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GTC2018 - Instructor-Led Lab: Best GPU Code Practices Combining OpenACC, CUDA, and OmpSs

This lab is shold to be executed inside nvidia-docker with hopobcn/gtc2018-fwi:us image

Lab Instructions

In this lab you will make modifications to a real world oil&gas mini application called FWI. You will compile and execute the newly enhanced code in each step. Along the way, solutions are provided, so you can check your work, or take a peek if you get lost.

After login into the machine you should see a folder called FWI containing each step and solution as a different branch:

~# git branch
* gtc2018-step1
  gtc2018-step1-sol
  gtc2018-step3
  gtc2018-step3-sol
  gtc2018-step4
  gtc2018-step4-sol
  gtc2018-step5
  gtc2018-step5-sol
  ompss-openacc

Use git diff <branch-name> to compare branches and git stash && git checkout <branch-name> to discard non-commited changes and change to another step (branch).

Execution setup info (examples):

Dual socket Intel(R) Xeon(R) E5-2630 v3 (Haswell) @ 2.40 GHz with 8 cores (16 core total, HT off) NVIDIA Tesla K80

To save time, this step has been moved to Appendix 0

To compare our GPU executions we will use serial & OpenMP executions:

  • Sequential execution with 1 core:
Number of frequencies 1
Number of shots 1
Number of gradient iterations 1
Number of test iterations 1
Output directory path: results
FWI Program finished in 1059.629833 seconds
  • OpenMP execution with 16 threads (1 per core)
Number of frequencies 1
Number of shots 1
Number of gradient iterations 1
Number of test iterations 1
Output directory path: results
FWI Program finished in 120.587904 seconds

Step 1: Express parallelism with OpenACC directives

In the initial step, you have to express parallelism available around expensive loops in the application. We assume you already have some OpenACC knowledge.

Detailed instructions

Simplified instructions:

  1. git checkout gtc2018-step1

  2. Add #pragma acc kernels pragmas to compute_component_vcell_TL:166 and compute_component_scell_TR:624 functions (from src/fwi_propagator.c file)

  3. Compile fwi:

    mkdir build && cd build
cmake -DCMAKE_C_COMPILER=pgcc -DUSE_OPENACC=ON ..

  4. Inspect the compiler output. Try to identify if the kernels were successfully generated or if you found messages like: Loop carried dependence due to exposed use of [array-name] prevents parallelization.

  5. Fix the previous issue providing #pragma acc loop independent to each loop-nest

  6. Compile and run fwi:

    $ make irun

Step 2: Profile the application

Step 2 will profile the application to find possible weaknesses and optimization opportunities. We could use NVIDIA Visual Profiler for a graphical assestment or pgprof/nvprof for a command-line visualization. For simplicity in this lab we are going to use nvprof

Detailed instructions

Simplified instructions:

  1. git checkout gtc2018-step2

  2. Profile fwi with:

    nvprof --dependency-analysis bin/fwi fwi_schedule.txt

    or

    NVVP

  3. Observe if:

    1. Which kernels take the most time in the critical path
    2. Is memory traffic due CUDA Unified Memory an issue?

Step 3: Express data movement: from CUDA Unified memory to OpenACC data directives

Unified Memory (UM) can be very inefficient in older GPU generations and an experienced programmer with detailed knowledge of the application algorithm could outperform the Unified Memory.

As we have seen in Step 2, FWI doesn't specially suffer from too much UM traffic. But as an example, in this step we'll migrate from UM to a movement of data using OpenACC directives.

Detailed instructions

Simplified instructions:

  1. git checkout gtc2018-step3

  2. Annotate, using OpenACC directives, which arrays will be used on the GPU: In alloc_memory_shot function (src/fwi_kernel.c), after all allocations, complete the OpenACC section:

    coeff_t cc = *c;
#pragma acc enter data create(cc)
#pragma acc enter data create(cc.c11[:cellsInVolume])
#pragma acc enter data create(cc.c12[:cellsInVolume])
#pragma acc enter data create(/* COMPLETEME */)
#pragma acc enter data create(/* COMPLETEME */)
... // continue with c13,c14,c15,c16, c22,c23,c24,c25,c26, c33,c34,c35,c36, c44,c45,c46, c55,c56, c66

    In free_memory_shot function, before all dealocations (free) we should first, deallocate the GPU memory with:

    #pragma acc wait

#pragma acc exit data delete(c->c11)
#pragma acc exit data delete(c->c12)
#pragma acc exit data delete(/* COMPLETEME */)
#pragma acc exit data delete(/* COMPLETEME */)
...
#pragma acc exit data delete(c)
...

  3. In compute_component_vcell_TL:166 and compute_component_scell_TR:625 functions. Specify that all the GPU arrays are already on the gpu with the present clause

  4. Compile and run fwi:

    $ make irun

Step 4: Optimize kernel scheduling with multiple streams

OpenACC follows the same semantics as CUDA regarding streams. By default, kernels and memory copies are executed in the default stream which imposes serialization between kernels and memory transfers.

Since we know that scell don't have dependences between each other (same for vcell functions) we could launch all the scell/vcell kernels to different streams.

Detailed instructions

Simplified instructions:

OpenACC uses the clause async that requires a nonnegative scalar integer as a parameter. In include/fwi/fwi_propagator.h::78 we have already prepared an enum that we are going to use as identifiers:

typedef enum {TR, TL, BR, BL} phase_t;

  1. git checkout gtc2018-step4

  2. Add the async clause to all OpenACC kernels (src/fwi_propagator.c)

    All scell & vcell functions provide a parameter named phase which can be provided to the OpenACC clause: async(phase)

  3. In velocity_propagator and stress_propagator

    Pass as the last paramter, the stream identifier from phase_t enum (TR,TL,BR or BL).

  4. Compile and run fwi:

    $ make irun

Step 5: Add glue code to call CUDA Kernels

FWI is a great candidate to take advantage of shuffle intrinsics and shared memory. OpenACC provides the directive cache to exploit shared memory, but it lacks a way to exploit CUDA intra-warp intrinsics.

In this step we provide a set of highly optimized kernels in src/fwi_propagator.cu file. Your task consists on adding the necessary glue code to get:

  • The GPU device pointers managed by the OpenACC runtime
  • The CUDA stream allocated by OpenACC

Detailed instructions

Simplified instructions:

  1. git checkout gtc2018-step5

  2. Add #pragma acc host_data use_device directives to forward the device pointers allocated by OpenACC to our CUDA kernels

  3. Pass the current stream to the CUDA kernel (with acc_get_cuda_stream)

  4. Compile and run fwi:

    $ make irun

FWI OmpSs/OpenACC

OmpSs/OpenACC - Detailed instructions

Appendix

Characterize FWI application

Detailed instructions