Cell SDK and Best Practices

Transcription

1 Cell SDK and Best Practices Stefan Lutz Florian Braune Hardware-Software-Co-Design Universität Erlangen-Nürnberg 1

2 Overview - Introduction Background SIMD - Programming Overview Programming strategies - Programming tips Cell BE system practices PPE programming practices Memory subsystem programming practices SPE programming practices - Software Development Kit (SDK) 2

3 Overview - The Simulator - Basics - Interacting with the simulator - Developing code for the CBE - References 3

4 Introduction - Background - PPE - SPE - SIMD - two different prozessor types - special requirements to the programmer - SIMD 4

5 Programming Overview PPE s instruction set is extended PowerPC instruction set - extended by Vector/SIMD Multimedia instruction set SPE s instructions set similar to PPE s set Although SPE s and PPE use the same instruction set two compilers have to be used Byte ordering is big-endian 5

6 Programming Overview Programming Strategies Problem: Partitioning the workload among the processor elements - Programming models - SPE centric - PPE centric - Multistage Pipline Model - Parallel Stage Model - Services Model 6

7 PPE centric Models - Multistage model is typically avoided - difficult load balanacing - increased data movement - Parallel Stage - load is partitioned an processed by the SPEs 7

8 PPE centric models - Services Model - each SPE is assigned a different service - fixed static allocation should be avoided 8

9 Programming Tips 9

10 Programming Tips Cell BE programming practices - Offload as much work onto the SPEs as possible - Choose an efficient workload partition strategy - Example: image processing height = CEIL(image_height / n); first_line = height * spe_num; last_line = MIN(image_height, first_line+height) 1; - Acommodate potential data type differences - PPE ILP32 or LP64 - SPE ILP32 always 10

11 Programming Tips PPE Programming Practices - Self-manage cache - pre-load the caches so that data is available when needed - VMX data stream instructures should not be used for pre_loading - self manage cache of SPEs 11

12 Memory subsystem programming - Initiate DMA transfers from the SPE - let the SPE pull the date instead of pushing it throught the PPE - the SPE command queue is twice as deep as the proxy command queue - Pulling uses less instructions than pushing the data - Allocate large data sets from large pages - three concurrent page sizes supported - 4kB and two of 64kb, 1Mb or 64Mb 12

13 SPE programming practices - Avoid external scalars - scalar loads are rotated into preferred vector element - Read, insert scalar, write (operation overhead) - convert scalars manually into quad-words - cluster scalars into groups and load them at one time using a quad-word memory access - Exploit SIMD - use of assembly für performance critical sections - daunting task for large, complicated code - compromise solution: use of intrinsics - essentialy inline assembler with C function call syntax 13

14 SPE programming practices - Unroll and pipeline loops - loops are foundation of nearly every programm - unroll loops as far as possible or even remove them if the number of iterations is constant 14

15 SPE programming practices - Eliminate or reduce branches - branches relatively expensive when misspredicted - exploit the select bits instruction Regular code: if (a > b) c+=1; else d = a+b; Branchless code: select = spu_cmpgt(a, b); c_plus_1 = spu_add(c, 1); a_plus_b = spu_add(a, b); c = spu_sel(c, c_plus_1, select); d = spu_sel(a_plus_b, d, select); 15

16 SPE programming practices - sometimes a branch can not be eliminated - try to reduce the number of misspredicts by using the builtin_expect language extension if( builtin_expect((a > b), 0)) c += 1; else d = a + b; 16

17 SPE programming practices - avoid integer multiplies - SPE contains only a 16x16bit multiplier - 32bit multiplies take five instructions - three 16bit multipilies and two addidtions - if operands are less than 16bit cast them to unsigned shorts prior to multiplication - make sure to cast constants (they are imlpicit int) - consider using a macro to perform 32bit operations 17

18 SPE programming practices - last tip - due to limited space of SPE local store all optimazations might not be possible for a given application. - utilize only these tips wich give the best performance results 18

19 Software Development Kit 19

20 Software Development Kit - The kit includes: - systemsim (simulator) - system root image for use in systemsim - GNU tools - C, C++ Compilers, linkers, assemblers, binary utils for PPE and SPE - IBM xlc (C,C++ compiler) - IBM xlf (Fortran compiler) - newlib for the SPU - standard library for use on embedded systems - gdb for PPE and SPE - PPC64 Linux with CBE enhancements - SPE Runtime Managment Library - Libraries to assist in the development and the execution of parallel applications - Performance tools: - oprofile, CellPerfCount, FDRPR-Pro, 20

21 Software Development Kit - An eclipse based IDE - Standardized SIMD math libraries - Mathematical Acceleration Subsystem (MASS) libraries - Cell optimized domain_specific application libraries - Basic Linear Algebra Subprograms (BLAS) library - Fast Fourier Transfor (FFT) library - Monte Carlo Random Number Generator library - Example libraries, example codes, demos and benchmarks 21

22 The Simulator - the simulator supports full functional simulation of: - PPE - PPE caches - bus - SPEs - MCFs - memory controller 22

23 The Simulator - Operating Modes: - Linux mode: - the simulator boots the Linux OS one the simulated system - Standalone Mode: - the simulator is configured and started without any OS running only the user-mode application - Interacting with the simulator - issuing commands to the simulated system - console window of simulated OS - issuing commands to the simulator - simulator command window 23

24 The Simulator - Simulator windows and layers 24

25 The Simulator - command line interface: - start with: PATH=/opt/ibm/systemsim-cell/bin:$PATH; systemsim - displays the prompt: systemsim % - to interact use the commands for the simulator: Simulator Command quit help mysim go mysim spu n set model mode mysim spu n display statistics Meaning Closes the Application and exits the simulator Displays a list of available commands Starts or continues the simulation Sets SPEn into model mode, where n is a value from 0 to 7 and mode is piplined or instruction Displays to the simulator command window the performance analysis collected, where n is a value from 0 to 7 25

26 The Simulator - Graphical user inteface 26

27 The Simulator - SPE modes - instruction mode - pipeline mode - fast mode - Statistics window - only in pipeline mode 27

28 Developing code for the CBE - Pretty good programming example in the Programming tutorial pages Explanation of every necessary step to: - Create working directories - Compile the sources - Load and start the simulator - Use and run the simulator - Debugging the code 28

29 References CBE Programming Tutorial v tips to optimal application performance - Cell Programming Tips & Techniques

30 END Questions?? Thanks for your attention 30