1. ECE669 L23: Parallel Compilation April 29, 2004 ECE 669 Parallel Computer Architecture Lecture 23 Parallel Compilation

2. ECE669 L23: Parallel Compilation April 29, 2004 Parallel Compilation °Two approaches to compilation Parallelize a program manually Sequential code converted to parallel code °Develop a parallel compiler Intermediate form Partitioning -Block based or loop based Placement Routing

3. ECE669 L23: Parallel Compilation April 29, 2004 Compilation technologies for parallel machines Assumptions: Input:Parallel program Output:“Coarse” parallel program & directives for: Which threads run in 1 task Where tasks are placed Where data is placed Which data elements go in each data chunk Limitation: No special optimizations for synchronization -- synchro mem refs treated as any other comm.

4. ECE669 L23: Parallel Compilation April 29, 2004 Toy example °Loop parallelization

5. ECE669 L23: Parallel Compilation April 29, 2004 Example °Matrix multiply °Typically, °Looking to find parallelism...

6. ECE669 L23: Parallel Compilation April 29, 2004 Choosing a program representation... °Dataflow graph No notion of storage problem Data values flow along arcs Nodes represent operations + a0a0 b0b0 c0c0 + a2a2 b2b2 c2c2 + a5a5 b5b5 c5c5

7. ECE669 L23: Parallel Compilation April 29, 2004 Compiler representation °For certain kinds of structured programs °Unstructured programs A Data X Task A Task B LOOP LOOP nest Data array Index expressions Communication weight Array

8. ECE669 L23: Parallel Compilation April 29, 2004 Process reference graph Nodes represent threads (processes) computation Edges represent communication (memory references) Can attach weights on edges to represent volume of communication Extension: precedence relations edges can be added too Can also try to represent multiple loop produced threads as one node P0P0 P1P1 P2P2 P5P5 Memory Monolithic memory or21......Not very useful

9. ECE669 L23: Parallel Compilation April 29, 2004 Allocate data items to nodes as well Nodes: Threads, data objects Edges: Communication Key: Works for both shared-memory, object-oriented, and dataflow systems! (Msg. passing) Process communication graph P0P0 A0A0 B0B0 C0C0 d1d1 d0d0 1 1 1 P1P1 A 01 A2A2 C1C1 d3d3 d2d2 1 1 1 P2P2 B0B0 d5d5 d4d4 P5P5

10. ECE669 L23: Parallel Compilation April 29, 2004 PCG for Jacobi relaxation Fine PCG Coarse PCG 4 4 4 4 4 : Computation : Data

11. ECE669 L23: Parallel Compilation April 29, 2004 Compilation with PCGs Fine process communication graph Partitioning Coarse process communication graph

12. ECE669 L23: Parallel Compilation April 29, 2004 Compilation with PCGs Fine process communication graph Coarse process communication graph Placement... other phases, scheduling. Dynamic? MP: Partitioning Coarse process communication graph

13. ECE669 L23: Parallel Compilation April 29, 2004 Parallel Compilation °Consider loop partitioning °Create small local compilation °Consider static routing between tiles °Short neighbor-to-neighbor communication °Compiler orchestrated

14. ECE669 L23: Parallel Compilation April 29, 2004 Flow Compilation °Modulo unrolling °Partitioning °Scheduling

15. ECE669 L23: Parallel Compilation April 29, 2004 Modulo Unrolling – Smart Memory °Loop unrolling relies on dependencies °Allow maximum parallelism °Minimize communication

16. ECE669 L23: Parallel Compilation April 29, 2004 Array Partitioning – Smart Memory °Assign each line to separate memory °Consider exchange of data °Approach is scalable

17. ECE669 L23: Parallel Compilation April 29, 2004 Communication Scheduling – Smart Memory °Determine where data should be sent °Determine when data should be sent

18. ECE669 L23: Parallel Compilation April 29, 2004 Speedup for Jacobi – Smart Memory °Virtual wires indicates scheduled paths °Hard wires are dedicated paths °Hard wires require more wiring resources °RAW is a parallel processor from MIT

19. ECE669 L23: Parallel Compilation April 29, 2004 Partitioning Use heuristic for unstructured programs For structured programs......start from: Arrays List of arrays A B C L0 L1 L2 Loop Nests List of loops

20. ECE669 L23: Parallel Compilation April 29, 2004 Notion of Iteration space, data space E.g. A Matrix j i Data space Iteration space Represents a “thread” with a given value of i,j

21. ECE669 L23: Parallel Compilation April 29, 2004 °Partitioning: How to “tile” iteration for MIMD M/Cs data spaces? Notion of Iteration space, data space E.g. A Matrix j i Data space Iteration space Represents a “thread” with a given value of i,j This thread affects the above computation +

22. ECE669 L23: Parallel Compilation April 29, 2004 Loop partitioning for caches °Machine model Assume all data is in memory Minimize first-time cache fetches Ignore secondary effects such as invalidations due to writes Memory Network Cache PPP A

23. ECE669 L23: Parallel Compilation April 29, 2004 Summary °Parallel compilation often targets block based and loop based parallelism °Compilation steps address identification of parallelism and representations Graphs often useful to represent program dependencies °For static scheduling both computation and communication can be represented °Data positioning is an important for computation