[Omp] slow performance

[Ruud van der Pas]

Hi Andrew,

> Second, as pointed out by Alexander Spiegel, I should declare "id" 
> variable as thread private, this really help a lot, although I am still 
> not fully understand that yet, anyway thanks Alex!

As "id" is shared now, multiple threads will access the same cache
line, more or less at the same time. The value of "id" is not only
undefined as a result, but the contention for the same cache line
will cause a performance loss as well. I think the same is true
for variable "pi".

Kind regards,
Ruud
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