Bert's blog

## Tools I use for scientific code execution

In a previous post, I already covered the main tools I use during the scientific code development process. These tools were all closely related to writing, testing and maintaining code. In this post, I will cover tools that help with running code efficiently. These are tools that allow you to run whatever you want to run more efficiently, faster and more securely, and that also allow you to rerun things in a more effective way if required.

# Automation tools

Automation tools are all about efficiency: they allow you to decrease the number of commands you need to manually enter or execute, and hence allow you to spend your time on other things, especially if you have a long list of commands that can be automated. Additionally, automation tools are inherently part of large shared computing facilities where jobs cannot be executed in real time but have to be scheduled in a job queue.

## bash loops and scripts

A shell is the interactive program that runs in the background when you run a command-line terminal and that interprets and executes your commands. There are various shells you can choose from, but most of them offer basic scripting functionality, i.e. they have support for control structures like loops and conditions that allow you to write simple programs (called scripts) that are executed by the shell. Ubuntu-systems by default ship with the bash shell, and that is also my personal favourite.

A first level of automation can already be achieved by just using these control structures as actual commands. Suppose for example that you have a list of 500 files (file001, file002, etc.) and you want to run a program (or script) compute_average_density on each of these. Instead of manually calling the command on each of these files:

> compute_average_density --file file001
Done.
> compute_average_density --file file002
Done.
...

you could automate this process using a for loop:

> for file in file???
> do
> compute_average_density --file $file > done Done. Done. ... In this case, you only need to enter a single command, and bash will automatically call the compute_average_density command 500 times. Note that the bash history (you can scroll through this history using the up and down arrows on your keyboard and search through it using CTRL+R) will store the entire loop as a single line, like this: > for file in file???; do compute_average_density --file$file; done

You can always use this syntax as well if you like it better.

The basic syntax for the for command allows you to specify a space-separated list of elements over which to iterate:

> for i in 0 2 4; do echo $i; done 0 2 4 But you can also list files (and folders) in the file system using wildcards that are converted into such a list by bash before the for command is actually called: > echo file??? file001 file002 ... > echo file* file0001 file001 file002 ... > echo file??[!2] file001 file003 ... Where I have illustrated some of the wildcards that are available: wildcardmeaning ?Replace by exactly one allowed character *Replace by any number of allowed characters [!...]Replace by exactly one allowed character that is not the character(s) listed in between brackets Additionally, you can generate ranges of (padded) values: > echo {0..10} 0 1 2 3 4 5 6 7 8 9 10 > echo {00..10} 00 01 02 03 04 05 06 07 08 09 10 It can be quite useful to know some of these commands; learning them is a bit tedious at first, but you quickly get used to them and they facilitate basic tasks enormously when used correctly. bash scripts (or more generally shell scripts) allow you to save the commands you would usually manually enter into a file (e.g. commands.sh). This file can then be executed by running > bash commands.sh This runs the commands in a new, isolated shell. You can also run the commands in the active shell: > source commands.sh The difference between both is quite technical. The first version will not allow changes to the shell environment made by the script to persist after the script finishes (e.g. setting an environment variable inside your script). Usually you don’t care about that. A final way to run a bash script is by converting it into an executable. This can be done by adding the following line to the top of the script: #! /bin/bash This tells the operating system to automatically execute the script with the bash program. You can then simply invoke the script as any other program: ./commands.sh Note that Unix-systems are quite specific about executing files and by default set the file permissions so that this is not allowed. You can manually set the permission to execute the script by running chmod +x commands.sh The advantage of using scripts is reproducibility: if you often rerun the same set of commands, then having them stored saves you the effort of re-entering them every time. And you easily rerun the same set of commands for different sets of files. Shell scripts are really like mini-programs, as they also allow you to use conditions: > for i in {0..10} > do > if [$i -lt 4 ]
> then
> echo $i > fi > done 0 1 2 3 The bash comparison operators themselves are unfortunately quite archaic. Here are some of them: comparison operatormeaning -eqEqual to -neqNot equal to -ltLess than -gtGreater than -leLess than or equal to -geGreater than or equal to Also note the spaces in between the brackets of the conditional statement, as they are important. There are additional commands like e.g. -f that can check if a file exists: > if [ -f file001 ] > then > echo "File exists!" > else > echo "File does not exist!" > fi And of course, there is also a not operator: > if [ ! -f file001] > then > echo "File does not exist!" > fi Inside a script, you can define variables: > message="hello" > echo$message
hello

And these variables can also be the result of another command if you enclose that command in $(): > message=$(uname -o)
> echo $message GNU/Linux Note that the uname command gives you information about the operating system. Lastly, bash scripts automatically receive additional command line arguments in special variables. The following script: #! /bin/bash echo "Number of command line argument:$#"
echo "First argument: $0" echo "Third argument:$2"

illustrates this:

> ./script.sh these are words
Number of command line argument: 3
First argument: ./script.sh
Third argument: are

The \$0 argument contains the name of the script.

The overview above is just the tip of the iceberg that is shell programming; there are a lot of good online tutorials that can teach you more.

## batch systems

Batch systems are very similar to the shells introduced above, except that you don’t usually use them interactively. Batch systems are inherently part of shared computing facilities, where you have to use them. A batch system shell script is referred to as a job script. You need to submit this job script to the batch system (using an appropriate command) and this will then put your job in a shared queue that contains all the jobs of all the other people that want to run commands on the facility. The batch system will then schedule your job on (a part of) the machine when there are sufficient resources available. Depending on how many jobs are in the queue, this can take a while. Most batch systems additionally assign a priority to each job that is scheduled, with higher priority jobs getting preference over low priority jobs. These priorities can depend on many factors, and usually funding is one of them (someone has to pay for the facility).

Unfortunately, there are many batch systems, and it is not up to you to pick one; the choice of batch system depends on the shared computing facility. Most batch systems have very similar features however, and they all use an underlying shell to actually execute your job. Knowing shell commands hence also pays off in this case. And knowing one batch system is usually enough to be able to use another one, if you can get a hold of the documentation for the system-specific feature syntax, that is.

Figuring out which batch system you are using can be tricky. In my experience, small facilities (especially computing nodes owned by university departments or research groups) don’t bother documenting their system very well. In this case, it helps to try locate system-specific commands. The table below can be useful:

commandslikely batch system
sbatch, sinfo, squeue, scancelSlurm Workload Manager
qsub, qstat, qdelPortable Batch System or TORQUE

If you can find these commands on the login shell of the remote facility, you can be quite confident that they are using the corresponding batch system.

Independent of the batch system, each system offers commands to submit (sbatch, qsub), monitor (sinfo and squeue, qstat) and cancel (scancel, qdel) jobs.

A batch script (job script) is very similar to a normal shell script, except for some batch system directives at the start. Below is an example script for the Slurm system:

#! /bin/bash
#
#SBATCH --job-name=test
#SBATCH --nodes=1
#SBATCH --output=test.out
#SBATCH --error=test.err

echo "Hello expensive computing facility!"

The lines starting with #SBATCH contain directives for the Slurm system, e.g. the name of the job that will be used in status reports, the number of computing nodes to use, the number of threads to use per computing node, and the names of the files in which the standard output and standard error output will be stored (this is the output you would normally see in the command line terminal if you were to execute the script in a shell). Most other batch systems have similar directives (but with different names). For the PBS and TORQUE systems, directive lines start with #PBS instead of #SBATCH.

Using a batch system can seem a bit intimidating at first, but once you realise it is just a somewhat more complicated shell, it is actually not that difficult. It is quite normal that things go wrong when you submit a script for the first time, and knowing how to interpret error output and to get information about how the script was executed can help a lot. Just make sure that you test your script properly before submitting 100 jobs using the same script!

# Performance tools

The tools below are similar to the automation tools above, except that they introduce a new introducing feature: parallelisation. All systems nowadays (even your laptop) have a number (usually something like 4, 8 or 16) cores to their disposal that can execute commands simultaneously. Unfortunately, you cannot normally use these cores manually from the command line; the responsibility for using the parallel environment correctly is left to the individual programs you use. If your program does not support using multiple threads (a thread is a single active core, or the equivalent of a single serial program), it will just run on a single core, and you will be wasting a large fraction of your available resources. The tools discussed below can help you exploit the additional available computing power.

## Python multiprocessing

The first tool is not really a tool in itself, but is a useful module that is part of Python: the multiprocessing module. This module allows you to execute a function within Python using multiple, completely independent threads, and hence offers you a lightweight way to introduce parallelisation into your Python scripts. Since a lot of commands I run nowadays are Python scripts, this addresses the problem with serial commands at the core: by making the commands themselves use the available threads efficiently.

Despite being useful, I recently stopped using this option in favour of the solution I will introduce next. The main reason is that parallel Python scripts usually have complex dependencies, which makes it harder to use them in workflows (see below). On top of that, it is quite tricky to interrupt parallel Python scripts; just killing the Python script does not properly close all the threads and leads to ghost Python processes still running in the background. I’m sure parallel processing within Python can be useful for some applications, but for the purposes I usually use Python (simulation analysis), it is not ideal.

## GNU parallel

GNU parallel is a more elegant solution for the problem that shells do not execute commands in parallel; it is a (powerful) program that takes a list of commands and then executes them in parallel within a copy of the shell and sends the output back to the shell.

A basic example looks like this

> parallel -j 4 echo ::: a b c d
a
c
d
b

The program takes the echo command, and then runs it with the four different arguments provided in the list that starts with :::. The -j 4 argument tells parallel to use 4 parallel threads to execute this command. The output will not necessarily be in the same order as the order of the list, as it depends on how fast the corresponding thread is at executing the echo command.

parallel has a lot more ways of specifying a list of commands, and I very much suggest that you consult its extensive tutorial to find out more. I usually use it as a parallel alternative for the shell loops introduced above:

> ls file??? | parallel -j 8 compute_average_density
Done.
Done.
...

In this case, the list generated by the ls command is piped directly into the parallel command and used as arguments for the compute_average_density program.

For more complicated commands that require multiple arguments, you can use argument substitution:

> ls file??? | parallel -j 8 plot_density --input {} --output {}.png
Plotting file001 as file001.png...
Plotting file004 as file004.png...
...

In this case, each {} in the command is replaced by the respective element in the list. An individual command issued by parallel will look like

plot_density --input file001 --output file001.png

There are many variants of the {} directive that allow you to apply filters to the list elements, e.g. filtering out file extensions, folder names…

To get a parallel equivalent for a loop that has a numerical counter, you can use the seq command:

> seq 0 3 | parallel -j 4 echo {}
0
2
1
3

To get the equivalent of a fixed width counter ({00..03}), you can use seq -w:

> seq -w 00 03 | parallel -j 4 echo {}
00
03
02
01

Note that if you use parallel for the first time, it will ask you to promise that you will cite the underlying publication if you use it for your work:

When using programs that use GNU Parallel to process data for publication

O. Tange (2011): GNU Parallel - The Command-Line Power Tool,
;login: The USENIX Magazine, February 2011:42-47.

This helps funding further development; AND IT WON'T COST YOU A CENT.
If you pay 10000 EUR you should feel free to use GNU Parallel without citing.

Once you do this, it will work as illustrated above.

# Workflow management

The tools above allow you to automate some of the tasks you want to do during code execution, and to make more efficient use of the available resources on your computer. They still require a lot of manual intervention however, and still limit you to using a single computer.

Workflow Management Systems (or WMSs) offer a more complete solution to automating and optimising your tasks. They do however require an additional step of abstraction, which we call a workflow.

I already briefly mentioned workflows in a previous post. In essence, a workflow is a structured representation of the commands you need to run in order to execute a task that details the dependencies between these commands: required input and output files, and software and hardware requirements. This representation is usually visualised as a flowchart, and allows you to see the order in which commands need to be run, and which files need to be present in order for a command to work. It also shows you which parts of your task get invalidated if a file changes, i.e. they make it possible to easily figure out which commands you need to rerun if a small part of your input files changed.

Workflows are incredibly powerful, and I should probably discuss them in more detail in a future post. For now, I will limit myself to the software that uses workflows: WMSs. In short, these WMSs take your workflow and execute it for you, using all available resources, including remote shared computing facilities, and in the most optimal way. This is obviously a quite complicated thing to do; and much depends on which WMS you use and how your computer and other facilities are configured. I am still very much new to these tools myself. Below is what I know so far.

## Makeflow

Makeflow is probably one of the most basic WMSs, and is also the only system I have thus far managed to successfully use myself. The tool itself is very similar to GNU make (hence its name), and is quite easy to manually install (which is usually the only way to make it work consistently on a number of different machines). It consists of the WMS itself, makeflow, and an additional batch system, workqueue, that is responsible for executing makeflow jobs efficiently (similar to a traditional batch system). makeflow can also be used in conjunction with existing batch systems, like the ones discussed above, and with a standard shell.

I will explain the usage of makeflow in more detail in a future post and limit myself to a very basic example. Suppose we have a single input file, input.txt, and we want to split this file into two parts using split. We then want to store the number of lines in each file in a file (using wc), and then join these two files back into a final output file output.txt (using cat). A corresponding Makeflow file, makeflow.makeflow looks like this:

input00 input01: input.txt
split -n 2 -d input.txt input

input00count: input00
wc input00 > input00count

input01count: input01
wc input01 > input01count

output.txt: input00count input01count
cat input00count input01count > output.txt

Despite its absurdity, this example shows you how Makeflow works: each block in this file has the general structure

OUTPUT: INPUT
COMMAND

(note that the COMMAND needs to be preceded by a tab). The OUTPUT should contain a list of all output that the command generates that you will use in other commands or want at the end of the task (output that is not listed will be deleted by Makeflow as soon as the command finishes). INPUT should specify all the input needed for the command (if this input is not present in the file system, Makeflow will look for a block that has the corresponding file as output and run that block first). The COMMAND specifies the actual command to run. Makeflow will complain if this command does not generate all of the expected output.

To run this flow using Makeflow, we need to do two things. First, we need to start the WMS, using the makeflow command:

> makeflow -T wq makeflow.makeflow -p 9000

The -T wq argument specifies the batch system to use to submit jobs; wq stands for WorkQueue, Makeflow’s own batch system. -p 9000 specifies the network port that Makeflow will use to communicate with WorkQueue. If you omit this argument, Makeflow will choose a random port and tell you its number. You need this number for the second step.

The second step is launching the batch system itself. This can be done using the work_queue_worker command (from a new command line terminal):

> work_queue_worker --cores 8 --memory 1000 --disk 1000 localhost 9000

This will launch a batch system with 8 threads (--cores 8). This system will use a maximum of 1000 MB of RAM memory (--memory 1000) and 1000 MB of hard drive space (--disk 1000), and will try to contact Makeflow on the localhost using network port 9000. Note that the system parameters do not have to match those that are actually available, but that WorkQueue will fail to launch if you use values that are larger than what is available. If WorkQueue has multiple threads available, it will use all of these to execute Makeflow jobs. Makeflow allows you to specify the computational requirements for a job in the Makeflow file.

If all went well, the Makeflow and WorkQueue processes will now start communicating with each other, and will execute the entire workflow. If something goes wrong, Makeflow will tell you about this. In this case, you will be able to restart the workflow where you left it.

You can also run the WorkQueue command on a remote cluster and still make it communicate with your local Makeflow command. This however requires appropriate network settings and is not so easy to set up. Makeflow has additional features that make it possible to execute a workflow using multiple WorkQueue systems on multiple clusters. This is just a very brief introduction to Makeflow, so I will tell you more about this in a future post.

## Other WMSs

Makeflow is quite limited in terms of what it can do and the amount of diagnostic output it generates. This makes it less suitable for really large projects. There are many other WMSs that are much more powerful, but unfortunately, I have not been able to successfully use any of these. Here is a list of some of these:

I would also recommend the Blue Waters webinar series on Scientific Workflow Management Systems.

Professional astronomer.