Re: [RFC][PATCH 2/3] mm: slab: move around slab ->freelist forcmpxchg

[Andi Kleen]

On Thu, Dec 12, 2013 at 05:46:02PM +0000, Christoph Lameter wrote:
> On Wed, 11 Dec 2013, Dave Hansen wrote:
> 
> >
> > The write-argument to cmpxchg_double() must be 16-byte aligned.
> > We used to align 'struct page' itself in order to guarantee this,
> > but that wastes 8-bytes per page.  Instead, we take 8-bytes
> > internal to the page before page->counters and move freelist
> > between there and the existing 8-bytes after counters.  That way,
> > no matter how 'stuct page' itself is aligned, we can ensure that
> > we have a 16-byte area with which to to this cmpxchg.
> 
> Well this adds additional branching to the fast paths.

The branch should be predictible and compare the cost of a branch
(near nothing on a modern OOO CPU with low IPC code like this when
predicted) to the cost of a cache miss (due to larger struct page)

-Andi

-- 
ak@xxxxxxxxxxxxxxx -- Speaking for myself only
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