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Comments on Co-Array Fortran

Comments on Co-Array Fortran. Robert W. Numrich Minnesota Supercomputing Institute University of Minnesota, Minneapolis. Philosophy Behind the CAF Model. A minimum number of new features that look and feel like Fortran.

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Comments on Co-Array Fortran

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  1. Comments on Co-Array Fortran Robert W. Numrich Minnesota Supercomputing Institute University of Minnesota, Minneapolis

  2. Philosophy Behind the CAF Model • A minimum number of new features that look and feel like Fortran. • The rules for co-dimensions are the same as those for normal dimensions with a few exceptions. • The CAF model is purely local. • The compiler performs normal optimization between synchronization points. • Compiler is not required to perform, is not expected to perform, is deliberately prevented from performing, global optimization. • Programmer is responsible for explicit data distribution, explicit communication, and explicit synchronization. • Programmer is responsible for memory consistency.

  3. The Essential Features of CAF Co-array syntax • Co-dimensions (How images are related to each other) • Data communication (How data moves between images) Synchronization • Full barrier • Pair-wise handshake Dynamic Memory management • Allocatable co-arrays • Allocatable/pointer components of co-array derived types

  4. Recommendations Delete all of Section 8.5 • Substitute SYNC[([myPal])] Delete everything on I/O except • Only image 1 reads stdin • Records from different images not mixed to stdout Delete all the stuff on collectives • Substitute the functions glbSum(x), glbMin(x), glbMax(x) • Argument is not necessarily a co-array.

  5. Ragged Arrays • The most important, the most powerful, feature of the CAF model. • Allows each image to have data structures with different sizes and different locations on each image. • No other model can handle this. • Fortran with CAF extensions handles it in a very natural way. • Allocatable/pointer components of co-array derived types type x real,allocatable,target :: z(:) real,pointer :: ptr(:) type(x) :: y[] end type x allocate(z(someRule(this_image))) y%ptr => z sync y[p]%ptr = 25.0 • Most difficult feature to implement on crummy hardware and/or crummy operating systems. • Good systems should be rewarded for getting this right. • We should not allow bad systems to drag everything down to the lowest common denominator.

  6. Memory Consistency • The programmer is responsible for maintaining memory consistency, not the compiler. • The rules must be very simple and very clear. • With just SYNC, it is very simple • The addition of NOTIFY/QUERY makes it not so simple • Segment boundary statements • Compiler may optimize between segment boundaries but not across segment boundaries • Processor must make co-arrays “visible” to all images across segment boundaries • The programmer must make sure that one and only one image defines a co-array variable “at the same time” • The programmer must make sure that no image tries to reference a co-array variable “at the same time” as another image is trying to define it.

  7. Input/Output • What’s the minimum needed? • stdin/stdout are special cases • Always connected to all images • Only image 1 can read stdin • System must not mix records to stdout from different images • Shared files • Allow each image to open to same unit • Direct access only • Allowing sequential access requires changes to backspace, rewind, etc. • open(unit=u,access=,…) • open(unit=u,access=connectList,…) • Do we need teams? • Do we need to sync?

  8. Collectives • Avoid language bloat • Not everything in the MPI Library needs to be reproduced in CAF as intrinsic procedures • What do we really need and want? • What should the interface be? • CAF is intended as a low-level language • Collectives are easy to write in CAF for any specific procedure • Throwing a long list of new intrinsic procedures over the wall may discourage vendors from adopting CAF • If anything, supply intrinsic functions: glbSum(x), glbMin(x), glbMax(x) argument x need not be a co-array • Propose a supporting library for CAF.

  9. VOLATILE Co-arrays? • What do we want the VOLATILE attribute to mean for co-arrays? • Inhibit optimization? • Always read from memory? • Always flush to memory? • Can VOLATILE make spin-loops work without the need for an artificial sync-memory() from the programmer? • Statements with co-arrays are segment boundary statements

  10. SYNC/BARRIER • One new statement SYNC • Implies full BARRIER for all images • No arguments • No SYNC_IMAGES • Synchronization between subsets of images can be done with NOTIFY/QUERY

  11. NOTIFY/QUERY • Why do we want NOTIFY/QUERY? • Split-phase sync • Subset sync • Master-slave work distribution • Should they match in pairs? • Should we expose which notify matches which query? • Maybe we really want EVENTS? • EVENT_POST(tag) • EVENT_WAIT(tag) • EVENT_CLEAR(tag)

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