Parallelizing nvcc

Edit (2021-04-29): According to the Stack Overflow:

nvcc from CUDA 11.3 finally supports this out of the box via the -t flag.

CUDA 11.3 was released on 2021-04-15.

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This post talks about fast_nvcc.py, which Nikita Shulga, Rong Rong, and I brought to the PyTorch repo in December, reducing wall-clock build time for builds that use nvcc to compile large CUDA files for multiple architectures at once. I first give the motivation and a broad overview of the approach, and then explore several technical details of the development process.

Overview

As documented by NVIDIA, nvcc is a "compiler driver" which manages the "several splitting, compilation, preprocessing, and merging steps for each CUDA source file." It parses CLI arguments, creates a plan for which tools to invoke to do its job (and the CLI arguments to pass to those tools), and then executes that plan.

The key to our story here is that we can pass nvcc the --dryrun argument to make it just tell us the plan, rather than actually executing it. For instance, let's say we want to compile this simple CUDA file:

#include <stdio.h>

__global__ void kernel() {
  printf("Hello World of CUDA\n");
}

int main() {
  kernel<<<1,1>>>();
  return cudaDeviceSynchronize();
}

We can run the following to make nvcc tell us what it would do to compile hello.cu for architectures 30, 50, 60, and 70 (and produce an executable called hello):

$ NVCC_ARGS='hello.cu -gencode arch=compute_30,code=sm_30 -gencode arch=compute_50,code=sm_50 -gencode arch=compute_60,code=sm_60 -gencode arch=compute_70,code=sm_70 -o hello'
$ nvcc --dryrun $NVCC_ARGS

This command runs almost instantly, produces no files, and prints to stderr something similar to this, listing all the commands nvcc would run and the environment variables it would set while running them.

So at this point we can reproduce the behavior of nvcc by running it with --dryrun, parsing the output, setting those environment variables, and running those commands. If we do that, we find that a significant portion of the total compilation time (see the "Flags" section below for numbers) is taken by the gcc, cicc, and ptxas commands, of which there is one of each, for each architecture (plus three extra gcc commands to do other things). These architecture-specific steps are essentially independent of each other, but nvcc runs them all sequentially, missing out on concurrency possibilities.

That's where fast_nvcc.py comes in: it serves as a (mostly) transparent drop-in replacement for nvcc, which works by taking the output from nvcc --dryrun and running commands concurrently whenever possible (by generating a dataflow dependency graph from the /tmp/tmpxft* filenames in the commands). If you have it on your PATH, you should be able to replace nvcc ... with fast_nvcc.py -- ... anywhere (with some exceptions; see the "Limitations" section below).

Implementation

This section discusses several (hopefully interesting?) details about the fast_nvcc.py implementation and development process.

Flags

Going along with the theme of working as a standalone script, fast_nvcc.py provides several CLI flags that I found invaluable while developing this diff. (If you have it on your PATH, you can see them by running fast_nvcc.py --help.) I left these in the final product in case someone finds them useful in the future.

$ mkdir files
$ fast_nvcc.py --graph=graph.dot --save=files --table=table.csv --verbose=verbose.txt -- $NVCC_ARGS
$ dot -Tpng graph.dot -o graph.png

This populates files and generates graph.dot, table.csv, and verbose.txt. And if you have dot installed, that last command generates graph.png which looks like this:

A few notes:

• The --verbose flag differs from nvcc's built-in --dryrun and --verbose flags because it shows the (modified) commands that will be run by fast_nvcc.py (rather than the originals from nvcc), and because it tries to make the commands more readable by numbering them and spreading each one across multiple lines using Python's shlex.split function. It's not very friendly for machine consumption, but it's useful for humans, and provides context for the other flags since the numbers are the same as those shown in the output of --graph, for instance.
• The arrows in the --graph point from dependencies to the commands that depend on them, so essentially, the graph shown in the picture above gets run from top to bottom.
• The --table flag was added before --save, so all its columns past the first two are a bit redundant (they just give the sizes of files, while --save gives the files themselves). Its second column was useful for finding what to parallelize and what to ignore: see the "Leftovers" section below.

Here's an Excel chart generated from the first two columns of the table.csv file. This shows us that we are indeed parallelizing the most time-consuming parts of the compilation:

Tweaks

While the high-level description in the "Approach" section above is mostly accurate, there were a few tweaks that we needed to make to get it to actually work. To clarify, the general rule is that if two commands refer to the same /tmp/tmpxft* filename, then we make the latter wait for the former before running. We had a bug at first because some of the filenames leave off the /tmp at the beginning (i.e. they're just relative paths starting with tmpxft) but it mostly works.

The first exception we found was for *.fatbin.c files. As mentioned in a comment in the fast_nvcc.py source code, the cicc commands actually refer to such files (which, incidentally, are the ones that don't start with /tmp) before the fatbinary command that generates them. This is because the cicc command simply generates a file that references (via #include) the *.fatbin.c file which will be created later, and then when the file generated by cicc is processed later, that is when the *.fatbin.c file needs to exist. So in this case, the rule is: if commands A and B (of which B is a fatbinary command) both reference the same *.fatbin.c file, and A also references a (different) file that is later referenced by command C, then C depends on B.

Another exception was for the *.module_id files used by cicc and cudafe++ commands. This one led me down a very deep rabbit hole. The issue here is that several commands (including one for each architecture) reference the same *.module_id file, with the first cicc command being passed the --gen_module_id_file flag to generate the file that is then used (without modifying) by the other commands.

• Running them in all parallel (even after waiting for the first to finish) doesn't immediately work, because even though none of them modify the file, for some reason they conflict when they access it at the same time.
• An easy "solution" would be to just run the first command, copy the one file into a bunch of different files (so they have different names) and then run each of the other commands with its own unique *.module_id filename, but that's suboptimal because it means that you have to run the first two cicc commands (which, remember, are some of the most expensive in nvcc runs) serially.
• At first we tried just giving each command its own unique *.module_id filename and the --gen_module_id_file flag, so they would each generate their own file and work completely independently. This actually works in most cases! All the *.module_id files get slightly different contents this way, since they're all generated by different commands, but usually that doesn't matter. However, it fails for aten/src/ATen/native/cuda/SoftMax.cu, which does a special thing that most CUDA files (at least in PyTorch) don't do.

So in the end we just looked at the contents of some *.module_id files (they're not very large), took a guess at the function used to generate them, and replaced the usage of the --gen_module_id_file flag with a bunch of echo commands that each create a *.module_id file with contents similar to what would have been created by the first cicc command. We know that the last 8 characters of the file contents are definitely wrong: they appear to probably be a hash of some sort (since they don't (usually) change when compiling the same file multiple times), but it's unclear what hash function is being used or what exactly is being fed to that hash function. So we just take an MD5 checksum (although it would almost certainly be better to use a cheaper hash function) of the preceding part of the *.module_id contents, stick it on the end, and call it a day.

(Fun detour: you can disable these *.module_id shenanigans via the --faithful flag, and by combining that with the --graph flag, you can see the less-parallel execution graph that results!)

$ fast_nvcc.py --faithful --graph=faithful.dot -- $NVCC_ARGS
$ dot -Tpng faithful.dot -o faithful.png

Leftovers

As is plainly visible from the above graphs, this diff doesn't parallelize all the per-architecture commands run by nvcc. Specifically, all the nvlink steps still run serially. But as the --table data show, this doesn't matter, since nvlink is basically the fastest part of the whole compilation. Interestingly, though, architecture 72 is an exception to this rule:

$ fast_nvcc.py --table=/dev/stdout -- hello.cu -gencode arch=compute_70,code=sm_70 -gencode arch=compute_72,code=sm_72 -o hello | grep nvlink | cut -d, -f1-2
14 nvlink,0.007828212808817625
15 nvlink,0.7020692219957709

As suggested above, nvlink for architecture 72 is two orders of magnitude slower than for any of the other architectures, and by far the slowest step for any compilation that includes it. Currently it seems to be the only such anomalously slow architecture, but if more are introduced in the future, the sequentiality of the nvlink steps in fast_nvcc.py may become important. It's unclear whether it's possible to parallelize those steps, though.

Limitations

Since fast_nvcc.py depends on being able to infer dataflow dependencies using /tmp/tmpxft* filenames, it assumes the /tmp prefix. According to the nvcc docs, there are a few ways that this assumption could be violated:

• if TMPDIR is set, its value is used instead
• if the OS is Windows, C:\Windows\temp (or the value of TEMP) is used instead
• if --objdir-as-tempdir is passed, the directory of the object file is used instead
• if --keep or --save-temps is passed, the current directory is used instead
• if --keep-dir is passed, its value is used instead

This limitation could be removed, but we haven't need to address it yet, so instead, fast_nvcc.py simply warns the user (linking to the nvcc docs) if any of the above conditions occur. As an aside, it doesn't appear immediately obvious what the precedence of these rules would be; perhaps fast_nvcc.py should still warn if multiple of them occur simultaneously.

Recap

We looked at fast_nvcc.py---which acts as a faster (wall time) replacement for nvcc when used to compile CUDA files for multiple architectures at once---and explored several of its implementation details. Thanks for reading! :)