Cheap Parallelisation

Table of Contents


I was recently creating a snakemake pipeline and needed to write a rule/process that would perform a multiple sequence alignment (MSA) on 2,582 fasta files. Usually, it is easy to parallelise this kind of task using snakemake. To cut a long story short; using snakemake to parallelise across the files was not feasible. I knew there were ways of doing this kind of thing with tools such as parallel, xargs, and find, but I had never really invested the time to get comfortable with them. This post is an attempt to document that process using one of my favourite CLI tools: fd. We’ll see how fd can be used to execute multiple MSAs (with MAFFT) simultaneously, and benchmark how much faster it is than a conventional “synchronous” approach.

What is fd?

Quoting from its GitHub repository:

fd is a simple, fast and user-friendly alternative to find.

and I would certainly agree with that. For the sake of brevity, I won’t dive into the full range of what fd does, but please take a minute to have a quick look at the README before reading on any further.

For the purposes of this post, the functionality of fd we are most interested in is -x, --exec and -j, --threads.

    -x, --exec <cmd>
            Execute a command for each search result.
            All arguments following --exec are taken to be arguments to the command until the argument ';' is encountered.
            Each occurrence of the following placeholders is substituted by a path derived from the current search result before the command is
              '{}':   path
              '{/}':  basename
              '{//}': parent directory
              '{.}':  path without file extension
              '{/.}': basename without file extension
    -j, --threads <num>
            Set number of threads to use for searching & executing (default: number of available CPU cores)

Using --exec and --threads together will allow us to run a given command on however many threads we like.

Baby steps: finding our files

The first thing we need to do before executing a command on multiple threads is to find the files we want to operate on. The toy dataset I’ll be using is eight small fasta files. If you’re going to follow along at home, you can download the data from here.

The directory tree we are working with is

├── fd_example
│  ├── f1.fa
│  ├── f2.fa
│  ├── f3.fa
│  ├── f4.fa
│  ├── f5.fa
│  ├── f6.fa
│  ├── f7.fa
│  └── f8.fa
└── msas

To find the fasta files with fd, we can use the following command

$ fd --extension fa . fd_example

--extension tells fd we want to filter out search results by the given extension - fa in this case. (I’ll use the abbreviated -e for this in subsequent commands). Next, we tell fd the pattern we are looking for is . - i.e. anything. Lastly, we want to search (recursively) under the fd_example directory.

An alternate solution for the above would be skipping the use of --extension and using the pattern on its own

$ fd '\.fa$' fd_example

The pattern is a regular expression this time that says “anything ending in .fa”.

Personally, I like to make commands like this as easy for others, and myself in the future, to understand. So I’ll stick with the --extension method.

Constructing execution commands

Now that we have a list of file paths, we need to do something with them. From earlier, we can see that --exec takes a command, <cmd>. The fd docs have examples of different conventions for writing these commands. This command will be executed for each file path that fd finds.

Simple: counting characters in files

We’ll start with something easy like counting characters in each file using wc. wc is a command-line utility that counts lines, words, and bytes (characters) in files.

$ fd -e fa --exec wc -c '{}' \; . fd_example
3689 fd_example/f5.fa
2155 fd_example/f8.fa
7324 fd_example/f1.fa
2503 fd_example/f2.fa
1433 fd_example/f7.fa
5701 fd_example/f3.fa
1766 fd_example/f6.fa
2530 fd_example/f4.fa

Let’s break down the juicy bit - --exec wc -c '{}' \;. We tell fd to execute the command wc -c on {}. If we refresh our memories on the --exec docs, {} refers to the path returned by fd (note: we add quotes around {} to prevent more complex paths failing). The \; bit at the end just tells fd where the execution command ends. Anything else after this is for fd, or the shell, to deal with.

An even more concise version of this command would be

$ fd -e fa -x wc -c \; . fd_example

as fd appends the path {} to the end of the command if we don’t add it ourselves.

Ok, so that was pretty easy. Let’s look at something a little more complicated.

Intermediate: changing file extensions

As a (somewhat contrived) second example, we’ll use the same approach to change the file extensions on all our fasta files from .fa to .fasta.

$ fd -e fa --exec mv '{}' '{.}'.fasta \; . fd_example
$ ls fd_example
f1.fasta  f2.fasta  f3.fasta  f4.fasta  f5.fasta  f6.fasta  f7.fasta  f8.fasta

Then only new thing here is the use of '{.}'. Again, from the help menu, {.} gives us the path without the file extension. So fd_example/f1.fa becomes fd_example/f1.

Advanced: redirecting stdout to a file within the command

Trying to redirect the standard output of a command to a file turns out to be a little more involved. For example, going with what we’ve seen so far, if we wanted to get the sequence identifier for each sequence in a fasta file and write them to file, our initial attempt might look like this

$ fd -e fa \
    --exec rg -P '^>(?P<id>[^\s]+)\s.*' --replace '$id' > '{/.}'.ids \; \
    . fd_example

In this command, we use ripgrep (rg) to grab the identifiers for the file and write them out to a file called '{/.}'.ids. The help menu for fd tells us that {/.} is the path basename without the file extension. So fd_example/f1.fa becomes f1.ids. However, when we run this, we get an error message like zsh: no such file or directory: {/.}.ids. What is effectively happening here is that before this command gets passed to fd, it is first interpreted by the shell. The shell thinks we are trying to write the output from everything up until the redirection operator > (i.e. fd -e fa --exec rg -P '^>(?P<id>[^\s]+)\s.*' --replace '$id') to a file called '{/.}'.ids \; . fd_example. The shell is correct, there is no file called {/.}.ids - this syntax is only valid inside an fd --exec command.

We can fix this in one of two ways: inline bash command execution or a shell script.

Inline bash command

This option is pretty ugly - unless you’re into one-liners…

$ fd -e fa \
    --exec sh -c "rg -P '^>(?P<id>[^\s]+)\s.*' --replace '\$id' '{}' > '{/.}'.ids" \; \
    . fd_example

As you can see, we need to invoke an inline shell command with sh -c. Part of the awkwardness here is also having to escape some characters to prevent the shell from trying to evaluate them, amongst other things. These inline commands can get very cumbersome very fast!

Shell script

Recommended. For complex examples, such as the one we are working with, I would suggest using this approach. The fd command itself does become slightly more obscure, but if you name the script accordingly, it should still be self-explanatory.

Replicating our example from the above example, we create a script

#!/usr/bin/env sh
rg -P '^>(?P<id>[^\s]+)\s.*' --replace '$id' "$1" > "$2"

and then execute the script with fd.

$ fd -e fa \
   --exec sh '{}' '{/.}'.ids \; \
   . fd_example

Much neater!

If we take a look inside one of the output files, we should see the identifiers

$ cat f1.ids

Putting it all together: Parallel MSA

Ok, armed with our fd --exec knowledge, let’s parallelise multiple sequence alignment on our samples and benchmark how much of a difference throwing more threads at fd makes.

To perform the MSA, we are going to use MAFFT. MAFFT writes its output to standard output. Luckily we know how to deal with this 😎. We’ll write a script and then execute that script with fd.

#!/usr/bin/env sh
mafft --thread 1 --auto "$1" > "$2"

Note: we are only specifying one thread for benchmarking purposes.

Our fd command will then be

$ fd -e fa --exec sh '{}' msas/'{/.}'.msa.fa \; . fd_example

Our directory tree now looks like

├── fd_example
│  ├── f1.fa
│  ├── f2.fa
│  ├── f3.fa
│  ├── f4.fa
│  ├── f5.fa
│  ├── f6.fa
│  ├── f7.fa
│  └── f8.fa
└── msas
   ├── f1.msa.fa
   ├── f2.msa.fa
   ├── f3.msa.fa
   ├── f4.msa.fa
   ├── f5.msa.fa
   ├── f6.msa.fa
   ├── f7.msa.fa
   └── f8.msa.fa


How much of a difference does more threads make? We’ll benchmark various numbers of threads using the tool hyperfine and its neat parameter scan functionality. The data used in this benchmarking is a little different to the toy dataset. I have restricted the files to those above 9kb in size (80 files in total) to try and see the full effects of multiple threads. The 100 threads option is just a roundabout way of saying “however many threads you can find.” The machine I ran this benchmark on has 16.

$ hyperfine -L threads 1,2,4,8,100 --export-markdown \
    "fd -j {threads} -S +9k -e fa --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data"


Benchmark #1: fd -j 1 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/
  Time (mean ± σ):     16.708 s ±  0.475 s    [User: 13.742 s, System: 6.550 s]
  Range (min … max):   15.972 s … 17.666 s    10 runs

Benchmark #2: fd -j 2 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/
  Time (mean ± σ):     10.003 s ±  0.064 s    [User: 15.850 s, System: 7.498 s]
  Range (min … max):    9.896 s … 10.115 s    10 runs

Benchmark #3: fd -j 4 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/
  Time (mean ± σ):      7.232 s ±  0.331 s    [User: 20.556 s, System: 10.231 s]
  Range (min … max):    6.943 s …  7.751 s    10 runs

Benchmark #4: fd -j 8 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/
  Time (mean ± σ):      6.118 s ±  0.233 s    [User: 22.905 s, System: 11.137 s]
  Range (min … max):    5.461 s …  6.250 s    10 runs

Benchmark #5: fd -j 100 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/
  Time (mean ± σ):      5.913 s ±  0.028 s    [User: 24.093 s, System: 11.508 s]
  Range (min … max):    5.880 s …  5.954 s    10 runs

  'fd -j 100 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/' ran
    1.03 ± 0.04 times faster than 'fd -j 8 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/'
    1.22 ± 0.06 times faster than 'fd -j 4 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/'
    1.69 ± 0.01 times faster than 'fd -j 2 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/'
    2.83 ± 0.08 times faster than 'fd -j 1 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/'
CommandMean [s]Min [s]Max [s]Relative
fd -j 1 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/16.708 ± 0.47515.97217.6662.83 ± 0.08
fd -j 2 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/10.003 ± 0.0649.89610.1151.69 ± 0.01
fd -j 4 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/7.232 ± 0.3316.9437.7511.22 ± 0.06
fd -j 8 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/6.118 ± 0.2335.4616.2501.03 ± 0.04
fd -j 100 -e fa -S +9k --exec sh '{}' msas/'{/.}'.msa.fa \; . bench_data/5.913 ± 0.0285.8805.9541.00


The benchmark I have run here is a little contrived and doesn’t really reflect the full benefit of multiple threads in fd. The fasta files I am performing MSA on are quite small/simple, so the benefit of multiple threads isn’t quite as drastic as it would be for more complex alignments. Part of the problem with benchmarking on more complex files is the single-threaded commands run far too slow, and this is not intended to be a full scale benchmarking post. For a real-world (anecdotal) example, it took ~3.5 minutes to run MAFFT on my 2,582 fasta files using 16 threads. You can see an example of how I embedded this in a snakemake pipeline here.

Final Remarks

I hope this post was insightful and useful. In the future, I hope to write more posts like this one (provided people find this one helpful) on whatever comes up in my work that I think might be interesting to others. In the meantime, feel free to get in touch if you have any questions or comments or complaints about this post.

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