Linux memory management. Linux uses demand paging where the virtual pages are only loaded into physical memory when needed by the process. Swapping occurs when a process needs to bring data from the virtual memory to the physical memory,

Linux uses demand paging where the virtual pages are only loaded into physical memory when needed by the process. Swapping occurs when a process needs to bring data from the virtual memory to the physical memory, if there is no space available an old page is swapped out of memory to bring in the new one. Linux the Least Recently Used (LRU) technique to efficiently choose pages which are to be removed from the system. This scheme requires every page in the system having an age which changes as the page is accessed. The more frequently that a page is accessed, the younger it is; the less that it is accessed the older it becomes. Old pages are good candidates for swapping. The Linux kernel is not run in the virtual address space instead it is solely allocated space in the physical memory address. The page table in Linux also controls access to the code as well as mapping, e.g. the processor has two modes user and kernel, you won’t allow access of kernel code to users and vice versa to protect your system. Linux also uses many different types of cache to make access to information faster to the processes

• Buffer cache = the buffer cache contains data buffers that are used by the block device drivers.
• Page cache = this is used to speed up access to data and image son the disk.
• Swap cache = only modified pages are saved in swap cache.
• Hardware cache = One commonly implemented hardware cache is in the processor; a cache of Page Table Entries.

Linux also uses a concept known as page tables, Linux uses three level page tables, Each Page Table accessed contains the page frame number of the next level of Page Table .the image below shows how a virtual address can be broken down into three fields, and each page provides an offset to a certain page table. A virtual address is translated into a physical one as each page table number is read until the page frame number of the physical page is found.

Linux also uses the buddy system algorithm to allocate and de-allocate blocks of pages. In this algorithm pages are allocated in blocks which are powers of 2 in size , this essentially means it can allocate a certain block 1 page , 2 pages , 4 pages etc ..As long as there is enough free space. In the de-allocation of pages, free blocks which are next to each other are combined together to form one big free block with more pages to allocate.

Dis – Advantages:

• The drawback of using caches, hardware or otherwise, is that in order to save effort Linux must use more time and space maintaining these caches and, if the caches become corrupted, the system will crash.

Advantages:

• Page tables are so successful that Linux uses the same page table manipulation code for the Alpha processor, which has three levels of page tables and for Intel x86 processors, which have two levels of page tables.
• Linux memory management is better for older computers as it can sometimes require less processing power than other operating systems.

Resources:

• Ehttp://www.makelinux.com/ldd3/chp-15-sect-1