20
Lesson 20
Handling large data files with memory mapping
Objective
By the end of this lesson, students will understand how to handle large data files efficiently using memory mapping in Python with the mmap module. They will learn the benefits of memory mapping for accessing file content without loading the entire file into memory.
1. Introduction to memory mapping:
Memory mapping allows a file to be mapped directly into the address space of a process, enabling efficient access to file contents.
It can be particularly beneficial for handling large files that may not fit entirely in memory.
2. The 'mmap' Module:
The mmap module provides a way to create memory-mapped file objects.
These objects can be accessed like byte arrays, allowing for direct manipulation of file data.
By the end of this lesson, students will understand how to handle large data files efficiently using memory mapping in Python with the mmap module. They will learn the benefits of memory mapping for accessing file content without loading the entire file into memory.
1. Introduction to memory mapping:
Memory mapping allows a file to be mapped directly into the address space of a process, enabling efficient access to file contents.
It can be particularly beneficial for handling large files that may not fit entirely in memory.
2. The 'mmap' Module:
The mmap module provides a way to create memory-mapped file objects.
These objects can be accessed like byte arrays, allowing for direct manipulation of file data.
import mmap
3. Creating a memory-mapped file:
You can create a memory-mapped file by opening a file and using the mmap() function.
Syntax: mmap.mmap(fileno, length, access=mmap.ACCESS_WRITE)
with open("largefile.txt", "r+b") as f:
# Create a memory-mapped file with the same size as the file
mmapped_file = mmap.mmap(f.fileno(), 0) # 0 means the whole file4. Accessing data with memory mapping:
You can read and write to the memory-mapped file using indexing and slicing, similar to working with a byte array.
This allows for efficient access to any part of the file without reading the entire file into memory.
with open("largefile.txt", "r+b") as f:
mmapped_file = mmap.mmap(f.fileno(), 0)
# Read a specific portion of the file
print(mmapped_file[0:10]) # Print the first 10 bytes5. Modifying data with memory mapping:
You can modify data directly in the memory-mapped file, which reflects changes in the underlying file.
with open("largefile.txt", "r+b") as f:
mmapped_file = mmap.mmap(f.fileno(), 0)
mmapped_file[0:5] = b'Hello' # Modify the first 5 bytes6. Advantages of memory mapping:
- Efficiency: Accessing parts of large files without loading the entire file into memory.
- Speed: Faster file I/O operations due to reduced overhead compared to traditional file handling.
- Convenience: Allows file content to be treated like an array, making it easy to read and manipulate.
7. Limitations of memory mapping:
Not suitable for all types of files, especially very small files where traditional methods may be faster.
Memory mapping can consume more virtual memory space, especially if many large files are mapped simultaneously.
Modifications may not be written back to the disk unless explicitly done, depending on the access mode.
8. Best Practices for Using 'mmap' :
- Always ensure the file is opened in a suitable mode ('r+b' for reading and writing).
- Unmap the memory-mapped file using mmapped_file.close() to free resources after use.
- Handle exceptions and errors, especially when working with large files, to avoid data corruption.
9. Exercises:
Exercise 1: Memory mapping a file
1. Write a program that memory maps a large text file and prints the first 20 bytes.
Exercise 2: Modify a file using memory mapping
1. Implement a script that modifies a specific portion of a memory-mapped file and verifies the changes by reading back the modified content.
Exercise 3: Performance comparison
1. Create two versions of a file processing function: one using standard file reading and one using memory mapping. Measure and compare their performance on a large file.
Exercise 4: Handling Binary files
1. Write a program that uses memory mapping to read and modify a binary file, ensuring the data integrity is maintained.
Conclusion
In this lesson, students learned how to handle large data files efficiently using the mmap module in Python. They explored the benefits of memory mapping for direct access to file content without loading it entirely into memory. Understanding these concepts will enable them to manage large datasets effectively in their applications, optimizing performance and resource usage.