On Fri, Jan 03, 2014 at 02:18:16PM -0800, Andrew Morton wrote: > On Fri, 03 Jan 2014 10:01:47 -0800 Dave Hansen <dave@xxxxxxxx> wrote: > > > This is a minor update from the last version. The most notable > > thing is that I was able to demonstrate that maintaining the > > cmpxchg16 optimization has _some_ value. > > > > Otherwise, the code changes are just a few minor cleanups. > > > > --- > > > > SLUB depends on a 16-byte cmpxchg for an optimization which > > allows it to not disable interrupts in its fast path. This > > optimization has some small but measurable benefits: > > > > http://lkml.kernel.org/r/52B345A3.6090700@xxxxxxxx > > So really the only significant benefit from the cmpxchg16 is with > cache-cold eight-byte kmalloc/kfree? 8% faster in this case? But with > cache-hot kmalloc/kfree the benefit of cmpxchg16 is precisely zero. Hello, I guess that cmpxchg16 is not used in this cache-hot kmalloc/kfree test, because kfree would be done in free fast-path. In this case, this_cpu_cmpxchg_double() would be called, so you cannot find any effect of cmpxchg16. Thanks. > > This is really weird and makes me suspect a measurement glitch. > > Even if this 8% is real, it's unclear that it's worth all the > complexity the cmpxchg16 adds. > > It would be really useful (hint :)) if we were to know exactly where > that 8% is coming from - perhaps it's something which is not directly > related to the cmpxchg16, and we can fix it separately. > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>