Hi Scott,
any news after this wine-meeting ?
Best,
Marek
Dne Thu, 27 Jun 2013 00:12:54 +0200 Scott Le Grand <varelse2005.gmail.com>
napsal/-a:
> Handed NVIDIA a defective Titan on Saturday. No word since. Having
> dinner
> with the perps tonight so we'll see what comes of it. Since I'm bringing
> the wine, I'll be sure to spike it sodium pentothal...
>
>
>
>
>
> On Wed, Jun 26, 2013 at 2:22 PM, Jonathan Gough
> <jonathan.d.gough.gmail.com>wrote:
>
>> Any Updates on the bug with the Titan cards?
>>
>>
>> On Thu, Jun 20, 2013 at 2:05 PM, Marek Maly <marek.maly.ujep.cz> wrote:
>>
>> > Thanks guys !
>> > Now it is all clear even to me :))
>> >
>> > So the key problem is here that one could not reproduce the exactly
>> the
>> > same
>> > "resource" conditions (e.g. state of individual cuda cores and
>> memory
>> > segments)
>> > during different runs of parallel code.
>> >
>> > Best wishes,
>> >
>> > Marek
>> >
>> >
>> >
>> >
>> >
>> > Dne Thu, 20 Jun 2013 19:36:25 +0200 Ross Walker
>> <ross.rosswalker.co.uk>
>> > napsal/-a:
>> >
>> > >
>> > > 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 |
>> > > ---------------------------------------------------------
>> > >
>> > > Note: Electronic Mail is not secure, has no guarantee of delivery,
>> may
>> > > not
>> > > be read every day, and should not be used for urgent or sensitive
>> issues.
>> > >
>> > >
>> > >
>> > >
>> > >
>> > >
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Received on Mon Jul 01 2013 - 04:30:02 PDT
