1. Understanding Operating Systems1 Operating Systems Virtual Memory Thrashing Single-User Contiguous Scheme Fixed Partitions Dynamic Partitions

2. Understanding Operating Systems2 Virtual Memory  Programs are often too large to fit into the primary memory.  To solve this problem, computer scientist came up with the idea of virtual memory – where part of the storage on the secondary memory (hard disk) is considered as part of the main memory (RAM).  The actual memory required by a program -- logical memory, is considered to be independent of the physical memory of the computer.  The following explains how OS implements this idea:  Let the physical memory be of size M.  M is partitioned into r equal parts each of size M/r.  Each of these partitions is called a page frame.  Let the logical Memory be L.  L is portioned into p equal parts each of size s=M/r.  Each of these partitions is called a page.  Notice that page frames and pages must be of the same size.  Parts of the pages that cannot fit into the physical memory are placed on the virtual memory.  As the program executes, and more page frames become available, the pages are swapped into page frames. This process is called paging.

3. Understanding Operating Systems3 Example Suppose the physical memory of a computer is 256KB and is partitioned into 8 page frames. If the logical memory required by a program is 5MB, what is the number of pages needed in the virtual memory. Solution: page size, s = M/p = 256KB/8 = 32KB therefore, number of pages, p = L/s = 5MB / 32KB = 5 * 2^10KB / 2^5KB = 5 * 2^5 = 5 * 32 = 160. Thus, 160 pages are needed in the virtual memory

4. Understanding Operating Systems4 Page Table  When a program is using virtual memory, the system keep tracks of which part is in memory and which part is in virtual memory through the page table.  The table consists of at least three columns as shown below:  The frame reference is a single bit flag indicating whether the page is in memory (1) on in virtual memory (0). The page address is the location of the page if it is on the disk and the page frame address is the location of the page if it has been transferred to memory.  At least one page is loaded into a page frame before the execution begins.  When the program demands access to the address of an instruction, the system first checks if it is in a page frame, if it is not, an OS interrupt called page fault occurs. This forces the OS to search for the next logical page and put it into a page frame. frame referencepage addresspage frame address 1 0 1 0

5. Understanding Operating Systems5 Thrashing  Virtual memory presents the problem of thrashing – high paging activity, which reduces the speed of a program.  If the size of a page is small, too many page faults can result which cause thrashing.  On the other hand, if the size of a page is too big, it can result in waste of memory. For example, if a page is 32KB, a 3KB program has to be stored in 32KB, wasting 29KB. However, if the size of a page is 4KB, then only 1KB is wasted.  Designers of OS often have to tradeoff between these two factors.  The greatest problem of thrashing is under-use of the CPU as it remains idle waiting for pages to be transferred into page frames.

6. Understanding Operating Systems6 Single-User Contiguous Scheme Each program loaded in its entirety into memory and allocated as much contiguous memory space as needed. If program was too large -- it couldn’t be executed. Minimal amount of work done by Memory Manager. Hardware needed : 1) register to store base address; 2) accumulator to track size of program as it is loaded into memory.

7. Understanding Operating Systems7 Fixed (Static) Partitions Attempt at multiprogramming using fixed partitions –one partition for each job –size of partition designated by reconfiguring the system –partitions can’t be too small or too large. Critical to protect job’s memory space. Entire program stored contiguously in memory during entire execution. Internal fragmentation is a problem.

8. Understanding Operating Systems8 Original StateAfter Job Entry 100KJob 1 (30K) Partition 1 Partition 2 25KJob 4 (25K) Partition 2 Partition 3 25K Partition 3 Partition 4 50KJob 2 (50K) Partition 4 Job List : J130K J250K J330K J425K Main memory use during fixed partition allocation of Job 3 must wait.

9. Understanding Operating Systems9 Dynamic Partitions Available memory kept in contiguous blocks and jobs given only as much memory as they request when loaded. Improves memory use over fixed partitions. Performance deteriorates as new jobs enter the system –fragments of free memory are created between blocks of allocated memory (external fragmentation).

10. Understanding Operating Systems10 Dynamic Partitioning of Main Memory & Fragmentation

11. Understanding Operating Systems11 Dynamic Partition Allocation Schemes First-fit: Allocate the first partition that is big enough. –Keep free/busy lists organized by memory location (low- order to high-order). –Faster in making the allocation. Best-fit: Allocate the smallest partition that is big enough – Keep free/busy lists ordered by size (smallest to largest). –Produces the smallest leftover partition. –Makes best use of memory.