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Efficient Compilation of the HPJava Language for HPC

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Han-Ku Lee

Department of Computer Science

Florida State University

Feb 19th, 2002

- Background - review of data-parallel languages
- HPspmd Programming Language Model
- HPJava

- The compilation strategies for HPJava
- Author’s contributions and Proposedwork
- Conclusions and Current Status

- Data-parallel programming and languages have played a major role in high-performance computing
- HPF – difficult (compilation)
- Library-based lower-level SPMD programming – successful
- HPspmd programming language model – a flexible hybrid of HPF-like data-parallel language and the popular, library-oriented, SPMD style
- Base-language for HPspmd model should be clean and simple object semantics, cross-platform portability, security, and popular – Java

- Efficient Compilation of the HPJava Language for HPC
- Main thrust of proposal work will be to explore effectiveness of optimizations in the HPspmd translator
- Continue to investigate which optimization strategies are most critical in a wider range of applications in High Performance Compilers

- Large data-structures, typically arrays, are split across nodes
- Each node performs similar computations on a different part of the data structure
- SIMD – Illiac IV and Connection Machine for example introduced a new concept, distributed arrays
- MIMD – asynchronous, flexible, hard to program
- SPMD – loosely synchronous model (SIMD+MIMD)
- Each node has its own local copy of program

- By early 90s, value of portable, standardized languages universally acknowledged.
- Goal of HPF Forum – a single language for High Performance programming. Effective across architectures—vector, SIMD, MIMD, though SPMD a focus.
- HPF - an extension of Fortran 90 to support the data parallel programming model on distributed memory parallel computers
- Supported by Cray, DEC, Fujitsu, HP, IBM, Intel, Maspar, Meiko, nCube, Sun, and Thinking Machines

Processors

Memory Area

Ideal data distribution

- Multi-processing and data distribution – communication and load-balance
- Introduced processor arrangement and Templates
- Data Alignment

- A language for parallel programming, especially suitable for massively parallel, distributed memory computers.
- Takes various ideas from HPF.
- e.g. - distributed array model

- In other respects, HPJava is a lower levelparallel programming language than HPF.
- explicit SPMD, needing explicit calls to communication libraries such as MPI or Adlib

- The HPJava system is built on Javatechnology.
- The HPJava programming language is an extension of the Java programming language.

- Translators are much easier to implement than HPF compilers. No compiler magic needed
- Attractive framework for library development, avoiding inconsistent parameterizations of distributed array arguments
- Better prospects for handling irregular problems – easier to fall back on specialized libraries as required
- Can directly call MPI functions from within an HPspmd program

- Java is an attractive language, but needs to be improved for large computational tasks
- Java provides an array of arrays => disadvantage
- Time consumption for out-of bounds checking
- The ability to alias rows of an array
- The cost of accessing an element

- HPJava introduces true multidimensional arrays and regular sections
- For example
int [[*,*]] a = new int [[5, 5]] ;

for (int i=0; i<4; i++) a [i, i+1] = 19 ;

foo ( a [[:, 0]] ) ;

0

1

p

Proces2 p = new Procs(2, 3) ;

on (p) {

Range x = new BlockRange(N, p.dim(0)) ;

Range y = new BlockRange(N, p.dim(1)) ;

float [[-,-]] a = new float [[x, y]] ;

float [[-,-]] b = new float [[x, y]] ;

float [[-,-]] c = new float [[x, y]] ;

… initialize ‘a’, ‘b’

overall (i=x for :)

overall (j=y for :)

c [i, j] = a [i, j] + b [i, j];

}

- An HPJava program is concurrently started on all members of some process collection – process groups
- on construct limits control to the active process group (APG), p

0

1

2

- The most important feature of HPJava
- A collective object shared by a number of processes
- Elements of a distributed array are distributed
- True multidimensional array
- Can form a regular section of an distributed array
- When N = 8 in the previous example code, the distributed array, ‘a’ is distributed like:

BlockRange

CyclicRange

Range

ExtBlockRange

IrregRange

CollapsedRange

Dimension

- HPJava provides further distribution formats for dimensions of distributed arrays without further extensions to the syntax
- Instead, the Range class hierarchy is extended
- BlockRange, CyclicRange, IrregRange, Dimension
- ExtBlockRange – a BlockRange distribution extended with ghost regions
- CollapsedRange – a range that is not distributed, i.e. all elements of the range mapped to a single process

overall (i = x for 1: N-2: 2)

a[i] = i` ;

- Distributed parallel loop
- i– distributed index whose value is symbolic location (not integer value)
- Index triplet represents a lower bound, an upper bound, and a step – all of which are integer expressions
- With a few exception, the subscript of a distributed array must be a distributed index, and x should be the range of the subscripted array (a)
- This restriction is an important feature, ensuring that referenced array elements are locally held

- HPJava supports subarrays modeled on the array sections of Fortran 90
- The new array section is a subset of the elements of the parent array
- Triplet subscript
- The rank of an array section is equal to the number of triplet subscripts
- e.g. float [[-,-]] a = new float [[x, y]] ;
float [[-]] b = a [[0, :]] ;

float [[-,-]] u = a [[0 : N/2-1, 0 : N-1 : 2]] ;

- Type signature of a distributed array
T [[attr0, …, attrR-1]] bras

where R is the rank of the array and each term attrr is either a single hyphen, - or a single asterisk, *, the term bras is a string of zero or more bracket pairs, []

- T can be any Java type other than an array type. This signature represents the type of a distributed array whose elements have Java type
T bras

- A distributed array type is not treated as a class type

- The HPJava system is not exactly a high-level parallel programming language – more like a tool to assist programmers generate SPMD parallel code
- This suggests the translations the system applies should be relatively simple and well-documented, so programmers can exploit the tool more effectively
- We don’t expect the generated code to be human readable or modifiable, but at least the programmer should be able to work out what is going on

- The HPJava specification defines the basic translation scheme as a series of schema

Source: T [[attr0, …, attrR-1]] a ;

TRANSLATION: T [] a ’dat ;

ArrayBase a ’bas ;

DIMENSION_TYPE (attr0) a ’0 ;

…

DIMENSION_TYPE (attrR-1) a ’R-1 ;

where DIMENSION_TYPE (attrr) ≡ ArrayDim if attrr is a hyphen, or

DIMENSION_TYPE (attrr) ≡ SeqArrayDim if attrr is a asterisk

e.g.

float [[-,*]] var ; float [] var__$DS ;

ArrayBase var__$bas ;

ArrayDim var__$0 ;

SeqArrayDim var__$1 ;

SOURCE: overall (i = x for e lo : e hi : e stp) S

TRANSLATION: Block b = x.localBlock(T [e lo], T [e hi], T [e stp]) ;

Group p = apg.restrict(x.dim(), apg) ;

for (int l = 0; l < b.count; l ++) {

int sub = b.sub_bas + b.sub_stp * l ;

int glb = b.glb_bas + b.glb_stp * l ;

T [S | p]

}

where: i is an index name in the source program,

x is a simple expression in the source program,

e lo, e hi, and e stpare expressions in the source,

S is a statement in the source program, and

b, p, l, sub and glb are names of new variables

- Based on the observations for parallel algorithms such as Laplace equation using red-black iterations, distributed array element accesses are generally located in inner overall loops.
- The complexity of the associated terms in the subscript expression of a distributed array element access.
- Strength Reduction - introducing the induction variables
- Loop-unrolling - hoisting the run-time support classes
- Common-subexpression elimination

- The novelty is in adapting these optimizations to make HPspmd practical

- Here we only consider strength reduction optimizations on the index expression
- Consider the nested overall and loop constructs
overall (i=x for :)

overall (j=y for :) {

float sum = 0 ;

for (int k=0; k<N; k++)

sum += a [i, k] * b [k, j] ;

c [i, j] = sum ;

}

Block bi = x.localBlock() ;

for (int lx = 0; lx<bi.count; lx ++) {

Block bj = y.localBlock() ;

for (int ly = 0; ly<bj.count; ly ++) {

float sum = 0 ;

for (int k = 0; k<N; k ++)

sum += a.dat() [a.bas() + (bi.sub_bas + bi.sub_stp * lx) * a.str(0) +

k * a.str(1)] *

b.dat() [b.bas() + (bj.sub_bas + bj.sub_stp * ly) * b.str(1) +

k * b.str(0)] ;

c.dat() [c.bas() + (bi.sub_bas + bi.sub_stp * lx) * c.str(0) +

(bj.sub_bas + bj.sub_stp * ly) * c.str(1)] = sum;

}

}

- The problem is the complexity of the associated terms in the subscript expressions
- The subscript expressions can be greatly simplified by application of strength-reduction optimization
- Eliminate complicated expressions involving multiplication from expressions in inner loops by introducing the induction variables:
- Which can be computed efficiently by increasing at suitable points with the induction increments:

- Before adapting optimization strategies in HPJava translator, need to benchmark hand-coded optimizations
- Need to prove distributed arrays in Java don’t introduce unacceptable overhead

- Benchmarked on Linux Red Hats 7.2 (Pentium IV 1.5 GHZ)
- Linpack, Matrix-Multiplication, Laplace Equations using red-black relaxation
- IMB Developer kits 1.3 (JIT)
- Compared Java and HPJava with GNU cc and Fortran77

- daxpy() kernel in Linpack
- N = 200, iter = 100000 with Maximal Optimization

- N = 100, iter =100 with Maximal Optimization
- HPJava uses a single-processor

- N = 500, count = 100 with Maximal Optimization

- Naïve HPJava is slow because allows for distributed arrays – complexity of subscripting
- Practical optimizations can remove these overhead
- HPJava results for a single processor – expected scale with multiple-processors
- Java is quite competitive with other languages

- Could say “performance of Fortran and C” are same
- But, depends upon compilers
- GNU Fortran 77 compiler generates more machine codes than GNU cc compiler does for main loop in Linpack

- Developing and maintaining the HPJava front-end and back-end environments at NPAC, CSIT, and Pervasive Technology Labs.
- Translator, Type-Checker, and Type-Analyzer of HPJava.
- Some of his early works at NPAC
- Unparser and Abstract Expression Node generator, and original implementation of the JNI interfaces of the run-time communication library, Adlib.

- Collaborated with Bryan Carpenter, Geoffrey Fox, Guansong Zhang, Sang Lim and Zheng Qiang
- The first fully functional HPJava translator (written in Java) is now operational
- Parser – JavaCC and JTB tools
- Has been tested and debugged against small test suite and 800-line multigrid code

- Efficient Compilation of the HPJava Language for HPC
- optimizations of HPJava

- Main thrust of proposal work will be to explore effectiveness of optimizations in the HPspmd translator
- First, need to know which optimization strategies should be applied, by experimenting with hand-coded optimizations in HPJava and need to benchmark on parallel machines such as SP3
- Next, develop the optimized HPJava translator, test codes and applications over next few months
- Will continue to investigate which optimization strategies are most critical in a wider range of applications in HPspmd compilers

- Han-Ku Lee, Bryan Carpenter, Geoffrey Fox, Sang Boem Lim. Benchmarking HPJava: Prospects for Performance. Feb 8, 2002. Submitted to Sixth Workshop on Languages, Compilers, and Run-time Systems for Scalable Computers(LCR2002). http://motefs.cs.umd.edu/lcr02/
- Bryan Carpenter, Geoffrey Fox, Han-Ku Lee, and Sang Lim. Node Performance in the HPJava Parallel Programming Language. Feb, 2002. The 16th Annual ACM International Conference on Super Computing(ICS2001). http://www.lcpcworkshop.org/LCPC2001/
- Bryan Carpenter, Geoffrey Fox, Han-Ku Lee, and Sang Lim. Translation of the HPJava Language for Parallel Programming. May 31, 2001. The 14th annual workshop on Languages and Compilers for Parallel Computing(LCPC2001). http://www.lcpcworkshop.org/LCPC2001/
- Bryan Carpenter, Guansong Zhang, Han-Ku Lee, and Sang Lim. Parallel Programming in HPJava. Draft of May 2001. http://aspen.csit.fsu.edu/pss/HPJava/

- Reviewed data-parallel languages such as HPF
- Introduced HPspmd programming language model – SPMD framework for using libraries based on distributed arrays
- Specific syntax, new control constructs, basic translation schemes, and basic optimization strategies for HPJava

- Proposed work:
- Efficient Compilation of the HPJava Language for HPC

- This work was supported in part by the National Science Foundation (NSF ) Division of Advanced Computational Infrastructure and Research
- Contract number – 9872125