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

Wed Jul 18 10:22:07 CDT 2012

i meant make the user cast.  i think you underestimate how often
scientists falsely think that == acts on floating point numbers in a
mathematically sensible way.

On Wed, Jul 18, 2012 at 10:18 AM, Underwood, Keith D
<keith.d.underwood at intel.com> wrote:
> Yes, the MPI library could certainly make that choice in how it used the hardware...
>
>> -----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: Wednesday, July 18, 2012 11:09 AM
>> To: MPI 3.0 Remote Memory Access working group
>> Subject: Re: [Mpi3-rma] notes from 5pm Tuesday meeting @ July 2012
>> Forum
>>
>> 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
>> >> >>
>> >> >> _______________________________________________
>> >> >> mpi3-rma mailing list
>> >> >> mpi3-rma at lists.mpi-forum.org
>> >> >> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma
>> >> >
>> >> > _______________________________________________
>> >> > 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
>> >> _______________________________________________
>> >> mpi3-rma mailing list
>> >> mpi3-rma at lists.mpi-forum.org
>> >> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma
>> >
>> > _______________________________________________
>> > 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
>> _______________________________________________
>> mpi3-rma mailing list
>> mpi3-rma at lists.mpi-forum.org
>> http://lists.mpi-forum.org/mailman/listinfo.cgi/mpi3-rma
>
> _______________________________________________
> 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