Gnu Parallel is a powerful command-line tool that allows you to run multiple commands in parallel. It is commonly used in the field of data processing, where it can significantly speed up tasks by utilizing the full capacity of your computer's CPU cores. However, like any software, you may encounter some issues while using Gnu Parallel. In this article, we will discuss a specific problem related to the order of attributes when using the --sshlogin option with the --keep-order option, and how to troubleshoot it.
The Problem
When using Gnu Parallel with the --sshlogin option to execute commands on remote servers, you may sometimes want to ensure that the output of the commands is displayed in the same order as the input. This can be achieved by using the --keep-order option, which preserves the order of the input when parallelizing the commands.
However, you may notice that the order of the --sshlogin attributes is not respected when using the --keep-order option. For example, consider the following command:
parallel --sshlogin server1,server2,server3 --keep-order 'echo {}' ::: 1 2 3
Instead of executing the commands in the order server1, server2, server3, you may observe that the commands are executed in a different order, such as server2, server1, server3. This behavior can be confusing and may lead to unexpected results.
The Explanation
The reason behind this behavior is related to how Gnu Parallel distributes the workload among the available resources. When using the --sshlogin option, Gnu Parallel establishes SSH connections to the specified servers and distributes the commands across them. However, the order in which the SSH connections are established is not guaranteed to be the same as the order of the --sshlogin attributes.
Instead, Gnu Parallel dynamically assigns commands to available resources as they become available. This means that the order in which the commands are executed may not match the order of the --sshlogin attributes, even when using the --keep-order option.
The Solution
To overcome this issue and ensure that the commands are executed in the desired order, you can use a workaround. Instead of relying on the order of the --sshlogin attributes, you can explicitly specify the order of the commands using the {#} replacement string. The {#} replacement string represents the sequence number of the job, which corresponds to the order of the input.
Here's an example of how you can modify the previous command to achieve the desired order:
parallel --sshlogin server1,server2,server3 --keep-order 'echo {}' ::: {1} {2} {3}
By explicitly specifying the sequence numbers as the input, you ensure that the commands are executed in the desired order, regardless of the order of the --sshlogin attributes. In this example, the commands will be executed in the order server1, server2, server3.
Conclusion
Gnu Parallel is a powerful tool for parallelizing tasks, but it may exhibit some unexpected behavior when using certain options. In this article, we discussed the issue of attribute order being ignored when using the --sshlogin option with the --keep-order option. We explained the reason behind this behavior and provided a workaround to ensure the desired order of execution. By using the {#} replacement string, you can explicitly specify the order of commands and overcome this issue.
References
| Reference | Link |
|---|---|
| Gnu Parallel Documentation | https://www.gnu.org/software/parallel/ |
| Gnu Parallel Tutorial | https://www.gnu.org/software/parallel/tutorial.html |