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Project 2 Stream Compaction Instructions

This is due Tuesday, September 27 2016 at midnight.

Summary: In this project, you'll implement GPU stream compaction in CUDA, from scratch. This algorithm is widely used, and will be important for accelerating your path tracer project.

Your stream compaction implementations in this project will simply remove 0s from an array of ints. In the path tracer, you will remove terminated paths from an array of rays.

In addition to being useful for your path tracer, this project is meant to reorient your algorithmic thinking to the way of the GPU. On GPUs, many algorithms can benefit from massive parallelism and, in particular, data parallelism: executing the same code many times simultaneously with different data.

You'll implement a few different versions of the Scan (Prefix Sum) algorithm. First, you'll implement a CPU version of the algorithm to reinforce your understanding. Then, you'll write a few GPU implementations: "naive" and "work-efficient." Finally, you'll use some of these to implement GPU stream compaction.

Algorithm overview & details: There are two primary references for details on the implementation of scan and stream compaction.

Your GPU stream compaction implementation will live inside of the stream_compaction subproject. This way, you will be able to easily copy it over for use in your GPU path tracer.

Part 0: The Usual

This project (and all other CUDA projects in this course) requires an NVIDIA graphics card with CUDA capability. Any card with Compute Capability 2.0 (sm_20) or greater will work. Check your GPU on this compatibility table. If you do not have a personal machine with these specs, you may use those computers in SIG Lab which have supported GPUs.

Useful existing code

  • stream_compaction/common.h
    • checkCUDAError macro: checks for CUDA errors and exits if there were any.
    • ilog2ceil(x): computes the ceiling of log2(x), as an integer.
  • main.cpp
    • Some testing code for your implementations.

Note 1: The tests will simply compare against your CPU implementation Do it first!

Note 2: The tests default to an array of size 256. Test with something larger (10,000? 1,000,000?), too!

Part 1: CPU Scan & Stream Compaction

This stream compaction method will remove 0s from an array of ints.

Do this first, and double check the output! It will be used as the expected value for the other tests.

In stream_compaction/cpu.cu, implement:

  • StreamCompaction::CPU::scan: compute an exclusive prefix sum.
  • StreamCompaction::CPU::compactWithoutScan: stream compaction without using the scan function.
  • StreamCompaction::CPU::compactWithScan: stream compaction using the scan function. Map the input array to an array of 0s and 1s, scan it, and use scatter to produce the output. You will need a CPU scatter implementation for this (see slides or GPU Gems chapter for an explanation).

These implementations should only be a few lines long.

Part 2: Naive GPU Scan Algorithm

In stream_compaction/naive.cu, implement StreamCompaction::Naive::scan

This uses the "Naive" algorithm from GPU Gems 3, Section 39.2.1. We haven't yet taught shared memory, and you shouldn't use it yet. Example 39-1 uses shared memory, but is limited to operating on very small arrays! Instead, write this using global memory only. As a result of this, you will have to do ilog2ceil(n) separate kernel invocations.

Since your individual GPU threads are not guaranteed to run simultaneously, you can't generally operate on an array in-place on the GPU; it will cause race conditions. Instead, create two device arrays. Swap them at each iteration: read from A and write to B, read from B and write to A, and so on.

Beware of errors in Example 39-1 in the chapter; both the pseudocode and the CUDA code in the online version of Chapter 39 are known to have a few small errors (in superscripting, missing braces, bad indentation, etc.)

Be sure to test non-power-of-two-sized arrays.

Part 3: Work-Efficient GPU Scan & Stream Compaction

3.1. Scan

In stream_compaction/efficient.cu, implement StreamCompaction::Efficient::scan

Most of the text in Part 2 applies.

  • This uses the "Work-Efficient" algorithm from GPU Gems 3, Section 39.2.2.
  • This can be done in place - it doesn't suffer from the race conditions of the naive method, since there won't be a case where one thread writes to and another thread reads from the same location in the array.
  • Beware of errors in Example 39-2.
  • Test non-power-of-two-sized arrays.

Since the work-efficient scan operates on a binary tree structure, it works best with arrays with power-of-two length. Make sure your implementation works on non-power-of-two sized arrays (see ilog2ceil). This requires extra memory

  • your intermediate array sizes will need to be rounded to the next power of two.

3.2. Stream Compaction

This stream compaction method will remove 0s from an array of ints.

In stream_compaction/efficient.cu, implement StreamCompaction::Efficient::compact

For compaction, you will also need to implement the scatter algorithm presented in the slides and the GPU Gems chapter.

In stream_compaction/common.cu, implement these for use in compact:

  • StreamCompaction::Common::kernMapToBoolean
  • StreamCompaction::Common::kernScatter

Part 4: Using Thrust's Implementation

In stream_compaction/thrust.cu, implement:

  • StreamCompaction::Thrust::scan

This should be a very short function which wraps a call to the Thrust library function thrust::exclusive_scan(first, last, result).

To measure timing, be sure to exclude memory operations by passing exclusive_scan a thrust::device_vector (which is already allocated on the GPU). You can create a thrust::device_vector by creating a thrust::host_vector from the given pointer, then casting it.

Part 5: Radix Sort (Extra Credit) (+10)

Add an additional module to the stream_compaction subproject. Implement radix sort using one of your scan implementations. Add tests to check its correctness.

Write-up

  1. Update all of the TODOs at the top of your README.md.
  2. Add a description of this project including a list of its features.
  3. Add your performance analysis (see below).

All extra credit features must be documented in your README.md, explaining its value (with performance comparison, if applicable!) and showing an example how it works. For radix sort, show how it is called and an example of its output.

Always profile with Release mode builds and run without debugging.

Questions

  • Roughly optimize the block sizes of each of your implementations for minimal run time on your GPU.

    • (You shouldn't compare unoptimized implementations to each other!)

  • Compare all of these GPU Scan implementations (Naive, Work-Efficient, and Thrust) to the serial CPU version of Scan. Plot a graph of the comparison (with array size on the independent axis).

    • You should use CUDA events for timing GPU code. Be sure not to include any initial/final memory operations (cudaMalloc, cudaMemcpy) in your performance measurements, for comparability. Note that CUDA events cannot time CPU code.
    • You can use the C++11 std::chrono API for timing CPU code. See this Stack Overflow answer for an example. Note that std::chrono may not provide high-precision timing. If it does not, you can either use it to time many iterations, or use another method.
    • To guess at what might be happening inside the Thrust implementation (e.g. allocation, memory copy), take a look at the Nsight timeline for its execution. Your analysis here doesn't have to be detailed, since you aren't even looking at the code for the implementation.

  • Write a brief explanation of the phenomena you see here.

    • Can you find the performance bottlenecks? Is it memory I/O? Computation? Is it different for each implementation?

  • Paste the output of the test program into a triple-backtick block in your README.

    • If you add your own tests (e.g. for radix sort or to test additional corner cases), be sure to mention it explicitly.

These questions should help guide you in performance analysis on future assignments, as well.

Submit

If you have modified any of the CMakeLists.txt files at all (aside from the list of SOURCE_FILES), mentions it explicity. Beware of any build issues discussed on the Google Group.

Open a GitHub pull request so that we can see that you have finished. The title should be "Project 2: YOUR NAME". The template of the comment section of your pull request is attached below, you can do some copy and paste:

  • Repo Link
  • Your PENNKEY
  • (Briefly) Mentions features that you've completed. Especially those bells and whistles you want to highlight
    • Feature 0
    • Feature 1
    • ...
  • Feedback on the project itself, if any.

GPU Gem 3 Ch 39 Patch

  • Example 1

  • Example 2

  • Figure-39-4

  • Figure-39-2. This image shows an inclusive scan while we should implement an exclusive scan here.