Process Memory Management in Linux
Process memory management is a crucial aspect of any operating system. In Linux, memory management system is designed to efficiently manage memory usage, allowing processes to access and use memory they require while preventing them from accessing memory they do not own. In this article, we will discuss process memory management in Linux in detail, covering various aspects such as memory allocation, virtual memory, memory mapping, and more.
Memory Allocation
Memory allocation is process of assigning memory to a process or program. In Linux, kernel provides two main methods for memory allocation: static and dynamic.
Static Memory Allocation
Static memory allocation is done at compile-time, where memory allocation for a program is fixed and cannot be changed during runtime. memory is allocated in program’s data section or stack segment. data section contains global variables and static variables, while stack segment contains local variables.
Dynamic Memory Allocation
Dynamic memory allocation is done during runtime, where memory allocation for a program can be dynamically adjusted based on program’s requirements. kernel provides various system calls such as malloc(), calloc(), and realloc() to dynamically allocate memory. These functions allocate memory from heap segment of program’s address space.
Virtual Memory
Virtual memory is a memory management technique that allows a program to use more memory than is physically available in system. In Linux, virtual memory is implemented using a combination of hardware and software. hardware component is Memory Management Unit (MMU), which is responsible for translating virtual memory addresses to physical memory addresses. software component is kernel’s Virtual Memory Manager (VMM), which manages allocation and deallocation of virtual memory.
Memory Mapping
Memory mapping is a technique that allows a process to access a file’s contents as if it were part of process’s memory. In Linux, memory mapping is implemented using mmap() system call. mmap() system call maps a file into a process’s virtual memory address space, allowing process to read and write to file’s contents as if it were part of its own memory. Memory mapping is commonly used in applications such as databases and multimedia players, where large files need to be accessed efficiently.
Shared Memory
Shared memory is a technique that allows multiple processes to access same portion of memory. In Linux, shared memory is implemented using shmget(), shmat(), and shmdt() system calls. shmget() system call creates a shared memory segment, shmat() attaches shared memory segment to a process’s address space, and shmdt() detaches shared memory segment from process’s address space. Shared memory is commonly used in inter-process communication, where multiple processes need to share data efficiently.
Swapping
Swapping is a technique that allows kernel to move pages of memory from RAM to a swap space on disk when system’s memory is low. In Linux, swapping is implemented using a combination of hardware and software. hardware component is disk, which is used as swap space. software component is kernel’s Swapping Manager, which manages swapping process. When system’s memory is low, Swapping Manager selects pages of memory to swap out to disk, freeing up memory for other processes.
Some additional concepts to consider include −
Kernel Memory Management
The Linux kernel itself also requires memory management, and it uses a separate set of memory management techniques to manage kernel memory. Kernel memory is used to store data structures and code required by kernel to operate. kernel uses techniques like memory mapping, page caching, and memory allocation to manage kernel memory.
Memory Protection
Memory protection is another critical aspect of memory management in Linux. Memory protection techniques prevent processes from accessing memory they are not authorized to access. MMU implements memory protection by using page tables, which map virtual memory addresses to physical memory addresses and track permissions for each memory page.
Memory Fragmentation
Memory fragmentation occurs when available memory is divided into small, non-contiguous chunks, making it difficult to allocate larger blocks of memory. Memory fragmentation can lead to performance issues and even crashes if system runs out of memory. Linux kernel uses several techniques to manage memory fragmentation, including memory compaction and defragmentation.
Memory Leak Detection
As mentioned earlier, failing to release dynamically allocated memory can result in memory leaks, where memory is not returned to system and can eventually cause program to crash due to insufficient memory. Detecting and fixing memory leaks is crucial for maintaining system stability and performance. Linux provides several tools for detecting memory leaks, including valgrind, which can detect memory leaks and other memory-related issues.
Conclusion
In conclusion, process memory management is a crucial aspect of any operating system, and Linux is no exception. Linux kernel provides a robust and efficient memory management system, allowing processes to access and use memory they require while preventing them from accessing memory they do not own. In this article, we discussed various aspects of process memory management in Linux, including memory allocation, virtual memory, memory mapping, shared memory, and swapping. Understanding these concepts is essential for any Linux developer or administrator to efficiently manage memory usage in their systems.
what is kernel mapping in linux?
I’m assuming you’re talking about memory mapping in linux kernel.
Memory mapping is a process of mapping kernel address space directly to users process’s address space.
Types of addresses :
- User virtual address : These are the regular addresses seen by user-space programs
- Physical addresses : The addresses used between the processor and the system’s memory.
- Bus addresses : The addresses used between peripheral buses and memory. Often, they are the same as the physical addresses used by the processor, but that is not necessarily the case.
- Kernel logical addresses : These make up the normal address space of the kernel.
- Kernel virtual addresses : Kernel virtual addresses are similar to logical addresses in that they are a mapping from a kernel-space address to a physical address.
High and Low Memory :
- Low memory : Memory for which logical addresses exist in kernel space. On almost every system you will likely encounter, all memory is low memory.
- High memory : Memory for which logical addresses do not exist, because it is beyond the address range set aside for kernel virtual addresses.This means the kernel needs to start using temporary mappings of the pieces of physical memory that it wants to access.
Kernel splits virtual address into two part user address space and kernel address space. The kernel’s code and data structures must fit into that space, but the biggest consumer of kernel address space is virtual mappings for physical memory. Thus kernel needs its own virtual address for any memory it must touch directly. So, the maximum amount of physical memory that could be handled by the kernel was the amount that could be mapped into the kernel’s portion of the virtual address space, minus the space used by kernel code.
Temporary mapping : When a mapping must be created but the current context cannot sleep, the kernel provides temporary mappings (also called atomic mappings). The kernel can atomically map a high memory page into one of the reserved mappings (which can hold temporary mappings). Consequently, a temporary mapping can be used in places that cannot sleep, such as interrupt handlers, because obtaining the mapping never blocks.