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Memory management in Linux

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Raghu Udiyar
Raghu Udiyar

The document discusses Linux memory management, describing how physical memory is divided into page frames and virtual memory allows processes to have a virtual view of memory mapped to physical memory using page tables, and covers topics like memory overcommit, page cache, swap space, and tools for monitoring memory usage.

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Memory management in Linux Raghu Udiyar Technical Support Engineer Red Hat
What is memory? It is just a form of storage
Can a disk be considered memory? Yes, in a way
Only difference is with : Capacity ¨C cost
Latency
Volatile
Hardware CPU registers - ~1 cycle
CPU cache (L1, L2..) - 3 ¨C 14 cycles
RAM (main memory) - ~250 cycles
Disk - ~40 million cycles
Physical Memory The map provides the physical addresses
These are then taken and divided into page frames of 4Kb
E.g. 4Gb = ~1 million pages
Virtual Memory The system never allocates physical memory directly
Every process has a virtual view of the memory
A process will think it is the only task running on the system
This virtual memory is then mapped to physical memory using page tables
Virtual Memory vi bash 2GB Physical Memory Virtual memory
Virtual Memory vi bash 2GB Physical Memory Virtual memory Page Table
Address Space On a 32bit system : 4Gb : 3 Gb userspace / 1 Gb kernel space On a 64bit system : 256 TB userspace / 256 TB kernel space
User and Kernel address space Kernel User Kernel User 32bit 64bit 3Gb 1GB 256TB 256TB
Memory Overcommit Allocate more then what is available
malloc(1G) = Ok
memset(1G) = Ok
malloc(4GB) = Ok
memset(4GB) = -ENOMEM
Kernel almost always grants Virtual memory

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Memory management in Linux