Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

[Waiman Long <waiman.long@hp.com>]

On 01/31/2014 10:08 AM, Peter Zijlstra wrote:
> 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 ?

I was testing on a RHEL6.4 system which has a CONFIG_NR_CPUS of 4096.

>
> 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 ...

I believe the speeds of the lock functions are about the same. However, 
qspinlock has a much simpler unlock function which probably account of 
most of the speed gain.

> 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.

Light contention is the only case where the qspinlock may not perform as 
good as the ticket spinlock. I know this is the most common case. 
However, I would argue that the slowdown, if any, will not be really 
noticeable. This is what I will try to find out.


> 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.

I don't get what you are trying to say.

Right now, I am using only bit 0 as a lock bit. I can use  bit 4, for 
instance, as a pending locker bit and spin until bit 0 is clear. So if 
there is only 1 other task spinning, it won't need to  fetch another 
cacheline. However, it will slow down the uncontended path as I can't 
assign a 0 byte to free the lock. I have to use an atomic subtraction or 
clear bit instead.

-Longman