Parallel Programming using thePGAS Approach
Outline • Introduction • Programming parallel systems: threading, message passing • PGAS as a middle ground • UPC (Unified Parallel C) • History of UPC • Shared scalars and arrays • Work-sharing in UPC: parallel loop • DASH – PGAS in the form of a C++ template library • A quick overview of the project • Conclusion
… Memory System Mem Mem Mem … Process/thread Physical memory Private data Shared data Memory Access (read/write) Explicit Message Programming Parallel Machines The two most widely used approaches for parallel programming: Shared Memory Programming using Threads Message Passing
… Memory System Process/thread Physical memory Private data Shared data Memory Access (read/write) Explicit Message Shared Memory Programming using Threads • Examples: • OpenMP, Pthreads, C++ threads, Java threads • Limited to shared memory systems • Shared data can be directly accessed • Implicit communication, direct reads, writes • Advantages • Typically easier to program, natural extension of sequential programming • Disadvantages • Subtle bugs, race conditions • False sharing as a performance problem
Mem Mem Mem … Process/thread Physical memory Private data Shared data Memory Access (read/write) Explicit Message Message Passing • Example • MPI (message passing interface) • Disadvantages • Complex programming paradigm • Manual data partitioning required • Explicit coordination of communication (send/receive pairs) • Data replication (memory requirement) • Advantages • Highly efficient and scalable (to the largest machines in use today) • Data locality is “automatic” • No false sharing, no race conditions • Runs everywhere
Process/thread PGAS Layer Physical memory Private data Shared data Memory Access (put/get) Explicit Message Partitioned Global Address Space • Best of both worlds • Can be used on large scale distributed memory machines but also on shared memory machines • A PGAS program looks much like a regular threaded program, but • Sharing data is declared explicitly • The data partitioning is made explicit • Both needed for performance! shared data space is partitioned!
Thread 0 Thread 1 Thread n-1 ours Shared mine mine mine … Global Address Space Private Partitioned Global Address Space • Example • Let’s call the members of our program threads • Let’s assume we use the SPMD (single program multiple data) paradigm • Let’s assume we have a new keyword “shared” that puts variables in the shared global address space This is how PGAS is ex- pressed in UPC (Unified Parallel C)! (more later) shared int ours; int mine; • n copies of mine (one per thread) • Each thread can only access its own copy • 1 copy of ours • Accessible by every thread
Thread 0 Thread 1 Thread 2 Thread 3 ours[0] ours[1] ours[2] ours[3] mine mine mine mine Shared Arrays • Example: a shared array (UPC) shared int[4] ours; int mine; Shared Global Address Space Private • Affinity – in which partition a data item “lives” • ours (previous slide) lives in partition 0 (by convention) • ours[i] lives in partition i
Local-view vs. Global-view • Two ways to organize access to shared data: • Global-viewE.g., Unified Parallel C • Local-viewE.g., Co-array Fortran • X is declared in terms of its global size • X is accessed in terms of global indices • process is not specified explicitly shared int X[100]; X[i]=23; Global size, Global index • a, b are declared in terms of their local size • a,b are accessed in terms of local indices • process (image) is specified explicitly (the co-index) integer :: a(100)[*], b(100)[*] b(17) = a(17)[2] Local size, Local index co-dimension / co-index in square brackets
UPC History and Status • UPC is an extension to ANSI C • New keywords, library functions • Developed in the late 1990s and early 2000s • Based on previous projects at UCB, IDA, LLNL, … • Status • Berkeley UPC • GCC version • Vendor compilers (Cray, IBM, …) • Most often used on graph problems, irregular parallelism
UPC Execution Model • A number of threads working independently in a SPMD fashion • Number of threads specified at compile-time or run-time; available as program variable THREADS • Note: “thread” is the UPC terminology. UPC threads are most often implemented as a full OS processes • MYTHREAD specifies thread index (0...THREADS-1) • upc_barrier is a global synchronization: all wait before continuing • There is a form of parallel loop (later) • There are two compilation modes • Static threads mode: • THREADS is specified at compile time by the user • The program may use THREADS as a compile-time constant • Dynamic threads mode: • Compiled code may be run with varying numbers of thread
Hello World in UPC • Any legal C program is also a legal UPC program • SPMD Model • If you compile and run it as UPC with N threads, it will run N copies of the program (same model as MPI) #include <upc.h> /* needed for UPC extensions */ #include <stdio.h> main() { printf("Thread %d of %d: hello UPC world\n", MYTHREAD, THREADS); } Thread 0 of 4: hello UPC world Thread 1 of 4: hello UPC world Thread 3 of 4: hello UPC world Thread 2 of 4: hello UPC world
r =1 A Bigger Example in UPC: Estimate p • Estimate Pi by throwing darts at a unit square • Calculate percentage that fall in the unit circle • Area of square = r2 = 1 • Area of circle quadrant = ¼ p r2 = p/4 • Randomly throw darts at (x,y) positions • If x2 + y2 < 1, then point is inside circle • Compute ratio R: • R = # points inside / # points total • p ≈ 4 R
Pi in UPC, First version #include <stdio.h> #include <math.h> #include <upc.h> main(int argc, char *argv[]) { int i, hits, trials = 0; double pi; if (argc != 2) trials = 1000000; else trials = atoi(argv[1]); srand(MYTHREAD*17); for (i=0; i < trials; i++) hits += hit(); pi = 4.0*hits/trials; printf("PI estimated to %f.", pi); } Each thread gets its own copy of these variables Each thread can use the input arguments Initialize RNG in math library hit() : get random numbers and return 1 if inside circle This program computes N independent estimates of Pi (when run with N threads)
Pi in UPC, Shared Memory Style • Problem with this program: race condition!Reading/writing to hits is not synchronized shared variable to record hits shared int hits=0; main(int argc, char **argv) { int i, my_trials = 0; int trials = atoi(argv[1]); my_trials = (trials + THREADS-1)/THREADS; srand(MYTHREAD*17); for (i=0; i < my_trials; i++) hits += hit(); upc_barrier(); if (MYTHREAD == 0) { printf("PI estimated to %f.", 4.0*hits/trials); } } divide up work evenly accumulate hits There is a problem with this program…
Fixing the Race Condition • A possible fix for the race condition • Have a separate counter per thread (use a shared array) • One thread computes the total sum int hits=0; shared int all_hits[THREADS]; main(int argc, char **argv) { // declarations and initialization code omitted for (i=0; i < my_trials; i++) all_hits[MYTHREAD] += hit(); upc_barrier(); if (MYTHREAD == 0) { for (i=0; i < THREADS; i++) hits += all_hits[i]; printf("PI estimated to %f.", 4.0*hits/trials); } } Shared array: 1 element per thread Each thread accesses its local element, no race condition, no remote communication Thread 0 computes overall sum
Other UPC Features • Locks • upc_lock_t: pairwise synchronization between threads • Can also be used to fix race condition in previous example • Customizable layout of one and multi-dimensional arrays • Blocked, cyclic, block-cyclic; cyclic is the default • Split-phase barrier • upc_notify() and upc_wait() instead of upc_barrier() • Shared pointer and pointer to shared • Work-sharing (parallel loop)
v1 … 0 1 2 3 0 1 2 3 0 v2 … 0 1 2 3 0 1 2 3 0 sum … 0 1 2 3 0 1 2 3 0 Worksharing: Vector Addition Example • Each thread iterates over the indices that it “owns” • This is a common idiom called “owner computes” • UPC supports this idiom directly with a parallel version of the for loop: upc_forall #include <upc_relaxed.h> #define N 100*THREADS sharedint v1[N], v2[N], sum[N]; void main() { int i; for(i=0; i<N; i++) { if(MYTHREAD == i%THREADS) { sum[i]=v1[i]+v2[i]; } } } Default layout: cyclic (round robin) Access local elements only: sum[i] has affinity to thread i
UPC work-sharing with forall • upc_forall • init, test, loop: same as regular C for loop:defines loop start, increment, and end • affinity: defines which iterations a thread is responsible for • Syntactic sugar for loop on previous slide: • Loop over all • Work on those with affinity to this thread • Programmer guarantees that the iterations are independent • Undefined if there are dependencies across threads • Affinity expression: two options • Integer: affinity%THREADS is MYTHREAD • Pointer: upc_threadof(affinity) is MYTHREAD upc_forall(init; test; loop; affinity) statement;
Vector Addition with upc_forall • The vector addition example can be rewritten as follows • Equivalent code could use „&sum[i]“ for the affinity test • The code would be correct (but slow) if the affinity expression is i+1 rather than i. #define N 100*THREADS shared int v1[N], v2[N], sum[N]; void main() { int i; upc_forall(i=0; i<N; i++; i) sum[i]=v1[i]+v2[i]; } Affinity expression
UPC Summary • UPC is an extention to C, implementing the PGAS model • Available as a gcc version, Berkeley UPC, from some vendors • Today most often used for graph problems, irregular parallelism • PGAS is a concept realized in UPC and other languages • Co-array Fortran, Titanium • Chapel, X10, Fortress (HPCS languages) • Not covered • Collective operations (reductions, etc. similar to MPI) • Dynamic memory allocation in shared space • UPC shared pointers
DASH array Node Node e.g., STL vector, array DASH – Overview • DASH – PGAS in the form of a C++ Template library • Focus on data structures • Array a can be stored in the memory of several nodes • a[i] transparently refers to local memory or to remote memory via operator overloading dash::array<int> a(1000); a[23]=412; std::cout<<a[42]<<std::endl; • Not a new language to learn • Can be integrated with existing (MPI) applications • Support for hierarchical locality • Team hierarchies and locality iterators
Hierarchical Machines • Machines are getting increasingly hierarchical • Both within nodes and between nodes • Data locality is the most crucial factor for performance and energy efficiency Source: LRZ SuperMUC system description. Source: Bhatele et al.: Avoiding hot-spots in two-level direct networks. SC 2011. Source: Steve Keckler et al.: Echelon System Sketch Hierarchical locality not well supported by current approaches. PGAS languages usually only offer a two-level differentiation (local vs. remote).
Tools and Interfaces DASH Application DASH C++ Template Library DASH Runtime Hardware: Network, Processor, Memory, Storage One-sided Communication Substrate MPI GASnet ARMCI GASPI DASH – Overview and Project Partners • Project Partners • LMU Munich (K. Fürlinger) • HLRS Stuttgart (J. Gracia) • TU Dresden (A. Knüpfer) • KIT Karlsruhe (J. Tao) • CEODE Beijing (L. Wang, associated) Component of DASH Existing component/ Software
Tools and Interfaces DASH Application DASH C++ Template Library DART API DASH Runtime (DART) Hardware: Network, Processor, Memory, Storage One-sided Communication Substrate MPI GASnet ARMCI GASPI DART: The DASH Runtime Interface • The DART API • Plain-C based interface • Follows the SPMD execution model • Defines Units and Teams • Defines a global memory abstraction • Provides a global pointer • Defines one-sided access operations (puts and gets) • Provides collective and pair-wise synchronization mechanisms
Units and Teams • Unit: individual participants in a DASH/DART program • Unit ≈ process (MPI) ≈ thread (UPC) ≈ image (CAF) • Execution model follows the classical SPMD (single program multiple data) paradigm • Each unit has a global ID that remains unchanged during the execution • Team: • Ordered subset of units • Identified by an integer ID • DART_TEAM_ALL represents all units in a program • Units that are members of a team have a local ID with respect to that team
Communication • Communication: One-sided puts and gets • Blocking and non-blocking versions Performance of blocking puts and gets closely matches MPI performance
DASH (C++ Template Library) • 1D array as the basic data type • DASH follows a global-view approach, but local-view programming is supported too • Standard algorithms can be used but may not yield best performance • lbegin(), lend() allow iteration over local elements
Data Distribution Patterns • A Pattern controls the mapping of an index space onto units • A team can be specified (the default team is used otherwise) • No datatype is specified for a pattern • Patterns guarantee a similar mapping for different containers • Patterns can be used to specify parallel execution
Accessing and Working with Data in DASH (1) • GlobalPtr<T> abstraction that serves as the global iterator • GlobalRef<T> abstraction “reference to element in global memory” that is returned by subscript and iterator dereferencing
Accessing and Working with Data in DASH (2) • Range based for works on the global object per default • Proxy object can be used instead to access the local part of the data
Summary • Parallel programming is difficult • PGAS is an interesting middle ground between message passing and shared memory programming with threads • Inherits the advantages of both but also shares some of the disadvantages – specifically race conditions • PGAS • Today mostly used when working with applications with irregular parallelism - random data accesses • UPC is the most widely used PGAS approach today • Co-array Fortran and other new PGAS languages • DASH and other C++ libraries Thank you for your attention!
