[Mpi3-rma] notes from 5pm Tuesday meeting @ July 2012 Forum

Wed Jul 18 10:09:13 CDT 2012

So your A==B comparison is at the level of bits?  Couldn't you just
cast double to long then?

Jeff

On Wed, Jul 18, 2012 at 10:03 AM, Underwood, Keith D
<keith.d.underwood at intel.com> wrote:
> success=0;
> while (!success) {
>         A=B; // B is remote, obtained using RMA
>         C=(A*D+E*F) / G; // Everything local, all are doubles
>         if (B == A) {B=C; success = 1}; // This is CAS
> }
>
> And now you have done a relatively complicated thing on B atomically, though you will suffer if there is contention...
>
> Keith
>
>> -----Original Message-----
>> From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-
>> bounces at lists.mpi-forum.org] On Behalf Of Jeff Hammond
>> Sent: Tuesday, July 17, 2012 11:03 PM
>> To: MPI 3.0 Remote Memory Access working group
>> Subject: Re: [Mpi3-rma] notes from 5pm Tuesday meeting @ July 2012
>> Forum
>>
>> What do you mean by "arbitrary atomics"?  Are you proposing to do
>> something evil with bitwise comparison on floating-point numbers to realize
>> something other than the obvious meaning of CAS on a float type?  If so, just
>> say it, then do it the right way.
>>
>> We should consider how users will interpret CAS on doubles, for example,
>> and not what some low-level network person can dream up to do with this
>> operation.
>>
>> What architecture implements remote CAS on floating-point types in
>> hardware right now?  Who is proposing it?  I invoke the standard (and a bit
>> worn out) argument about implementing MPI-3 entirely in hardware and say
>> that unless someone knows how to do CAS for doubles, it cannot be in the
>> standard.
>>
>> And to be perfectly honest, I have utterly no idea what you really mean right
>> now so it would be very helpful if you could be very explicit about what you
>> mean by CAS on floating point types.  What measure are you using for
>> comparison?  Does this have any reason meaning in the context of floating
>> point arithmetic?
>>
>> Jeff
>>
>> On Tue, Jul 17, 2012 at 9:40 PM, Underwood, Keith D
>> <keith.d.underwood at intel.com> wrote:
>> > Floating-point CAS is valid as a way to implement "if this hasn't changed,
>> put the results of this operation in place".  It gives you a (moderately
>> expensive, not very fair) way to build a form of arbitrary atomics.
>> >
>> > Keith
>> >
>> >> -----Original Message-----
>> >> From: mpi3-rma-bounces at lists.mpi-forum.org [mailto:mpi3-rma-
>> >> bounces at lists.mpi-forum.org] On Behalf Of Jeff Hammond
>> >> Sent: Tuesday, July 17, 2012 7:26 PM
>> >> To: MPI 3.0 Remote Memory Access working group
>> >> Subject: Re: [Mpi3-rma] notes from 5pm Tuesday meeting @ July 2012
>> >> Forum
>> >>
>> >> I don't know that floating-point compare is well-defined.  You really
>> >> have ask "if abs(x-y)<tolerance" and not "if x==y".
>> >>
>> >> I think only fixed-point types should be valid for CAS.
>> >>
>> >> Jeff
>> >>
>> >> On Tue, Jul 17, 2012 at 6:14 PM, Dave Goodell <goodell at mcs.anl.gov>
>> wrote:
>> >> > Discussed the complex types in COMPARE_AND_SWAP issue.  "Fortran
>> >> Integer" category is permitted, but "Complex" category is not,
>> >> primarily because of width.  Since "Fortran Integer" contains wide
>> >> types, shouldn't we just permit "Complex" and "Floating point" as
>> >> well?  Consensus was to stick with the existing text which permits
>> >> only "C integer, Fortran integer, Logical, Multi-language types, or Byte".
>> >> >
>> >> > Group review (esp. Jim & Sreeram):
>> >> > https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/187
>> >> >
>> >> > incorporate Jim suggested change (Torsten):
>> >> > https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/271
>> >> >
>> >> > we think we are unaffected, but need a second check (Jim):
>> >> > https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/278
>> >> >
>> >> > Double-check that C++ is not referenced in the RMA chapter (Pavan):
>> >> > https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/281
>> >> >
>> >> > Needs review (Dave):
>> >> > https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/300
>> >> >
>> >> > Think unaffected, but slim chance of Rput/Rget being affected (Pavan):
>> >> > https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/317
>> >> >
>> >> > Need to check implementation of various (4?) "flush is non-local"
>> >> > changes (Dave):
>> >> > https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/270
>> >> >
>> >> > Need to check disp_unit change (Jim & Sreeram):
>> >> > https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/284
>> >> >
>> >> > After the above items have all been dealt with, all chapter
>> >> > committee
>> >> members should re-read the whole chapter in the *clean* document (the
>> >> one _without_ the changebars and colored text) to look for obvious
>> >> typos and inconsistencies.
>> >> >
>> >> > -Dave
>> >> >
>> >> >
>> >> > _______________________________________________
>> >> > mpi3-rma mailing list
>> >> > mpi3-rma at lists.mpi-forum.org
>> >> > http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma
>> >>
>> >>
>> >>
>> >> --
>> >> Jeff Hammond
>> >> Argonne Leadership Computing Facility University of Chicago
>> >> Computation Institute jhammond at alcf.anl.gov / (630)
>> >> 252-5381 http://www.linkedin.com/in/jeffhammond
>> >> https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond
>> >>
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>> >
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>>
>>
>>
>> --
>> Jeff Hammond
>> Argonne Leadership Computing Facility
>> University of Chicago Computation Institute jhammond at alcf.anl.gov / (630)
>> 252-5381 http://www.linkedin.com/in/jeffhammond
>> https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond
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>
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-- 
Jeff Hammond
Argonne Leadership Computing Facility
University of Chicago Computation Institute
jhammond at alcf.anl.gov / (630) 252-5381
http://www.linkedin.com/in/jeffhammond
https://wiki.alcf.anl.gov/parts/index.php/User:Jhammond