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

Christoph Lameter
2009-02-23T15:52:17Z

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. 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.

Maybe we can come up with a version of gfp_zone that has no branches and
no lookup?