CHAPTER THREE MEMORY MANAGEMENT Basics of memory Management

CHAPTER THREE MEMORY MANAGEMENT • • Basics of memory Management Swapping Virtual memory Page replacement • Segmentation

Basics of memory management • Program must be brought into memory and placed within a process for it to be run • Input queue – collection of processes on the disk that are waiting to be brought into memory to run the program • User programs go through several steps before being run • Subdividing memory to accommodate multiple processes • Memory needs to be allocated to ensure a reasonable supply of ready processes to consume available processor time

Basics of memory management • Memory management is the task carried out by the OS and hardware to accommodate multiple processes in main memory • If only a few processes can be kept in main memory, then much of the time all processes will be waiting for I/O and the CPU will be idle • Hence, memory needs to be allocated efficiently in order to pack as many processes into memory as possible

Basics of memory management Memory Management Requirements • Relocation – Programmer does not know where the program will be placed in memory when it is executed – While the program is executing, it may be swapped to disk and returned to main memory at a different location (relocated) – Memory references must be translated in the code to actual physical memory address

Basics of memory management Memory Management Requirements • Protection – Processes should not be able to reference memory locations in another process without permission – Impossible to check absolute addresses at compile time Must be checked at run time – Memory protection requirement must be satisfied by the processor (hardware) rather than the operating system (software) • Operating system cannot anticipate all of the memory references a program will make

Basics of memory management Memory Management Requirements • Sharing – Allow several processes to access the same portion of memory – if a number of processes are executing the same program, it is advantageous to allow each process access to the same copy of the program rather than have their own separate copy – Processes that are cooperating on some task may need to share access to the same data structure

Basics of memory management Memory Management Requirements • Logical Organization – Main memory in a computer system is organized as a linear, or one-dimensional, address space, consisting of a sequence of bytes or words – But programs are written in modules – If HW and OS can deal with user programs and data in the form of modules of some sort, then a number of advantages can be realized • Modules can be written and compiled independently • Different degrees of protection given to modules (read-only, execute-only) • Share modules among processes

Basics of memory management Memory Management Requirements • Physical Organization – Secondary memory of large capacity can be provided for long-term storage of programs and data, while a smaller main memory holds programs and data currently in use – Memory available for a program plus its data may be insufficient • Overlaying allows various modules to be assigned the same region of memory with a main program responsible • for switching the modules in and out as needed – Programmer does not know how much space will be available – moving information between these two levels of memory is a major concern of memory management (OS)

Basics of memory management • Memory management systems can be divided into two classes: – Those that move processes back and forth between main memory and disk during execution (swapping and paging) – Those that do not sufficient main memory to hold all the programs at once • Two simple memory management schemes – Monoprogramming – Multiprogramming

Basics of memory management • Monoprogramming – to run just one program at a time, sharing the memory between that program and the operating system – Three variations: OS RAM User Prog ROM Device Drivers User RAM Prog OS ROM

Basics of memory management Multiprogramming • Although the following simple memory management techniques are not used in modern OS, they lay the ground for a proper discussion of virtual memory – fixed partitioning – dynamic partitioning – simple paging – simple segmentation

Basics of memory management Fixed Partitioning • Partition main memory into a set of non overlapping regions called partitions • Partitions can be of equal or unequal sizes

Basics of memory management Equal-sized fixed partitions • Any program, no matter how small, occupies an entire partition. • If there is an available partition, a process can be loaded into that partition • because all partitions are of equal size, it does not matter which partition is used • If all partitions are occupied by blocked processes, choose one process to swap out to make room for the new process • Main memory use is inefficient. – This is called internal fragmentation.

Basics of memory management Unequal-sized fixed partitions • Any process whose size is less than or equal to a partition size can be loaded into the partition • If all partitions are occupied, the operating system can swap a process out of a partition • Two types: – Using multiple queue – Usingle queue • Multiple queue – When a job arrives, it can be put into the input queue for the smallest partition large enough to hold it – Problem • the queue for a large partition is empty but the queue for a small partition is full

Basics of memory management Unequal-sized fixed partitions • Assign each process to the smallest partition within which it will fit • A queue for each partition size tries to minimize internal fragmentation Partion 4 900 k Partion 3 500 k Partion 2 Partion 1 OS 300 k 200 k

Basics of memory management Unequal-sized fixed partitions • Single queue – maintain a single queue and whenever a partition becomes free, the job closest to the front of the queue that fits in it could be loaded into the empty partition and run – Problem • waste a large partition on a small job – Solution • search the whole input queue whenever a partition becomes free and pick the largest job that fits • But this algorithm discriminates against small jobs as being unworthy of having a whole partition, whereas usually it is desirable to give the smallest jobs (often interactive jobs) the best service, not the worst.

Basics of memory management Unequal-sized fixed partitions • Single queue – Ways to allow small jobs • To have at least one small partition around. – Such a partition will allow small jobs to run without having to allocate a large partition for them

Basics of memory management Unequal-sized fixed partitions

Basics of memory management Dynamic Partitioning • Partitions are of variable length and number • Each process is allocated exactly as much memory as it requires • Eventually holes are formed in main memory. – This is called external fragmentation • Must use compaction to shift processes so they are contiguous and all free memory is in one block

Basics of memory management Dynamic Partitioning • A hole of 64 K is left after loading 3 processes: not enough room for another process • Eventually each process is blocked. • The OS swaps out process 2 to bring in process 4

Basics of memory management Dynamic Partitioning • Another hole of 96 K is created • Eventually each process is blocked. • The OS swaps out process 1 to bring in again process 2 and another hole of 96 K is created. . . • Compaction would produce a single hole of 256 K

Basics of memory management Dynamic Partitioning • Used to decide which free block to allocate to a process • Goal: to reduce usage of compaction • Possible algorithms: – Best-fit: choose smallest hole – First-fit: choose first hole from beginning – Next-fit: choose first hole from last placement – Worst-fit: Choose the largest available hole

Basics of memory management Dynamic Partitioning

CHAPTER THREE MEMORY MANAGEMENT • • Basics of memory Management Swapping Virtual memory Page replacement • Segmentation

Swapping • Two general approaches to memory management can be used – Swapping: consists of bringing in each process in its entirety, running it for a while, then putting it back on the disk. – Virtual memory: allows programs to run even when they are only partially in main memory – Concerns: • how much memory should be allocated for a process when it is created or swapped in. – If processes are created with a fixed size that never changes, then the allocation is simple: • the operating system allocates exactly what is needed, no more and no less – However, a problem occurs whenever a process tries to grow

Swapping • When a memory grows: – If a hole is adjacent to the process, it can be allocated and the process allowed to grow into the hole – If the process is adjacent to another process • moved the process to a hole in memory large enough for it • one or more processes will have to be swapped out to create a large enough hole – If it is expected that most processes will grow as they run, it is probably a good idea to allocate a little extra memory whenever a process is swapped in or moved – when swapping processes to disk, only the memory actually in use should be swapped

Swapping

Swapping Reasons for swapping in and out • Time quantum expired – When a quantum expires, the memory manager will start to swap out the process that just finished and to swap another process into the memory space that has been freed • Higher priority process arrives – If a higher-priority process arrives and wants service, the memory manager can swap out the lower-priority process and then load and execute the higher-priority process

Swapping Requirements for swapping • Address binding – If binding is done at assembly or load time, then the process cannot be easily moved to a different location – a process that is swapped out will be swapped back into the same memory space it occupied previously • backing store – Secondary storage must be large enough to accommodate copies of all memory images for all users, and it must provide direct access to these memory images • Swapping time – The memory block with smallest swapping time is swapped out • Idle – A process should be completely idle, to be swapped out

CHAPTER THREE MEMORY MANAGEMENT • • Basics of memory Management Swapping Virtual memory Page replacement • Segmentation

Virtual Memory • Programs are usually too big to fit in the available memory • virtual memory is a mechanism in which the combined size of the program, data, and stack may exceed the amount of physical memory available for it. • The operating system keeps those parts of the program currently in use in main memory, and the rest on the disk • Virtual memory can also work in a multiprogramming system • Most virtual memory systems use a technique called paging

Paging • Both unequal fixed-size and variable-size partitions are inefficient in the use of memory – Unequal fixed-sized internal fragmentation – Equal fixed-sized internal fragmentation • Main memory is partitioned into equal relatively small fixed-size chunks known as pages. • Each process is also divided into small fixed-size chunks of the same size known as frames or page frames • Pages could be assigned to available chunks of memory

Paging

Paging

Paging

Paging • At a given point in time, a list of free frames is maintained by the operating system • Does the unavailability of sufficient contiguous memory space prevent the operating system from loading a process? – No – different pages can be stored on different frames • Memory address used by programs are mapped into real memory address called physical address • These program-generated addresses are called virtual addresses and form the virtual address space

Paging • If the processor encounters a virtual address that is not in main memory, it generates an interrupt indicating a memory access fault, known as page fault. • The operating system puts the interrupted process in a blocking state and takes control • The operating system maintains a page table which shows a frame location for each page of the process. • Each process will have its own page table • Consider a computer with 16 bit (0 -64 K) addressing, 32 KB of physical memory and 4 K frame size – 64 KB of virtual address space – 16 virtual pages – 8 page frames

Paging

Paging • • • When the program tries to access address 0, for example, using the instruction MOV REG, 0 virtual address 0 falls in page 0 (0 to 4095), which according to its mapping is page frame 2 (8192 to 12287) physical address Similarly, MOV REG, 8192 (p 2) MOV REG, 24576 (p 6) MOV REG, 32780 page foult trap

Virtual Memory • • • A process executes only in main memory, that memory is referred to as real memory or physical memory A programmer or user perceives a potentially much larger memory — that which is allocated on disk — referred to as virtual memory In the previous example a user assumes 64 K memory, but actually we have only 32 K memory

Virtual Memory

CHAPTER THREE MEMORY MANAGEMENT • Basics of memory Management • Swapping • Virtual memory • Page replacement • Segmentation

Segmentation • The virtual memory is one-dimensional because the virtual addresses go from 0 to some maximum address, one address after another. • For many problems, having two or more separate virtual address spaces may be much better than having only one • These completely independent address spaces are called segments • A user program can be subdivided using segmentation, in which the program and its associated data are divided into a number of segments • Different segments may have different lengths • A logical address using segmentation consists of two parts, in this case a segment number and an offset

Segmentation

Segmentation • Segmentation is similar to dynamic partitioning in the use of unequal-size segments – with segmentation a program may occupy more than one partition, and these partitions need not be contiguous – With dynamic partitioning, a program occupies contiguous memory partitions – Segmentation eliminates internal fragmentation – Segmentation and dynamic partitioning suffers from external fragmentation

Segmentation • Whereas paging is invisible to the programmer, segmentation is usually visible and is provided as a convenience for organizing programs and data – Analogous to paging, a simple segmentation scheme would make use of a segment table for each process and a list of free blocks of main memory – Each segment table entry would have to give the • starting address in main memory of the corresponding segment • length of the segment, to assure that invalid addresses are not used

Segmentation

Segmentation • Consider an address of n X m bits, where the leftmost n bits are the segment number and the rightmost m bits are the offset, in the previous figure, n= 4 and m=12. • Thus the maximum segment size is 212 = 4096. • The following steps are needed for address translation: – Extract the segment number as the leftmost n bits of the logical address. – Use the segment number as an index into the process segment table to find the starting physical address of the segment. – Compare the offset, expressed in the rightmost m bits, to the length of the segment. If the offset is greater than or equal to the length, the address is invalid. – The desired physical address is the sum of the starting physical address of the segment plus the offset.

Segmentation • In our example, we have the logical address 0001001011110000, which is segment number 1, offset 752. • Suppose that this segment is residing in main memory starting at physical address 00100000. – Then the physical address is 00100000 001011110000 00100010000 • To summarize, with simple segmentation, a process is divided into a number of segments that need not be of equal size. • When a process is brought in, all of its segments are loaded into available regions of memory, and a segment table is set up.

Segmentation • Advantages: – simplifying the handling of data structures that are growing or shrinking – linking up of procedures compiled separately on different segments is simplified – changing one procedure’s size doesn’t affect the starting address of others – facilitates sharing procedures or data between several processes • E. g. shared library – different segments can have different kinds of protection.

Segmentation

Quiz 1. What is the reason for performing swapping? 2. What are the requirement of Swapping? 3. From the given logical and physical memory in the previous example ü Logical adrress 9 mapss to -------physical address ü Logical address 5 mapps to-------physical address