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Re: [PATCH] Remove atomic operations from malloc.c


On 11-02-2015 11:29, Leonhard Holz wrote:
>> I did get into the changes itself, but at least for powerpc (POWER8/16c/128T)
>> I am not seeing improvements with the patch.  In fact it seems to increase
>> contention:
>>
>>             time per iteration
>> nths       master     patch
>> 1           51.422    75.046
>> 8          53.077    78.507
>> 16         57.430    89.385
>> 32         71.206   108.359
>> 64        114.370   172.115
>> 128       251.731   330.924
>>
>
> Thank you for testing! Maybe the costs of a mutex_lock are higher on PowerPC than on i686? Anyway it looks like I have to take a different approach...
>
PowerPC uses now the default implementation at sysdeps/nptl/lowlevellock.h which 
basically translates to acquire CAS followed by a futex operation in contention
case.  So I think the gain is for powerpc (specially with high SMT), busy-wait
using like a spinlock yields better performance than possible issuing a futex
operations.



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