Re: [PATCH] mm : kill combined_idx

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

On Mon, Dec 21, 2009 at 02:31:39PM +0900, KAMEZAWA Hiroyuki wrote:
> On Mon, 21 Dec 2009 11:32:27 +0800
> Huang Shijie <shijie8@gmail.com> wrote:
> 
> > In more then half of all the cases, `page' is head of the buddy pair
> > {page, buddy} in __free_one_page. That is because the allocation logic
> > always picks the head of a chunk, and puts the rest back to the buddy system.
> > 
> > So calculating the combined page is not needed but waste some cycles in
> > more then half of all the cases.Just do the calculation when `page' is
> > bigger then the `buddy'.
> > 
> > Signed-off-by: Huang Shijie <shijie8@gmail.com>
> 
> Hmm...As far as I remember, this code design was for avoiding "if".
> Is this compare+jump is better than add+xor ?
> 

Agreed. It's not clear that a compare+jump is cheaper than the add+xor.
How often it's the case that the page is the higher or lower half of the
buddy would depend heavily on the allocation/free pattern making it
hard, if not possible, to predict which is the more common case.

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

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