How can I keep Dask workers busy when processing large datasets to prevent them from running out of tasks?

I'm trying to process a large dataset (around 1 million tasks) using Dask distributed computing in Python. (I am getting data from a database to process it, and I am retriving around 1M rows). Here I have just made a simpler version of my code:

Each task simulates some computation, and I want to efficiently distribute these tasks among multiple workers to maximize resource utilization.

Here's a minimal reproducible example of my code:

from dask.distributed import Client, as_completed
from tqdm import tqdm
import time
import random

# Dummy computational function
def compute_task(data):
    # Simulate some computation
    time.sleep(random.uniform(0.01, 0.02))  # Simulate computation time
    return data * data

# Function to process a chunk of data
def process_chunk(chunk):
    results = []
    for item in chunk:
        result = compute_task(item)
        results.append(result)
    return results

def main(scheduler_address, num_tasks=1000000, chunk_size=100, max_concurrent_tasks=1000):
    client = Client(scheduler_address)
    print(f"Connected to Dask scheduler at {scheduler_address}")

    try:
        # Generate dummy data
        data = list(range(num_tasks))
        total_chunks = (num_tasks + chunk_size - 1) // chunk_size

        # Create a generator for chunks
        def chunk_generator():
            for i in range(0, len(data), chunk_size):
                yield data[i:i + chunk_size]

        chunks = chunk_generator()
        active_futures = []

        # Initial submission of tasks
        for _ in range(min(max_concurrent_tasks, total_chunks)):
            try:
                chunk = next(chunks)
                future = client.submit(process_chunk, chunk)
                active_futures.append(future)
            except StopIteration:
                break

        completed_chunks = 0
        with tqdm(total=total_chunks, desc="Processing data") as pbar:
            for completed_future in as_completed(active_futures):
                results = completed_future.result()
                # Here we could do something with the results
                pbar.update(1)
                completed_chunks += 1

                # Submit new tasks to keep the pipeline full
                try:
                    chunk = next(chunks)
                    future = client.submit(process_chunk, chunk)
                    active_futures.append(future)
                except StopIteration:
                    pass

                # Remove completed future from the list
                active_futures.remove(completed_future)

        print("Processing complete.")

    finally:
        client.close()
        print("Client closed.")

if __name__ == "__main__":
    main(scheduler_address='tcp://localhost:8786')

Explanation:

• compute_task: A dummy function that simulates computational work by sleeping for a short random duration and returning the square of the input data.
• process_chunk: Applies compute_task to each item in a chunk.
• The main function:
  • Generates a list of numbers as dummy data.
  • Splits the data into chunks.
  • Submits tasks to workers, aiming to keep a certain number of tasks (tasks_per_worker) queued per worker.
  • Processes results as tasks complete and tries to replenish the workers' queues.

The Problem:

Despite this setup, the workers quickly run out of tasks and become idle. And the worker pool becomes deprived of tasks. It seems that my logic for submitting and replenishing tasks isn't keeping the workers sufficiently occupied, leading to inefficient utilization of resources. The workers process tasks faster than new tasks are being submitted, causing them to become idle while waiting for more tasks.

My Questions:

• How can I improve my task submission logic to ensure that Dask workers remain busy until all data is processed?
• Is there a more efficient way to distribute tasks among workers to maximize throughput and resource utilization?

I suspect that the overhead in my task submission and management logic is causing delays. Managing per-worker queues and specifying workers in client.submit might be introducing unnecessary complexity and latency. Considering letting Dask handle the worker assignment by removing the workers parameter, but I'm unsure how to adjust my code accordingly.

Any guidance or suggestions would be greatly appreciated!

The following line is blocking your code:

results = completed_future.result()

It forces the main process to wait for the result (including transfer over the network if the work is distributed across a network). From the code you are not doing anything useful with the result anyway, so it should be safe to remove that line.

If in your actual code you are doing something useful with the result, then it's best to move the relevant procedure into a separate function (or integrate into an existing one) to allow for parallel, non-blocking processing.