Understanding and Managing Linux Memory and Disk Usage

The process of managing the computer's memory resources, including memory allocation, deallocation, and maintenance for active apps, the operating system, and other system processes, is referred to as memory management in Linux.

Multiple applications can operate simultaneously on Linux without interfering with one another due to the virtual memory model it uses to map memory addresses from a program to physical memory locations. In Linux, the kernel is in charge of managing memory allocation and deallocation. It also ensures that each task has access to the memory it requires to run.

So this whole process is known as memory management in Linux. You will learn more about the process and operation of memory management in Linux further in this article.

Key Linux Memory Management Concepts Overview

The following are some of the key concepts that are involved in memory management in Linux.

Virtual Memory Primer

Virtual memory is a technique that allows a computer to use more memory than it physically has by using hard disk space as an extension of physical memory. In Linux, virtual memory is implemented using the Paging mechanism. Paging divides the memory into smaller chunks called pages, which can be allocated or deallocated as needed.

When an application requires more memory than is available in physical memory, the Linux kernel uses the hard disk as an extension of physical memory, which is known as swap space. This technique in the memory management of Linux also helps prevent memory-related crashes and improves system stability.

Next, you need to know about the concept of Memory Pages which is known to be a crucial segment in the Linux operating system.

Concept of Memory Pages

Memory pages are the fundamental unit of memory allocation and management in the Linux operating system. On most modern hardware architectures, a page is a fixed-sized block of contiguous memory that is usually 4KB in size. The Linux kernel divides the system's accessible physical memory into pages, each with its physical address.

These pages are then mapped to virtual addresses, allowing each process to have its address area that corresponds to physical memory. The Virtual Memory Manager in the kernel is in charge of this allocation. In the context of Linux memory management, memory pages are an essential part that allows the effective allocation and administration of memory resources for individual processes operating within the system.

So that was a brief definition of Memory Pages, now we will move on to understand the types of memory pages and we will start with Huge Pages.

Huge Pages

Huge pages in Linux are large memory pages that can significantly improve system performance by reducing the overhead of memory management. They are typically 2MB or 1GB in size and can be allocated dynamically by the kernel or manually by system administrators.

Huge pages in the memory management of Linux are particularly useful for applications with large working sets, such as databases and scientific computing, because they reduce the number of page table entries needed to manage a given amount of memory, which results in faster data access, reduced CPU usage, and improved overall system performance.

Next, let's explore Zones in Linux.

Zones

In Linux, a zone is a logical grouping of memory pages with similar characteristics, such as access permissions or physical location. Each zone is managed separately by the kernel and can be used for a specific purpose, such as allocating memory for user processes or the kernel itself. There are several types of zones in Linux, including the user zone, kernel zone, and I/O zone. Each zone has its memory management policies and can be further subdivided into smaller zones for more fine-grained control.

Using zones in Linux enables more efficient memory allocation and can help prevent memory fragmentation, which can lead to performance issues. Zones in the memory management of Linux also provide greater flexibility and control over memory usage, allowing system administrators to optimize memory allocation for specific applications or workloads.

The next one is Page Cache, an essential concept for managing a system's memory.

Page Cache

Page cache in Linux is a mechanism for caching data from disk in memory to speed up access times. When data is read from a disk, it is stored in the page cache to be quickly accessed if needed. It can significantly improve system performance and reduce the required disk reads.

The kernel manages the page cache, which allocates memory to the cache as needed and evicts data when memory is needed for other purposes. It is commonly used for file system operations but also for other types of I/O operations, such as network I/O. Page cache in the memory management of Linux is an important tool for optimizing system performance and reducing I/O latency.

Now we will learn about Nodes, the module which connects memory chunks.

Nodes

In the memory management of Linux, a node refers to a physical or logical grouping of memory identified by a unique address within the system. Each node can be managed separately by the kernel and allocated for specific purposes, such as user processes or the kernel itself.

In simple words, Nodes are a way of dividing the system memory into discrete chunks. Each node has its memory and is responsible for managing its memory allocations. And Nodes are useful in systems with multiple processors, where the processors can access memory in their node more quickly than in another node.

The next topic is Anonymous Memory, and the name says it all, let's find out.

Anonymous Memory

In Linux, anonymous memory refers to a type of memory allocated dynamically by the kernel at runtime and not associated with a specific file or device. Processes use it to store data that does not need to be permanently stored on disk, such as program stack and heap memory. The use of anonymous memory in Linux memory management allows processes to assign and free memory actively without needing to know the physical location of the memory.

It makes memory management more efficient and reduces the probability of memory fragmentation. Anonymous memory is also used for interprocess communication, whereas shared memory allows multiple processes to access the same data. It can improve performance and reduce memory overhead by reducing the need for interprocess communication over the network.

We now move on to OOM Killer.

OOM Killer

The Out-Of-Memory (OOM) Killer is a Linux kernel feature responsible for reclaiming memory from processes that have exhausted the system's memory resources. When a system runs low on available memory, it can start experiencing performance issues or even become unresponsive. The OOM killer is designed to prevent this by automatically terminating processes that consume a disproportionate amount of memory.

The OOM killer in Linux memory management is triggered when the system has no more free memory available and the kernel cannot allocate any more memory to processes. At this point, the kernel selects a process to kill based on a set of questions that aim to identify the process causing the most memory pressure on the system. The questionnaire considers the amount of memory the process uses, age, and priority, among other factors.

Let's learn about Compaction.

Compaction

In Linux memory management, Compaction is a kernel mechanism used to defragment a system's memory. It relocates memory pages to create larger adjoining blocks of free memory. It helps to reduce memory fragmentation and improve overall system performance.

The compaction process works by identifying memory pages that are not currently in use and relocating them to create larger gaps between active memory pages. It can be particularly useful when a system is experiencing memory pressure and needs to allocate new pages quickly.

The next concept that we need to understand is Reclaim. Let's follow.

Reclaim

In Linux, reclaim refers to the process of freeing up memory that is no longer being actively used by a process or application. When a process or application allocates memory, it reserves a portion of the system's memory for use. The Linux kernel provides several mechanisms for memory reclaim, including page reclamation, slab reclamation, and direct reclaim.

Page reclamation involves freeing up individual memory pages that are no longer actively used. Slab reclamation involves freeing up kernel memory for data structures such as caches and buffers. Direct reclaim involves reclaiming memory directly from processes when the system is under memory pressure.

We will find out about CMA Debugfs Interface which is known for memory allocation.

CMA Debugfs Interface

The CMA (Contiguous Memory Allocator) Debugfs Interface in Linux provides a way to view and modify CMA parameters and settings. It allows developers and administrators to monitor CMA usage, view allocation statistics, and adjust allocation policies. The Debugfs interface provides a convenient way to troubleshoot and optimize CMA usage in systems that require contiguous memory allocation, such as video processing, graphics rendering, and other multimedia applications.

By using the CMA Debugfs interface, administrators can fine-tune memory allocation and ensure that the system uses memory efficiently. It provides various information, such as the amount of memory allocated, the number of pages used, and the amount of free memory available.

HugeTLB Pages

HugeTLB (Transparent Huge Pages) is a feature that enables processes to allocate memory in large, contiguous blocks known as huge pages. These pages are typically much larger than the standard page size used by the system, which can help to reduce memory overhead and improve system performance in certain use cases, such as database management, scientific computing, and high-performance computing.

The kernel manages HugeTLB pages, which can dynamically allocate and release memory as needed, and provides tools and interfaces for configuring and monitoring HugeTLB usage.

Idle Page Tracking

Idle Page Tracking is a technique that allows the kernel to identify memory pages that are not being used by any application. These pages can be reclaimed by the kernel and used for other purposes. It is a memory management feature in Linux that tracks pages of memory not currently being used by any process or application. The kernel periodically scans memory to identify idle pages and marks them as such.

This information can be used by other memory management features, such as compaction and swap, to optimize memory usage and reduce fragmentation. IPT is particularly useful in virtualized environments, where memory overcommitment can be a significant issue. The kernel can more effectively manage memory allocation and prevent memory pressure by tracking idle pages.

Kernel Samepage Merging

Kernel Samepage Merging (KSM) is a memory-saving feature in Linux that allows multiple processes to share identical memory pages. KSM identifies and merges identical memory pages into a single page, thereby reducing the amount of memory used in the system. This feature is particularly useful in virtualized environments where multiple virtual machines run on the same physical server.

By sharing identical memory pages, KSM reduces the amount of memory required by each virtual machine, allowing more virtual machines to run on a single server. KSM is enabled by default in many Linux distributions and can be configured to suit specific requirements. It is a crucial part of memory management in Linux.

Configuration of Kernel

Kernel configuration for memory management in Linux involves selecting the appropriate options and parameters to optimize available memory resources. The configuration options include memory allocation algorithms, page replacement policies, swap settings, and other memory-related parameters. For instance, depending on the specific use case, the kernel can be configured to use different memory allocation algorithms, such as SLAB, SLUB, or SLOB.

The page replacement policy can also be configured with options such as Least Recently Used (LRU), Clock, and Random. Proper kernel configuration for memory management is crucial for efficiently using memory resources, ensuring system stability, and preventing memory-related issues such as crashes, hangs, and performance degradation.

Further Key Points on No-MMU Memory

No-MMU (Memory Management Unit) memory in Linux is a type of memory that does not support memory protection and virtual memory management. This type of memory is commonly found in embedded systems and other devices with limited resources.

Further key points to note about no-MMU memory in the memory management of Linux include the following:

• No-MMU memory is accessed directly by the processor without virtual memory management.
• Any process can access any memory location without memory protection, potentially causing stability and security issues.
• Applications running on no-MMU systems must be designed to work within the system's memory limitations.
• No-MMU systems typically have a smaller memory footprint and are more power-efficient than systems with MMU-enabled memory.
• Linux provides support for no-MMU memory through special kernel modules and configuration settings.

Memory Map

Memory mapping is a fundamental concept in Linux that allows applications to access memory more efficiently and flexibly. Memory mapping involves creating a virtual memory map that maps the application's memory address space to the physical memory on the system. It allows the application to access memory locations as if they were part of its own process space without knowing the actual physical address.

The Linux kernel provides several system calls that allow applications to create memory maps, such as mmap() and munmap(). Memory mapping is used extensively in Linux for various purposes, such as file I/O, inter-process communication, and shared memory.

Multi-level Page Table vs Huge Page

Multi-level page tables and huge pages are two memory management techniques used in Linux, each with advantages and disadvantages.

Multi-level page tables use a hierarchical structure to manage memory, with each level representing a different page size. It allows for more efficient use of memory, as smaller pages can be used for smaller memory allocations, while larger pages can be used for larger allocations.

On the other hand, huge pages use much larger page sizes than standard pages, typically between 2MB and 1GB, as mentioned earlier in this article. It can reduce the overhead of managing large amounts of memory, as fewer page table entries are required. However, huge pages can also lead to wasted memory and reduced flexibility in memory allocation.

Virtual Memory Space Distribution

In Linux, virtual memory space is divided into several parts, each with a specific purpose. The lowest part of the virtual memory space is reserved for the kernel, which contains the operating system's code and data structures. The upper part of the virtual memory space is reserved for user-space applications, which are divided into two parts: the text segment (containing the executable code) and the data segment (containing initialized and uninitialized data).

A stack segment is also used for storing local variables and function calls, while a heap segment is used for dynamic memory allocation. The virtual memory space distribution in Linux is carefully designed to ensure the efficient use of memory resources and prevent memory-related issues.

Memory Allocation and Reclamation

Memory allocation and reclamation are essential aspects of memory management in Linux. Memory allocation involves reserving a portion of memory for a specific purpose, such as storing data or executing code. It can be done using malloc(), kmalloc(), or other memory allocation functions.

Memory reclamation, however, involves freeing up memory that is no longer needed or in use. It can use free(), kfree(), or other memory deallocation functions. In Linux, the kernel carefully manages memory allocation and reclamation to prevent memory leaks, fragmentation, and other memory-related issues that can lead to system instability and performance degradation.

How to Check Memory Usage in Linux?

In Linux, several commands can be used to check memory usage. The most commonly used command is free, which displays the system's total amount of free and used physical and swap memory. Another useful command is top, which displays real-time information about memory usage by running processes.

Check Memory Usage with the "ps" Command

The "ps" command can also display memory usage information about specific processes. Other commands, such as "vmstat" and "sar", provide more detailed information about system memory usage over time. Knowing how to check memory usage in Linux is essential for system administrators and users, as it allows them to monitor system performance and diagnose memory-related issues.

Let's learn more about the two most commonly used commands in the memory management of Linux to check memory usage.

Check Memory Usage with "free" Command

The command free displays the total amount of free and used physical and swap memory on the system and the buffers and cache used by the kernel. The output is displayed in kilobytes by default but can be changed using the -m or -g options to display the information in megabytes or gigabytes, respectively.

The free command is a useful tool for monitoring system performance and diagnosing memory-related issues, as it provides real-time information about the system's memory usage.

Check Memory Usage with "top" Command

The top command displays real-time information about system processes and their resource utilization, including memory usage. When executed, the "top" command displays a list of running processes in order of resource utilization, with the most resource-intensive processes listed first. The command also displays information about the system's overall resource usage, including CPU and memory usage.

The "top" command can monitor system performance, identify resource-intensive processes, and diagnose performance issues. To exit the top, press the Q key.

Disk Usage Management in Linux

Disk usage management is another critical aspect of system administration in Linux. Proper disk usage management ensures enough space on the system for storing data and running applications. The following are some tips for managing disk usage in Linux.

Monitor Disk Usage Regularly

It is important to monitor disk usage regularly to ensure enough space is available on the system. You can use tools such as df, du, and ncdu to monitor disk usage.

Remove Unnecessary Files

Removing unnecessary files is an effective way to free up disk space. You can use the rm command to remove no longer needed files.

Compress Files

Compressing files is another effective way to free up disk space. You can use tools such as gzip and tar to compress files.

Use a Disk Quota System

A disk quota system is a good way to manage disk usage and prevent users from using too much disk space. The disk quota system allows you to limit the amount of disk space users can use.

Use a File System with Built-in Compression

A file system with built-in compression is another effective way to save disk space. File systems such as ZFS and Btrfs support built-in compression, which can save a significant amount of disk space.

Use a Separate Partition for User Data

A separate partition for user data is a good way to isolate user data from system files. It makes managing disk usage easier and prevents users from using too much disk space.

Use a Disk Space Analyzer Tool

Using a disk space analyzer tool is a good way to identify files and directories using too much disk space. Tools such as Baobab and KDirStat provide a graphical representation of disk usage, which can be very useful for identifying large files and directories.