On 12/16/2013 04:01 PM, Andrew Morton wrote: > On Fri, 13 Dec 2013 15:59:03 -0800 Dave Hansen <d...@sr71.net> wrote: >> SLUB depends on a 16-byte cmpxchg for an optimization. For the >> purposes of this series, I'm assuming that it is a very important >> optimization that we desperately need to keep around. > > What if we don't do that.
I'll do some testing and see if I can coax out any delta from the optimization myself. Christoph went to a lot of trouble to put this together, so I assumed that he had a really good reason, although the changelogs don't really mention any. I honestly can't imagine that a cmpxchg16 is going to be *THAT* much cheaper than a per-page spinlock. The contended case of the cmpxchg is way more expensive than spinlock contention for sure. fc9bb8c768's commit message says: > The doublewords must be properly aligned for cmpxchg_double to work. > Sadly this increases the size of page struct by one word on some > architectures. > But as a resultpage structs are now cacheline aligned on x86_64. I'm not sure what aligning them buys us though. I think I just demonstrated that cache footprint is *way* more important than alignment. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
