Research Topics 2

Virtual memory in UNIX

Virtual memory is an internal “trick” that relies on the fact that not every executing task is always referencing it’s RAM memory region. Since all RAM regions are not constantly in-use, UNIX has developed a paging algorithm that move RAM memory pages to the swap disk when it appears that they will not be needed in the immediate future.

RAM demand paging in UNIX;

As memory regions are created, UNIX will not refuse a new task whose RAM requests exceeds the amount of RAM. Rather, UNIX will page out the least recently referenced RAM memory page to the swap disk to make room for the incoming request. When the physical limit of the RAM is exceeded UNIX can wipe-out RAM regions because they have already been written to the swap disk. When the RAM region is been removed to swap, any subsequent references by the originating program require UNIX copy page in the RAM region to make the memory accessible. UNIX page in operations involve disk I/O and are a source of slow performance. Hence, avoiding UNIX page in operations is an important concern for the Oracle DBA.

Memory management

Memory management is the act of managing computer memory. In its simpler forms, this involves providing ways to allocate portions of memory to programs at their request, and freeing it for reuse when no longer needed.

Virtual memory systems separate the memory addresses used by a process from actual physical addresses, allowing separation of processes and increasing the effectively available amount of RAM using disk swapping. The quality of the virtual memory manager can have a big impact on overall system performance.

Garbage collection is the automated allocation, and deallocation of computer memory resources for a program. This is generally implemented at the programming language level and is in opposition to manual memory management, the explicit allocation and deallocation of computer memory resources.

Relocation

In systems with virtual memory, programs in memory must be able to reside in different parts of the memory at different times. This is because when the program is swapped back into memory after being swapped out for a while it can not always be placed in the same location. Memory management in the operating system should therefore be able to relocate programs in memory and handle memory references in the code of the program so that they always point to the right location in memory.

Protection

Processes should not be able to reference the memory for another process without permission. This is called memory protection, and prevents malicious or malfunctioning code in one program from interfering with the operation of other running programs.

Sharing

Even though the memory for different processes is protected from each other different processes should be able to share information and therefore access the same part of memory.

Logical organization

Programs are often organized in modules. Some of these modules could be shared between different programs, some are read only and some contain data that can be modified. The memory management is responsible for handling this logical organization that is different from the physical linear address space. One way to arrange this organization is segmentation.

Physical organization

Memory is usually divided into fast primary storage and slow secondary storage. Memory management in the operating system handles moving information between these two levels of memory.

DOS memory managers

In addition to standard memory management, the 640 KB barrier of MS-DOS and compatible systems led to the development of programs known as memory managers when PC main memories started to be routinely larger than 640 KB in the late 1980s (see conventional memory). These move portions of the operating system outside their normal locations in order to increase the amount of conventional or quasi-conventional memory available to other applications. Examples are EMM386, which was part of the standard installation in DOS's later versions, and QEMM. These allowed use of memory above the 640 KB barrier, where memory was normally reserved for RAMs, and high and upper memory.