Wie man in Python Multiprocessing Argumente übergibt

Introduction

Python's multiprocessing module enables parallel computing by leveraging multiple CPU cores, which can significantly improve computational performance for CPU-intensive tasks. This lab explores different techniques for passing arguments to processes in the multiprocessing module, addressing common challenges in concurrent programming and demonstrating practical strategies for effective parallelization.

By the end of this lab, you will understand how to use different methods to pass data to parallel processes and apply these techniques to real-world scenarios.

Skills Graph

Introduction to Multiprocessing in Python

In this step, we will learn the fundamentals of multiprocessing in Python and create our first parallel program.

What is Multiprocessing?

Multiprocessing is a Python module that allows us to run multiple processes in parallel, effectively using multiple CPU cores. This is different from threading, which is limited by the Global Interpreter Lock (GIL) that prevents true parallel execution of Python code.

Here's a simple comparison:

Creating Your First Multiprocessing Program

Let's create a simple multiprocessing example. Open a new file in the WebIDE and name it simple_process.py:

import multiprocessing
import time
import os

def worker():
    """Simple function to demonstrate a process"""
    print(f"Worker process id: {os.getpid()}")
    print(f"Worker parent process id: {os.getppid()}")
    time.sleep(1)
    print("Worker process completed")

if __name__ == "__main__":
    ## Print information about the main process
    print(f"Main process id: {os.getpid()}")

    ## Create a process
    process = multiprocessing.Process(target=worker)

    ## Start the process
    print("Starting worker process...")
    process.start()

    ## Wait for the process to complete
    process.join()

    print("Main process completed")

Now let's run this program:

python3 simple_process.py

You should see output similar to:

Main process id: 12345
Starting worker process...
Worker process id: 12346
Worker parent process id: 12345
Worker process completed
Main process completed

The exact process IDs will differ on your system. Notice how the worker process has a different process ID from the main process, confirming they are separate processes.

Understanding Process Creation

When we create a multiprocessing Process and start it, Python:

1. Creates a new process
2. Imports the module containing the target function
3. Executes the target function in the new process
4. Returns control to the parent process

The if __name__ == "__main__": guard is important when working with multiprocessing in Python. This prevents the creation of duplicate processes when the module is imported.

Multiple Processes

Let's modify our example to create multiple processes. Create a new file named multiple_processes.py:

import multiprocessing
import time
import os

def worker(worker_id):
    """Worker function that accepts an argument"""
    print(f"Worker {worker_id} (PID: {os.getpid()}) started")
    time.sleep(1)
    print(f"Worker {worker_id} completed")
    return worker_id * 2  ## This return value won't be captured

if __name__ == "__main__":
    print(f"Main process PID: {os.getpid()}")

    ## Create multiple processes
    processes = []
    for i in range(3):
        p = multiprocessing.Process(target=worker, args=(i,))
        processes.append(p)
        p.start()

    ## Wait for all processes to complete
    for p in processes:
        p.join()

    print("All processes completed")

Run this code:

python3 multiple_processes.py

You should see output similar to:

Main process PID: 12345
Worker 0 (PID: 12346) started
Worker 1 (PID: 12347) started
Worker 2 (PID: 12348) started
Worker 0 completed
Worker 1 completed
Worker 2 completed
All processes completed

Notice how we passed an argument (worker_id) to each process using the args parameter. This is our first example of passing arguments to a process.

Also notice that the processes might not necessarily execute in a predictable order, as they are running in parallel.

Basic Argument Passing with Pool

In the previous step, we passed a simple argument to a process. Now, let's explore a more efficient way to handle multiple tasks using the Pool class and learn about different ways to pass arguments.

The Pool Class

The Pool class provides a convenient way to parallelize the execution of a function across multiple input values. It distributes the input data across processes and collects results.

Let's create a simple example to understand how Pool works. Create a new file named pool_example.py:

import multiprocessing
import time

def square(x):
    """Function that squares a number and returns the result"""
    print(f"Processing {x}...")
    time.sleep(1)  ## Simulate a time-consuming operation
    return x * x

if __name__ == "__main__":
    ## Create a list of numbers to process
    numbers = [1, 2, 3, 4, 5]

    ## Create a Pool with 2 processes
    with multiprocessing.Pool(processes=2) as pool:
        ## Map the square function to the numbers
        start_time = time.time()
        results = pool.map(square, numbers)
        end_time = time.time()

        print(f"Results: {results}")
        print(f"Time taken: {end_time - start_time:.2f} seconds")

    ## Compare with sequential processing
    start_time = time.time()
    sequential_results = [square(n) for n in numbers]
    end_time = time.time()

    print(f"Sequential results: {sequential_results}")
    print(f"Sequential time: {end_time - start_time:.2f} seconds")

Run the code:

python3 pool_example.py

You should see output similar to:

Processing 1...
Processing 2...
Processing 3...
Processing 4...
Processing 5...
Results: [1, 4, 9, 16, 25]
Time taken: 3.01 seconds
Processing 1...
Processing 2...
Processing 3...
Processing 4...
Processing 5...
Sequential results: [1, 4, 9, 16, 25]
Sequential time: 5.01 seconds

Notice how the Pool is faster because it processes multiple numbers in parallel. With a pool size of 2, it can process 2 numbers at a time, whereas the sequential approach processes them one after another.

Passing Single Arguments with Pool.map()

The map() method is perfect for when your function takes a single argument. It applies the function to each item in an iterable, in parallel.

Let's explore this more with another example. Create a file named pool_map.py:

import multiprocessing
import time
import os

def process_item(item):
    """Process a single item"""
    process_id = os.getpid()
    print(f"Process {process_id} processing item {item}")
    time.sleep(1)  ## Simulate work
    return item * 10

if __name__ == "__main__":
    ## Create a list of items to process
    items = list(range(10))

    ## Get the number of CPU cores available
    num_cores = multiprocessing.cpu_count()
    print(f"System has {num_cores} CPU cores")

    ## Create a Pool with the number of available cores
    with multiprocessing.Pool(processes=num_cores) as pool:
        print("Starting parallel processing...")
        start_time = time.time()

        ## Process all items in parallel
        results = pool.map(process_item, items)

        end_time = time.time()
        print(f"All processing completed in {end_time - start_time:.2f} seconds")
        print(f"Results: {results}")

Run the code:

python3 pool_map.py

Your output should show the items being processed in parallel across multiple processes, with a time significantly shorter than 10 seconds (which would be the sequential processing time for 10 items with a 1-second delay each).

Passing Multiple Arguments with Pool.starmap()

What if your function needs multiple arguments? This is where starmap() comes in.

Create a file named pool_starmap.py:

import multiprocessing
import time

def process_data(id, value, multiplier):
    """Process data with multiple arguments"""
    print(f"Processing item {id}: {value} with multiplier {multiplier}")
    time.sleep(1)  ## Simulate work
    return value * multiplier

if __name__ == "__main__":
    ## Create a list of argument tuples
    ## Each tuple contains all arguments for one function call
    arguments = [
        (1, 5, 2),    ## id=1, value=5, multiplier=2
        (2, 10, 3),   ## id=2, value=10, multiplier=3
        (3, 15, 2),   ## id=3, value=15, multiplier=2
        (4, 20, 4),   ## id=4, value=20, multiplier=4
        (5, 25, 5)    ## id=5, value=25, multiplier=5
    ]

    ## Create a Pool with 3 processes
    with multiprocessing.Pool(processes=3) as pool:
        print("Starting parallel processing with multiple arguments...")
        start_time = time.time()

        ## Process all items in parallel using starmap
        results = pool.starmap(process_data, arguments)

        end_time = time.time()
        print(f"All processing completed in {end_time - start_time:.2f} seconds")
        print(f"Results: {results}")

Run the code:

python3 pool_starmap.py

You should see output showing each item being processed with its own set of arguments, and the final results list containing all the calculated values.

Key Points About Pool

• Pool automatically handles the distribution of tasks across processes
• It manages a queue of tasks and assigns them to available worker processes
• It collects results and returns them in the same order as the input data
• The number of processes can be optimized based on your system's CPU cores and the nature of your task

Advanced Argument Passing Techniques

In the previous steps, we learned about basic argument passing with Process objects and the Pool class. Now, let's explore more advanced techniques for passing arguments in multiprocessing scenarios.

Passing Complex Data with apply() and apply_async()

Sometimes you need more flexibility than what map() and starmap() provide. The apply() and apply_async() methods let you execute a function with specific arguments and handle results more dynamically.

Create a file named pool_apply.py:

import multiprocessing
import time
import random

def process_task(task_id, duration, data):
    """Process a task with multiple arguments including complex data"""
    print(f"Starting task {task_id} with duration {duration}s and data: {data}")
    time.sleep(duration)  ## Simulate variable work time
    result = sum(data) * task_id
    print(f"Task {task_id} completed")
    return result

if __name__ == "__main__":
    ## Create a Pool with 2 processes
    with multiprocessing.Pool(processes=2) as pool:
        print("Starting tasks with apply()...")

        ## Execute tasks with apply() - blocking
        result1 = pool.apply(process_task, args=(1, 2, [1, 2, 3]))
        print(f"Result 1: {result1}")

        result2 = pool.apply(process_task, args=(2, 1, [4, 5, 6]))
        print(f"Result 2: {result2}")

        print("\nStarting tasks with apply_async()...")

        ## Execute tasks with apply_async() - non-blocking
        async_result1 = pool.apply_async(process_task, args=(3, 3, [7, 8, 9]))
        async_result2 = pool.apply_async(process_task, args=(4, 2, [10, 11, 12]))

        ## Do other work while tasks are running
        print("Tasks are running in the background...")
        time.sleep(1)
        print("Main process is doing other work...")

        ## Get results when needed (will wait if not yet available)
        print(f"Result 3: {async_result1.get()}")
        print(f"Result 4: {async_result2.get()}")

Run the code:

python3 pool_apply.py

The output will show how apply() is blocking (sequential) while apply_async() allows tasks to run in parallel.

Using functools.partial for Partial Function Application

When you have a function that takes multiple arguments but you want to fix some of them, functools.partial is very useful.

Create a file named partial_example.py:

import multiprocessing
from functools import partial
import time

def process_with_config(data, config_a, config_b):
    """Process data with specific configuration parameters"""
    print(f"Processing {data} with config_a={config_a}, config_b={config_b}")
    time.sleep(1)
    return data * config_a + config_b

if __name__ == "__main__":
    ## Data to process
    data_items = [1, 2, 3, 4, 5]

    ## Configuration values
    config_a = 10
    config_b = 5

    ## Create a partially applied function with fixed config values
    partial_process = partial(process_with_config, config_a=config_a, config_b=config_b)

    ## Create a Pool
    with multiprocessing.Pool(processes=3) as pool:
        ## Use map with the partially applied function
        print("Starting processing with partial function...")
        results = pool.map(partial_process, data_items)
        print(f"Results: {results}")

Run the code:

python3 partial_example.py

The output will show how each data item is processed with the same configuration values that were fixed with partial().

Passing Arguments via Shared Memory

For large data that shouldn't be copied for each process, shared memory can be more efficient. Let's explore this technique.

Create a file named shared_memory_example.py:

import multiprocessing
import numpy as np
import time

def process_array_chunk(array, start_idx, end_idx):
    """Process a chunk of a shared array"""
    print(f"Processing chunk from index {start_idx} to {end_idx}")

    ## Simulate processing each element
    for i in range(start_idx, end_idx):
        array[i] = array[i] ** 2

    time.sleep(1)  ## Simulate additional work
    return start_idx, end_idx

if __name__ == "__main__":
    ## Create a shared array
    array_size = 1000000
    shared_array = multiprocessing.Array('d', array_size)  ## 'd' for double precision

    ## Fill the array with initial values
    temp_array = np.frombuffer(shared_array.get_obj())
    temp_array[:] = np.arange(array_size)

    print(f"Array initialized with size {array_size}")
    print(f"First 5 elements: {temp_array[:5]}")

    ## Define chunk size and create tasks
    num_processes = 4
    chunk_size = array_size // num_processes
    tasks = []

    for i in range(num_processes):
        start = i * chunk_size
        end = start + chunk_size if i < num_processes - 1 else array_size
        tasks.append((start, end))

    ## Process chunks in parallel
    with multiprocessing.Pool(processes=num_processes) as pool:
        ## Create arguments for each task
        args = [(shared_array, start, end) for start, end in tasks]

        print("Starting parallel processing...")
        start_time = time.time()

        ## Using starmap to pass multiple arguments
        results = pool.starmap(process_array_chunk, args)

        end_time = time.time()
        print(f"Processing completed in {end_time - start_time:.2f} seconds")

    ## Verify results
    print(f"First 5 elements after processing: {temp_array[:5]}")
    print(f"Results summary: {results}")

Run the code:

python3 shared_memory_example.py

The output will show how a large array is processed in chunks by different processes, with all processes having access to the same shared memory.

Performance Comparison

Let's compare these different techniques for a real task.

Create a file named performance_comparison.py:

import multiprocessing
import time
import numpy as np
from functools import partial

def process_data_simple(data):
    """Simple processing function"""
    return sum(x ** 2 for x in data)

def process_data_with_config(data, multiplier):
    """Processing with additional configuration"""
    return sum(x ** 2 for x in data) * multiplier

def run_test(test_name, func, data_chunks, pool_size=4, **kwargs):
    """Run a test and measure performance"""
    start_time = time.time()

    with multiprocessing.Pool(processes=pool_size) as pool:
        if kwargs:
            partial_func = partial(func, **kwargs)
            results = pool.map(partial_func, data_chunks)
        else:
            results = pool.map(func, data_chunks)

    end_time = time.time()
    print(f"{test_name} completed in {end_time - start_time:.4f} seconds")
    return results

if __name__ == "__main__":
    ## Create test data
    chunk_size = 1000000
    num_chunks = 8
    data_chunks = [np.random.rand(chunk_size) for _ in range(num_chunks)]

    print(f"Created {num_chunks} data chunks of size {chunk_size} each")

    ## Test 1: Simple map
    results1 = run_test("Simple map", process_data_simple, data_chunks)

    ## Test 2: Using partial
    results2 = run_test("Map with partial", process_data_with_config,
                        data_chunks, multiplier=2)

    ## Test 3: Sequential processing for comparison
    start_time = time.time()
    seq_results = [process_data_simple(chunk) for chunk in data_chunks]
    end_time = time.time()
    print(f"Sequential processing completed in {end_time - start_time:.4f} seconds")

    ## Validate results
    print(f"\nFirst result from each test:")
    print(f"Test 1 first result: {results1[0]:.4f}")
    print(f"Test 2 first result: {results2[0]:.4f} (should be double of Test 1)")
    print(f"Sequential first result: {seq_results[0]:.4f}")

Run the performance comparison:

python3 performance_comparison.py

The output will show the performance differences between the approaches, demonstrating the benefits of parallel processing and the overhead of different argument passing techniques.

Real-World Application: Parallel Image Processing

Now that we understand various techniques for passing arguments in Python multiprocessing, let's apply these concepts to a real-world scenario: parallel image processing. This is a common use case where multiprocessing can significantly improve performance.

Setting Up the Environment

First, let's install the required packages:

pip install Pillow numpy

Creating Sample Images

Let's create a script to generate some sample images for our processing. Create a file named create_images.py:

from PIL import Image
import numpy as np
import os

def create_sample_image(filename, width=800, height=600):
    """Create a sample image with random data"""
    ## Create random array data
    data = np.random.randint(0, 255, (height, width, 3), dtype=np.uint8)

    ## Create image from array
    img = Image.fromarray(data, 'RGB')

    ## Save the image
    img.save(filename)
    print(f"Created image: {filename}")

if __name__ == "__main__":
    ## Create a directory for sample images
    os.makedirs("sample_images", exist_ok=True)

    ## Create 5 sample images
    for i in range(5):
        create_sample_image(f"sample_images/sample_{i+1}.png")

    print("All sample images created successfully")

Run this script to create sample images:

python3 create_images.py

Sequential Image Processing

Let's first implement a sequential version of our image processing tasks. Create a file named sequential_image_processing.py:

from PIL import Image, ImageFilter
import os
import time

def apply_filters(image_path, output_dir):
    """Apply multiple filters to an image and save the results"""
    ## Load the image
    image_name = os.path.basename(image_path)
    print(f"Processing image: {image_name}")
    img = Image.open(image_path)

    ## Apply filters
    ## 1. Blur filter
    start_time = time.time()
    blurred = img.filter(ImageFilter.GaussianBlur(radius=5))
    blurred.save(os.path.join(output_dir, f"blur_{image_name}"))

    ## 2. Edge detection filter
    edges = img.filter(ImageFilter.FIND_EDGES)
    edges.save(os.path.join(output_dir, f"edges_{image_name}"))

    ## 3. Emboss filter
    emboss = img.filter(ImageFilter.EMBOSS)
    emboss.save(os.path.join(output_dir, f"emboss_{image_name}"))

    ## 4. Sharpen filter
    sharpen = img.filter(ImageFilter.SHARPEN)
    sharpen.save(os.path.join(output_dir, f"sharpen_{image_name}"))

    end_time = time.time()
    processing_time = end_time - start_time

    print(f"Completed processing {image_name} in {processing_time:.2f} seconds")
    return image_name, processing_time

def process_images_sequentially(image_dir, output_dir):
    """Process all images in a directory sequentially"""
    ## Ensure output directory exists
    os.makedirs(output_dir, exist_ok=True)

    ## Get all image files
    image_files = [os.path.join(image_dir, f) for f in os.listdir(image_dir)
                   if f.endswith(('.png', '.jpg', '.jpeg'))]

    print(f"Found {len(image_files)} images to process")

    ## Process each image sequentially
    start_time = time.time()
    results = []

    for image_path in image_files:
        result = apply_filters(image_path, output_dir)
        results.append(result)

    end_time = time.time()
    total_time = end_time - start_time

    print(f"\nSequential processing completed")
    print(f"Total processing time: {total_time:.2f} seconds")
    return results

if __name__ == "__main__":
    ## Define directories
    image_dir = "sample_images"
    output_dir = "processed_sequential"

    ## Process images
    results = process_images_sequentially(image_dir, output_dir)

    ## Print summary
    print("\nProcessing summary:")
    for image_name, proc_time in results:
        print(f"  {image_name}: {proc_time:.2f} seconds")

Run the sequential processing:

python3 sequential_image_processing.py

Parallel Image Processing

Now, let's implement parallel image processing using the techniques we've learned. Create a file named parallel_image_processing.py:

from PIL import Image, ImageFilter
import multiprocessing
import os
import time

def apply_filters(args):
    """Apply multiple filters to an image and save the results"""
    image_path, output_dir = args

    ## Load the image
    image_name = os.path.basename(image_path)
    print(f"Process {os.getpid()} processing image: {image_name}")
    img = Image.open(image_path)

    ## Apply filters
    ## 1. Blur filter
    start_time = time.time()
    blurred = img.filter(ImageFilter.GaussianBlur(radius=5))
    blurred.save(os.path.join(output_dir, f"blur_{image_name}"))

    ## 2. Edge detection filter
    edges = img.filter(ImageFilter.FIND_EDGES)
    edges.save(os.path.join(output_dir, f"edges_{image_name}"))

    ## 3. Emboss filter
    emboss = img.filter(ImageFilter.EMBOSS)
    emboss.save(os.path.join(output_dir, f"emboss_{image_name}"))

    ## 4. Sharpen filter
    sharpen = img.filter(ImageFilter.SHARPEN)
    sharpen.save(os.path.join(output_dir, f"sharpen_{image_name}"))

    end_time = time.time()
    processing_time = end_time - start_time

    print(f"Process {os.getpid()} completed {image_name} in {processing_time:.2f} seconds")
    return image_name, processing_time

def process_images_in_parallel(image_dir, output_dir, num_processes=None):
    """Process all images in a directory in parallel"""
    ## Ensure output directory exists
    os.makedirs(output_dir, exist_ok=True)

    ## Get all image files
    image_files = [os.path.join(image_dir, f) for f in os.listdir(image_dir)
                   if f.endswith(('.png', '.jpg', '.jpeg'))]

    print(f"Found {len(image_files)} images to process")

    ## Determine number of processes
    if num_processes is None:
        num_processes = min(multiprocessing.cpu_count(), len(image_files))

    print(f"Using {num_processes} processes for parallel processing")

    ## Create arguments for each task
    task_args = [(image_path, output_dir) for image_path in image_files]

    ## Process images in parallel
    start_time = time.time()

    with multiprocessing.Pool(processes=num_processes) as pool:
        results = pool.map(apply_filters, task_args)

    end_time = time.time()
    total_time = end_time - start_time

    print(f"\nParallel processing completed")
    print(f"Total processing time: {total_time:.2f} seconds")
    return results

if __name__ == "__main__":
    ## Define directories
    image_dir = "sample_images"
    output_dir = "processed_parallel"

    ## Process images
    results = process_images_in_parallel(image_dir, output_dir)

    ## Print summary
    print("\nProcessing summary:")
    for image_name, proc_time in results:
        print(f"  {image_name}: {proc_time:.2f} seconds")

Run the parallel processing:

python3 parallel_image_processing.py

Advanced Parallel Processing with Different Filters

Now, let's implement a more complex version where we apply different filters to the same image in parallel. Create a file named advanced_parallel_processing.py:

from PIL import Image, ImageFilter
import multiprocessing
import os
import time
from functools import partial

def apply_filter(filter_info, image_path, output_dir):
    """Apply a specific filter to an image"""
    filter_name, filter_obj = filter_info

    ## Load the image
    image_name = os.path.basename(image_path)
    print(f"Process {os.getpid()} applying {filter_name} to {image_name}")

    ## Open image each time to avoid potential issues with sharing PIL objects
    img = Image.open(image_path)

    ## Apply filter
    start_time = time.time()
    filtered = img.filter(filter_obj)

    ## Save the filtered image
    output_filename = f"{filter_name}_{image_name}"
    output_path = os.path.join(output_dir, output_filename)
    filtered.save(output_path)

    end_time = time.time()
    processing_time = end_time - start_time

    print(f"Process {os.getpid()} completed {filter_name} on {image_name} in {processing_time:.2f} seconds")
    return filter_name, image_name, processing_time

def process_image_with_multiple_filters(image_path, output_dir, filters):
    """Process an image with multiple filters in parallel"""
    ## Create a partial function with fixed image_path and output_dir
    apply_filter_to_image = partial(apply_filter,
                                   image_path=image_path,
                                   output_dir=output_dir)

    ## Process the image with multiple filters in parallel
    with multiprocessing.Pool() as pool:
        results = pool.map(apply_filter_to_image, filters)

    return results

def main():
    ## Define directories
    image_dir = "sample_images"
    output_dir = "processed_advanced"
    os.makedirs(output_dir, exist_ok=True)

    ## Define filters to apply
    filters = [
        ("blur", ImageFilter.GaussianBlur(radius=5)),
        ("edges", ImageFilter.FIND_EDGES),
        ("emboss", ImageFilter.EMBOSS),
        ("sharpen", ImageFilter.SHARPEN),
        ("contour", ImageFilter.CONTOUR),
        ("detail", ImageFilter.DETAIL),
        ("smooth", ImageFilter.SMOOTH),
        ("smoothmore", ImageFilter.SMOOTH_MORE)
    ]

    ## Get all image files
    image_files = [os.path.join(image_dir, f) for f in os.listdir(image_dir)
                   if f.endswith(('.png', '.jpg', '.jpeg'))]

    print(f"Found {len(image_files)} images to process with {len(filters)} filters each")

    ## Process each image with all filters
    all_results = []
    overall_start = time.time()

    for image_path in image_files:
        image_name = os.path.basename(image_path)
        print(f"\nProcessing {image_name} with {len(filters)} filters in parallel")

        start_time = time.time()
        results = process_image_with_multiple_filters(image_path, output_dir, filters)
        end_time = time.time()

        image_time = end_time - start_time
        print(f"Completed all filters for {image_name} in {image_time:.2f} seconds")

        all_results.extend(results)

    overall_end = time.time()
    total_time = overall_end - overall_start

    ## Print summary
    print("\nProcessing Summary:")
    print(f"Total processing time: {total_time:.2f} seconds")
    print(f"Total operations: {len(all_results)}")

    ## Calculate average time per filter
    avg_time = sum(time for _, _, time in all_results) / len(all_results)
    print(f"Average processing time per filter: {avg_time:.2f} seconds")

if __name__ == "__main__":
    main()

Run the advanced parallel processing:

python3 advanced_parallel_processing.py

Comparing Performance

Let's create a script to compare the performance of the different approaches. Create a file named compare_performance.py:

import os
import time
import subprocess
import matplotlib.pyplot as plt
import numpy as np

def run_script(script_name):
    """Run a Python script and measure its execution time"""
    print(f"Running {script_name}...")
    start_time = time.time()

    ## Run the script
    process = subprocess.run(['python3', script_name],
                            capture_output=True, text=True)

    end_time = time.time()
    execution_time = end_time - start_time

    print(f"Completed {script_name} in {execution_time:.2f} seconds")
    print(f"Output: {process.stdout[-200:]}") ## Show last part of output

    return execution_time

def main():
    ## Scripts to compare
    scripts = [
        'sequential_image_processing.py',
        'parallel_image_processing.py',
        'advanced_parallel_processing.py'
    ]

    ## Run each script and measure time
    times = []
    for script in scripts:
        if os.path.exists(script):
            times.append(run_script(script))
        else:
            print(f"Script {script} not found!")
            times.append(0)

    ## Plot results
    if any(times):
        labels = ['Sequential', 'Basic Parallel', 'Advanced Parallel']
        x = np.arange(len(labels))

        plt.figure(figsize=(10, 6))
        plt.bar(x, times, color=['red', 'blue', 'green'])
        plt.xlabel('Processing Method')
        plt.ylabel('Execution Time (s)')
        plt.title('Image Processing Performance Comparison')
        plt.xticks(x, labels)

        ## Add execution time as text on bars
        for i, v in enumerate(times):
            plt.text(i, v + 0.1, f"{v:.2f}s", ha='center')

        ## Save the plot
        plt.tight_layout()
        plt.savefig('performance_comparison.png')
        print("Performance comparison plot saved as 'performance_comparison.png'")
    else:
        print("No valid execution times to plot")

if __name__ == "__main__":
    main()

Let's run the comparison:

python3 compare_performance.py

Review the results and examine the performance_comparison.png image to see the differences in execution time between the different approaches.

Key Takeaways

From this real-world example, you can observe several important aspects of multiprocessing:

1. Parallel processing can significantly reduce execution time when dealing with CPU-intensive tasks like image processing.

2. The method of passing arguments affects both code complexity and performance:

   • Simple map() for straightforward single-argument functions
   • Partial functions for fixing certain parameters
   • Advanced techniques for complex workflows

3. Considerations for real-world applications:

   • Resource management (CPU, memory)
   • Task distribution strategies
   • Error handling in parallel environments
   • Coordination between processes

This practical example demonstrates how to apply the multiprocessing concepts we've learned throughout this lab to solve real-world problems efficiently.

Summary

In this lab, you learned about Python multiprocessing and different techniques for passing arguments to parallel processes. Here's a summary of what you've accomplished:

1. Basics of Multiprocessing: You created your first multiprocessing program and understood the difference between processes and threads.

2. Argument Passing Techniques: You explored various methods for passing arguments:

   • Basic argument passing with Process.args
   • Single argument passing with Pool.map()
   • Multiple arguments with Pool.starmap()
   • Dynamic execution with apply() and apply_async()
   • Using functools.partial for partial function application
   • Shared memory for efficient data sharing

3. Real-World Application: You applied these concepts to a practical image processing example, demonstrating significant performance improvements through parallelization.

The skills you've learned in this lab are valuable for any CPU-intensive Python application, allowing you to leverage the full computational power of multi-core systems. As you develop more complex applications, remember to consider the trade-offs between different argument passing techniques and choose the most appropriate method for your specific use case.