Linux Memory Management: Understanding Page Tables, Swapping, and Memory Allocation

Introduction

Memory management is a key aspect in modern operating systems, ensuring efficient allocation and release of system memory. As a powerful and widely used operating system, Linux uses sophisticated techniques to efficiently manage memory. Understanding key concepts such as page tables, swaps, and memory allocation is critical to system administrators, developers, and anyone using Linux at the bottom.

This article discusses Linux memory management in depth, explores the complexity of page tables, the role of exchange, and different memory allocation mechanisms. After reading, readers will gain insight into how Linux handles memory and how to optimize memory for better performance.

Understand Linux page tables

What is virtual memory? Like most modern operating systems, Linux implements virtual memory, providing the process with an illusion of a huge continuous memory space. Virtual memory is able to multitask efficiently, isolate processes, and access more memory than physically available memory. The core mechanism of virtual memory is the page table , which maps virtual addresses to physical memory locations.

How page tables Work Page tables are data structures used by the Linux kernel to convert virtual addresses into physical addresses. Since memory is managed in fixed-size blocks called pages (usually 4KB in size), each process maintains a page table that tracks which virtual pages correspond to which physical pages.

Multi-level page table

Due to the large address space in modern computing (for example, a 64-bit architecture), single-level page tables are inefficient and consume too much memory. Therefore, Linux uses a hierarchical multi-level page table method:

1. Single-level page table (used in old 32-bit systems with a small amount of memory)
2. Two-level page table (improving efficiency by breaking page tables into smaller chunks)
3. Level 3 page table (for better scalability in some architectures)
4. Level 4 page table (standard in modern 64-bit Linux systems that break the address into smaller parts)

Each level helps to find the next part of the page table until the last entry, which contains the actual physical address.

Page table entry (PTE) and its components* Page table entry (PTE)**Contains basic information, such as:

• Physical page frame number.
• Access control bit (read/write/execute permission).
• A bit exists (indicates whether the page is in RAM or has been swapped to disk).
• Dirty (indicates whether the page has been modified).
• Reference bits (for page permutation algorithm).

Performance Note: Convert Bypass Buffer (TLB) Because it is slow to traverse multi-level page tables every memory access, modern CPUs use hardware caches called Convert Bypass Buffer (TLB) . TLB stores recent virtual to physical address translations, greatly improving performance by reducing the number of memory accesses required.

Swap in Linux: Extend memory beyond physical limitations

What is exchange? Swap is a mechanism in which Linux moves infrequently used memory pages from RAM to disk (swap space) when memory is insufficient. This process allows the system to handle workloads that exceed the available physical memory.

How swap works Linux reserves dedicated swap space, which can be:

• Swap partition (a separate disk partition dedicated to swap).
• Swap files (files on file systems used as swap space).

When the process requires more memory than available, the kernel uses the page permutation algorithm to decide which pages to swap out.

Page Permutation Algorithm Linux uses different algorithms to decide which pages to swap out:

• Least recently used (LRU) : The longest unused page is first exchanged.
• Recently Not Used (NRU) : Classify pages based on page access and modification bits.
• Clock Algorithm : A simplified version of LRU that can effectively approximate usage.

Manage exchange usage** swappiness parameter controls the positiveness of Linux exchange pages. This value ranges from 0 to 100**:

• Low value (for example, 10-20): Keep the page in RAM as long as possible.
• High values ​​(e.g., 60-100): More aggressively exchange to release RAM.

To check and adjust swappiness:

 <code>cat /proc/sys/vm/swappiness sudo sysctl vm.swappiness=30</code>

To monitor exchange usage:

 <code>free -m vmstat 2 swapon -s</code>

Optimize Switching Performance - Use fast SSDs for swap storage to reduce performance degradation.

• Make sure there is enough RAM to minimize swaps.
• Adjust swappiness according to workload requirements.

Memory allocation in Linux

Physical memory and virtual memory allocation Linux divides memory into three areas:

• DMA (Direct Memory Access) : Reserved for hardware that requires direct memory access.
• Normal area : memory available to the kernel and user processes.
• High memory : Used when physical memory exceeds the direct addressable range.

Kernel Memory Allocation Mechanism 1. Partner System : Allocate memory in power of 2 blocks to reduce fragmentation. 2. Slab allocator : Effectively manage small objects that are frequently allocated/released. 3. SLOB and SLUB allocators : Alternative allocation strategies optimized for different workloads.

Userspace memory allocation - malloc(): Allocate memory in userspace.

• brk() & sbrk(): Resize the process heap.
• mmap(): Allocate large memory areas directly from the kernel.

Handling Out of Memory (OOM) Situation When memory is exhausted, Linux OOM Killer selects and terminates the process to free up RAM. The logs can be checked by:

 <code>dmesg | grep -i 'oom'</code>

Practical insights and best practices

Monitor memory usage - use top and htop for real-time monitoring.

• Use free -m to get memory statistics.
• Use /proc/meminfo to get detailed information.
• Use pmap to get the memory map for a specific process.

Optimize memory performance - Tune swappiness to balance RAM and swap usage.

• Use memory cgroups to limit the memory consumption of processes.
• Use huge pages for large memory allocation.
• Optimize application memory footprint to prevent overswap.

in conclusion

Understanding Linux memory management—page tables, swaps, and memory allocation—enables system administrators and developers to effectively optimize performance and troubleshoot problems. Through tools and technologies to monitor, tune, and enhance memory processing, Linux remains a powerful and flexible operating system for a variety of workloads.

By mastering these concepts, you can ensure that your system runs efficiently and responds well under memory constraints, improving overall performance and reliability.

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