miniwdl run optimization

Follow these guidelines for optimal performance and reliability from miniwdl run.

WDL guidelines

Consolidate very short tasks

If your production workload uses many tasks that take only seconds each, consider restructuring the workflow to consolidate them. For example, if you’re scattering over a large array of very small items, try processing the items in batches, with tasks internally parallelized using GNU parallel or the like. There’s overhead in scheduling and provisioning each task container, which makes it inefficient to deploy large numbers of very short tasks.

Avoid unnecessary large WDL data structures

Except as needed for workflow orchestration (e.g. scatter arrays), pass large datasets through Files instead of WDL data structures (Arrays, Maps, structs, etc.). Excessively large WDL data structures might bottleneck the workflow, as they’re handled within the miniwdl orchestrator process.

Passing large Array[File] (more than dozens of files, regardless of their individual sizes) through task inputs can be especially costly, due to overhead localizing and mounting each file into the task’s container. Instead, consider Directory inputs/outputs (available in the WDL development version), or consolidating the files into tar or zip archives to extract inside the task.

Read-only input files

By default, miniwdl mounts task input files & directories read-only, blocking attempts to open them for writing, or more commonly, to move or rename them. Tasks that do so must be run with --copy-input-files, which should be avoided because it consumes more time and disk space.

If you just need to rename an input file to satisfy some tool’s convention, try symlinking the desired name to the input file at the beginning of the task command script.

Write scratch files under $TMPDIR

Scratch files (created but never output) should be written somewhere under $TMPDIR instead of the task’s initial working directory. Common examples include intermediate files, external sorting buffers, and decompressed reference databases.

Output files should be written directly into the initial working directory, which might be on a network file system to facilitate downstream reuse.

Scale runtime resources to input size

Miniwdl can schedule multiple task containers on one host concurrently, but only if it has enough CPU and memory resources to fulfill their total requirements (as declared in their runtime sections). If you run a workflow on small inputs (e.g. for testing) but the task runtime sections have hard-coded high CPU and memory requirements, the scheduler tends to serialize them unnecessarily.

Therefore, consider using WDL expressions to scale the runtime resource reservations to the input sizes:

task res {
  input {
    Array[File]+ input_files

    Int cpu = if length(input_files) < 16 then length(input_files) else 16
    Int memGB = cpu*2

  command {
    cat "~{write_lines(input_files)}" | xargs -i -P ~{cpu} echo {}

  runtime {
    cpu: cpu
    memory: "~{memGB}GB"

WDL’s size() function to measure File size (or total Array[File] size) can also be useful here. Placing the expressions among the inputs allows the caller to override the reservations if needed.

Control multithreading explicitly

With multiple concurrent containers, each one might “see” all of the host processors (for example, using nproc or Python multiprocessing.cpu_count()) even if its runtime.cpu reserved fewer. If the tasks use parallel tools that default their thread/process count to the detected processor count, they might cause an overload. Therefore, tasks should explicitly control their internal multithreading to match their reserved CPU count, as illustrated in the previous example with xargs -P.

bash shell configuration

In any task whose command consists of a non-trivial script, set -euxo pipefail at the beginning to configure the bash shell in a way that usually improves robustness and debuggability.

The record of invocations left by set -x goes well with miniwdl run --verbose, which includes them in the task log along with timestamps. You can also put arbitrary messages into this log by writing them into standard error, e.g. >&2 echo "progress report"

Host configuration

Enable call and download caches

Miniwdl can cache task & workflow call outputs and downloaded URIs for reuse across multiple runs, but these features must be enabled in the runner configuration.

Use local disks for Docker storage

Docker images and containers should reside on fast local disks, rather than a network file system, to optimize container startup and scratch I/O performance. These typically reside under /var/lib/docker, which on a cloud instance would usually be on the network-attached root file system. Suppose your instance has a local scratch disk mounted to /mnt. You can change the Docker storage location using a procedure like this:

systemctl stop docker    # or appropriate command to stop dockerd
mv /var/lib/docker /mnt
ln -s /mnt/docker /var/lib/docker
systemctl start docker

If the host has multiple scratch disks, consider first assembling them into a RAID0 array with all available space and IOPS.

How to miniwdl run inside a Docker container

It’s possible to operate miniwdl run inside a Docker container, with the following requirements:

  1. The host’s Docker socket must be mounted inside the container.
  2. Input files and the workflow run directory must reside on the host file system, mounted inside the miniwdl container at identical paths.

For example from the host,

mkdir /tmp/wdl
echo Alice > /tmp/wdl/alice
echo 'version 1.0
      task hello {
        input {
          File who
        command {
          echo "Hello, ~{read_string(who)}!" | tee message.txt
        output {
          File message = "message.txt"
      }' > /tmp/wdl/hello.wdl
docker run --rm -it -v /var/run/docker.sock:/var/run/docker.sock -v /tmp/wdl:/tmp/wdl continuumio/miniconda3 \
  bash -c 'conda install -c conda-forge -y miniwdl &&
    miniwdl run /tmp/wdl/hello.wdl who=/tmp/wdl/alice --dir /tmp/wdl'

If the input files aren’t already located within the desired run directory, then it’d be necessary to mount them with additional docker run -v arguments.