Run a list of commands on a set of SSH nodes. With a bit of optional parametrization.

Demo

Features

• Passwordless SSH is all you need.
• Simple config for simple use cases, flexible config for advanced ones.
• Two modes:
  • Broadcast mode runs each command on every node.
  • Queue mode runs each command once on the next free node.
• Modify the file-based queue (queue.yaml) while Pegasus is running.
• Parametrize hosts and commands.

Getting Started with Examples

To use Pegasus,

1. Install Pegasus, either from GitHub Release or cargo install pegasus-ssh.
2. Setup passwordless SSH for your nodes.
3. Populate hosts.yaml and queue.yaml, and run Pegasus.

Pegasus will remove one entry at a time from the top of queue.yaml and move it to consumed.yaml as it begins to execute it.

Queue Mode: Getting a Bag of Jobs Done

Run four Python commands using two nodes.

```yaml

hosts.yaml

• node-1
• node-2 ```

```yaml

queue.yaml

• . /opt/miniconda3/etc/profile.d/conda.sh; python train.py --bs 8
• . /opt/miniconda3/etc/profile.d/conda.sh; python train.py --bs 16
• . /opt/miniconda3/etc/profile.d/conda.sh; python train.py --bs 32
• . /opt/miniconda3/etc/profile.d/conda.sh; python train.py --bs 64 ```

console $ pegasus q # stands for Queue

Broadcast Mode: Terraforming Nodes

Run identical commands for multiple nodes.

```yaml

queue.yaml

• mkdir workspace
• cd workspace && git clone https://github.com/jaywonchung/dotfiles.git
• . workspace/dotfiles/install.sh ```

console $ pegasus b # stands for Broadcast

Parallelizing Execution with Node Parameters

Split nodes into sub-nodes that run commands in parallel. Below, four SSH connections are kept, and four commands run in parallel.

```yaml

hosts.yaml

• hostname:
  • node-1
  • node-2 container:
  • gpu0
  • gpu1 ```

When parametrizing nodes, just make sure you specify the hostname key.

You can use these parameters in your commands. By the way, the templating engine is Handlebars.

```yaml

queue.yaml

• docker exec {{ container }} python train.py --bs 8
• docker exec {{ container }} python train.py --bs 16
• docker exec {{ container }} python train.py --bs 32
• docker exec {{ container }} python train.py --bs 64 ```

Four sub-nodes and four jobs. So all jobs will start executing at the same time.

Parametrizing Commands for Conciseness

If you can parametrize nodes, why not commands?

```yaml

queue.yaml

• command:
  • docker exec {{ container }} python train.py --bs {{ bs }} bs: [8, 16, 32, 64] ```

This results in the exact same jobs with the example above. When parametrizing commands, just make sure you specify the command key.

Quiz

How many commands will execute in Queue mode?

```yaml

hosts.yaml

• hostname:
  • node-1
  • node-2 laziness:
  • 1
• hostname:
  • node-3 laziness:
  • 2 ```

```yaml

queue.yaml

• echo hi from {{ hostname }}
• command:
  • for i in $(seq {{ low }} {{ high }}); do echo $i; sleep {{ laziness }}; done
  • echo bye from {{ hostname }} low:
  • 1
  • 2 high:
  • 3
  • 4 ```

Note that although echo bye from {{ hostname }} doesn't really use the low or high parameters, it will run 2 * 2 = 4 times regardless.

The answer is 1 + 2 * 2 * 2.

Lock Mode: Modifying the Queue

queue.yaml is actually the queue.

Pegasus removes the first entry in queue.yaml whenver there's a free host available. If you delete entries before Pegasus pulls it, they will not execute. If you add entreis to queue.yaml, they will execute.

Q. Why do I need this?

Think about when the number of remaining commands is less than the number of free nodes. Without a way to submit more jobs to Pegasus, those free nodes will stay idle until all the commands finish and you start a fresh new instance of Pegasus.

By providing a way to add to the queue while commands are still running, users may achieve higher node utilization. Being able to delete from the queue is just a byproduct; adding to the queue is the key feature.

Q. But that's a race condition on queue.yaml.

Lock mode will lock queue.yaml and launch a command line editor for you.

console $ pegasus l --editor nvim # l stands for Lock

Editor priority is --editor > $EDITOR > vim. When you save and exit, the queue lock is released and Pegasus is allowed access to queue.yaml.

Q. What if Pegasus terminates before I add to queue.yaml?

Enable daemon mode, and Pegasus will not terminate even if queue.yaml is empty. It will stand waiting for you to populate queue.yaml again, and execute them.

console $ pegasus q --daemon

Details

queue.yaml

This is the queue file. Entries in queue.yaml are consumed from the top, one by one. Also, entries are consumed only when a new host is available to execute new commands. Consumed entries are immediately appended to consumed.yaml in "canonical form", where every entry has a command key. Thus you might do something like tail -n 2 consumed.yaml > queue.yaml to re-execute your previous single-line command.

As mentioned earlier, always use the Lock Mode when you need to modify queue.yaml.

Broadcast Mode

In broadcast mode, hosts are kept in sync with each other. That is, the next command is fetched from queue.yaml and executed on all hosts when all the hosts are done executing the previous command.

Consider the following situation:

fast-host slow-host - command1 success success - command2 success fail! - command3 success - command4 running

In this case, we would want to prepend a undo command for command2 (e.g., rm -rf repo || true) and restart from that, but fast-host is already far ahead, making things complicated. Thus, especially when you're terraforming nodes with Pegasus, keeping hosts in sync should be beneficial.

There is also a -e or --error-aborts flag in Broadcast Mode, which aborts Pegasus automatically when a host fails on a command.

Cancelling and killing

It is very difficult to find a generic way to cancel commands that started running via SSH (See #11). Therefore, the caveat of Pegasus at the moment is that it works very well when things go well, but it's difficult to cancel and kill when things go not quite as planned. You need to walk into every node and manually kill commands. That said, you can still use Broadcast mode to automate that.