Messages in this thread | | | Date | Fri, 31 Jan 2014 16:08:32 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation |
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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.
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