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

Mel Gorman
2009-02-23T16:40:58Z

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?

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