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Date: 22 Oct 2009 13:35:46 +0100
From: "N.M. Maclaren" <nmm1@cam.ac.uk>
To: SC22WG5 <sc22wg5@open-std.org>
Subject: Re: [ukfortran] (SC22WG5.4111) (j3.2006) Standard	intrinsics	and coarrays
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On Oct 22 2009, Bill Long wrote:
>
>>     Systems with suitable compare-and-swap hardware can implement
>> parallel use of a shared RANDOM_NUMBER safely without locking.  Adding
>> RANDOM_SEED to the mix makes that much trickier, and it's not something
>> that any sane programmer would do, anyway.
>
>As a (my) performance rule of thumb, if you can generate 1.e9 64-bit 
>values per second you are OK. In some cases, 1.e8 might be considered. 
>Even with hardware compare-and-swap, a shared generator on an even 
>modest sized system will have trouble with this. The scheme has no 
>chance of scaling to large numbers of images.  And it still fails the 
>reproducibility requirement.

Oh, yes.  It's a crazy approach for scalable HPC systems - no dissention
there.  But I don't think that it should be forbidden, as it's a very
plausible approach for single-chip, non-HPC implementations.  In those
cases, 1.0e7 values a second would be adequate, or even 1.0e6.

Regards,
Nick.