0x02u)
#define __GFP_HIGHMEM ((__force gfp_t)0x04u)
#define __GFP_MOVABLE ((__force gfp_t)0x08u)

#define GFP_MAGIC (0400030102) ) #depends on config.

gfp_zone(mask) = ((GFP_MAGIC >> ((mask & 0xf)*3) & 0x7)

Thx
-Kame