38:30
MM101: Introduction to Linux Memory Management - Christopher Lameter, Jump Trading LLC
The Linux Foundation
Overview
This video introduces the fundamental concepts of memory management in Linux, explaining how the operating system handles physical and virtual memory. It details the role of pages, page tables, and the Memory Management Unit (MMU) in translating virtual addresses to physical ones. The presentation covers on-demand paging, memory mapping, copy-on-write, swapping, and the importance of page caching. It also guides viewers on how to monitor memory usage, configure system memory behavior like overcommit, and where to find further information for deeper exploration.
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Chapters
- Linux kernel manages memory in fixed-size units called pages, typically 4KB.
- Physical memory is divided into page frames, each corresponding to a page.
- The Memory Management Unit (MMU) in hardware uses page frames and offsets to access physical memory.
Understanding pages and frames is crucial because it's the fundamental unit of memory management, enabling efficient allocation and access.
A 4GB system with 4KB pages has over a million page frames.
- Each process has its own virtual address space, providing isolation and security.
- Page tables map virtual addresses used by a process to physical page frame numbers.
- This mapping allows different processes to use the same virtual address but refer to different physical memory locations.
Virtual memory and page tables allow multiple processes to run concurrently without interfering with each other, enhancing system stability and security.
Two processes can both use address 0x1000, but this address might map to physical memory frame 5 for one process and physical memory frame 20 for another.
- Memory is loaded into physical RAM only when it's actually accessed (on-demand paging).
- When a process accesses a page not yet in memory, a page fault occurs, and the OS loads it from disk.
- Memory mapping (mmap) allows files or devices to be mapped directly into a process's address space, treating disk content as memory.
On-demand paging and memory mapping allow processes to use more memory than physically available, improving efficiency and enabling larger applications.
A large executable file is only loaded into physical memory as its code sections are executed, not all at once.
- Shared libraries or binaries can occupy a single physical page frame, mapped into multiple processes' virtual address spaces to save memory.
- Copy-on-Write (COW) is a technique where shared pages are initially mapped read-only; a write attempt triggers a page fault, causing the OS to create a private copy for the writing process.
- Process forking heavily utilizes COW, as the child process initially shares memory with the parent until it modifies it.
Sharing memory and using copy-on-write significantly reduce memory consumption and improve performance, especially when creating new processes.
When a process forks, the new process inherits the parent's memory mappings, but if the child writes to a shared page, a unique copy is created for it.
- When physical memory is scarce, the kernel can move less-used pages from RAM to a swap area on disk (swapping).
- Pages that can be easily re-read from disk (like read-only file data) can be discarded from memory without swapping.
- Swapping significantly degrades system performance due to the slow speed of disk I/O.
Swapping allows the system to continue operating even when memory is over-committed, but it comes at a significant performance cost.
If a system runs out of RAM, infrequently accessed application data might be written to the swap partition on the hard drive.
- The page cache stores recently accessed data from disk in memory to speed up future reads.
- Modified pages that haven't been written back to disk are called 'dirty pages'.
- The system periodically writes dirty pages back to disk to maintain data consistency, especially before a shutdown or when memory is needed.
Page caching optimizes disk I/O, while managing dirty pages ensures data integrity and prevents data loss.
After modifying a file, it becomes a 'dirty page' in memory; the `sync` command ensures all dirty pages are written to disk.
- Tools like `/proc/meminfo`, `free`, `top`, and `vmstat` provide insights into system memory usage.
- The `overcommit_memory` setting controls how the kernel handles requests for more virtual memory than physical RAM is available.
- Kernel parameters in `/proc/sys/vm/` allow fine-tuning of memory management behaviors like dirty page writeback thresholds.
Monitoring and configuration allow administrators to understand memory behavior, diagnose performance issues, and tune the system for specific workloads.
Checking `/proc/meminfo` shows total memory, free memory, cached memory, and swap usage.
- Individual process memory usage can be inspected via `/proc/<pid>/status` and tools like `top`.
- Key metrics include virtual memory size (VM_SIZE), resident set size (RSS), and swap usage (VM_SWAP).
- The `ulimit` command or system configuration can set resource limits for processes, such as maximum virtual memory or locked memory.
Understanding individual process memory footprints helps in identifying memory leaks and optimizing application performance by setting appropriate limits.
A process might have a large virtual memory size but a small actual physical memory usage due to sharing and zero pages.
Key takeaways
- Linux memory management is built around the concept of pages, which are fixed-size blocks of memory.
- Virtual memory provides each process with its own isolated address space, mapped to physical memory via page tables.
- On-demand paging and memory mapping allow systems to use more memory than physically available by loading data only when needed.
- Techniques like shared pages and copy-on-write optimize memory usage by avoiding redundant data storage.
- Swapping to disk is a fallback mechanism for low memory situations but severely impacts performance.
- Page caching speeds up I/O by keeping frequently accessed disk data in RAM.
- Monitoring tools and kernel parameters are essential for understanding and tuning system memory behavior.
- Resource limits can be set per process to prevent runaway memory consumption.
Key terms
PagePage FrameMemory Management Unit (MMU)Page TableVirtual MemoryPhysical MemoryPage FaultOn-Demand PagingMemory Mapping (mmap)Copy-on-Write (COW)SwappingPage CacheDirty PageOvercommitProcess ID (PID)
Test your understanding
- How does the Linux kernel use pages and page frames to manage physical memory?
- What is the role of page tables in translating virtual addresses to physical addresses for each process?
- Explain the concept of on-demand paging and why it's beneficial for system performance and memory utilization.
- How do techniques like shared pages and copy-on-write contribute to efficient memory management?
- What are the performance implications of swapping, and when does the system resort to this mechanism?