On Tue, Jan 28, 2014 at 01:19:10PM -0500, Waiman Long wrote: > For single-thread performance (no contention), a 256K lock/unlock > loop was run on a 2.4Ghz Westmere x86-64 CPU. The following table > shows the average time (in ns) for a single lock/unlock sequence > (including the looping and timing overhead): > > Lock Type Time (ns) > --------- --------- > Ticket spinlock 14.1 > Queue spinlock (Normal) 8.8* What CONFIG_NR_CPUS ? Because for CONFIG_NR_CPUS < 128 (or 256 if you got !PARAVIRT), the fast path code should be: ticket: mov $0x100,eax lock xadd %ax,(%rbx) cmp %al,%ah jne ... although my GCC is being silly and writes: mov $0x100,eax lock xadd %ax,(%rbx) movzbl %ah,%edx cmp %al,%dl jne ... Which seems rather like a waste of a perfectly good cycle. With a bigger NR_CPUS you do indeed need more ops: mov $0x10000,%edx lock xadd %edx,(%rbx) mov %edx,%ecx shr $0x10,%ecx cmp %dx,%cx jne ... Whereas for the straight cmpxchg() you'd get something relatively simple like: mov %edx,%eax lock cmpxchg %ecx,(%rbx) cmp %edx,%eax jne ... Anyway, as soon as you get some (light) contention you're going to tank because you have to pull in extra cachelines, which is sad. I suppose we could from the ticket code more and optimize the uncontended path, but that'll make the contended path more expensive again, although probably not as bad as hitting a new cacheline. -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
