Re: [Xen-devel] [PATCH v5 3/8] qspinlock, x86: Add x86 specific optimization for 2 contending tasks

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Tue, Mar 04, 2014 at 12:48:26PM -0500, Waiman Long wrote:
> Peter,
> 
> I was trying to implement the generic queue code exchange code using
> cmpxchg as suggested by you. However, when I gathered the performance
> data, the code performed worse than I expected at a higher contention
> level. Below were the execution time of the benchmark tool that I sent
> you:
> 
>                 [xchg]        [cmpxchg]
>   # of tasks    Ticket lock     Queue lock      Queue Lock
>   ----------    -----------     -----------     ----------
>        1          135            135              135
>        2          732           1315            1102
>        3         1827           2372            2681
>        4         2689           2934             5392
>        5         3736           3658             7696
>        6         4942           4434            9876
>        7         6304           5176           11901
>        8         7736           5955           14551
> 

I'm just not seeing that; with test-4 modified to take the AMD compute
units into account:

root@interlagos:~/spinlocks# LOCK=./qspinlock-pending-opt ./test-4.sh ; 
LOCK=./qspinlock-pending-opt2 ./test-4.sh
 4: 50783.509653
 8: 146295.875715
16: 332942.964709
 4: 51033.341441
 8: 146320.656285
16: 332586.355194

And the difference between opt and opt2 is that opt2 replaces 2 cmpxchg
loops with unconditional ops (xchg8 and xchg16).

And I'd think that 4 CPUs x 4 Nodes would be heavy contention.

I'll have another poke tomorrow; including verifying asm tomorrow, need
to go sleep now.

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