On 6/20/13 9:57 AM, "Marek Maly" <marek.maly.ujep.cz> wrote:
>Dne Thu, 20 Jun 2013 18:57:18 +0200 Scott Le Grand
><varelse2005.gmail.com>
>napsal/-a:
>
>> You're overthinking it. Neither NAMD nor GROMACS produce deterministic
>> outputs because they accumulate in 32-bit single precision in an
>> arbitrary order rather than do so in a deterministic order or use an
>> associative
>
>OK, but what is the reason of that "ARBITRARY order" ?
>
>Why the order of the numbers accumulation is in each run of the same code
>
>on the
>same machine different ? I would naturally assume that the order of all
>operations will be the same in each run unless is from some reason
>defined
>using some pseudorandom number generator which is not reset or is even
>impossible to "reset" it (if necessary) for each code run.
Because these calculations are NOT being run in serial. GPUs are massively
threaded architectures running hundreds of thousands of threads, even when
using a single GPU. These threads are dispatched across multiple streaming
compute units and essentially things are executed whenever the required
memory arrives. It is a VERY different situation from running single
threaded on CPUs. I would suggest reading a couple of books on CUDA and
GPUs and that should make the differences very apparent.
Essentially CPUs are going the same way now, pretty much nothing is serial
anymore so unless you take steps to deliberately control the way things
are rounded when an array is summed in an arbitrary order (either by use
of things like atomic operations, or various sync and locks, which make
your code slow) you will always get different answers from different runs.
All the best
Ross
/\
\/
|\oss Walker
---------------------------------------------------------
| Associate Research Professor |
| San Diego Supercomputer Center |
| Adjunct Associate Professor |
| Dept. of Chemistry and Biochemistry |
| University of California San Diego |
| NVIDIA Fellow |
|
http://www.rosswalker.co.uk |
http://www.wmd-lab.org |
| Tel: +1 858 822 0854 | EMail:- ross.rosswalker.co.uk |
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Received on Thu Jun 20 2013 - 11:00:02 PDT
