The optimal working set of a process is the set of pages that will be needed in the immediate future and thus should be resident in the physical memory. The size of the set will be based upon the size of the process and what it needs. To determine the size of the working window, the OS…
A reference string is the sequence of page numbers referenced by an executing program during a given time interval. Reference strings are used to compare different page replacement algorithms by counting the number of page faults generated. Since page faults are time consuming, the optimal page replacement algorithm selects the page that will not be referenced for the longest…
Virtual Memory (VM) is a collection of one or more logical address spaces, each of which may exceed the size of physical memory. To access this space, we use a virtual address. A paged virtual memory creates a single large contiguous address space per process. Demand paging is the principle of loading a page into memory only…
With paging, all components of a program, including code, static data, the stack, and other data structures, are combined into one contiguous address space divided into fixed-size pages. Segmentation addresses the problem by providing multiple logical address spaces for each process, where each segment can have a different size. A segment is a variable-size block of a…
The main disadvantage of dividing memory into variable partitions is external fragmentation, which requires the searching of memory for holes large enough to satisfy each request. As we’ve mentioned previously, swapping or compaction is necessary when no hole large enough is available. A page is a fixed-size contiguous block of a logical address space identified by the…
One guarantee is that if you add RAM, you’ll find a way to use it up. Between running more programs and more complex programs, RAM is always a resource that is in short supply. External memory fragmentation is the loss of usable memory space due to holes between allocated blocks of variable sizes. The 50% rule…
Logical vs Physical memory A computer’s physical memory (RAM) is a hardware structure consisting of a linear sequence of words that hold a program during execution. A word is a fixed-size unit of data. The size of a word is reflected in many aspects of a computer’s structure and operation. A typical word size is 4 to 8 bytes…
A deadlock in a resource allocation graph can be detected by executing a graph reduction algorithm by repeating this process: Select an unblocked process p. Remove p, including all request and allocation edges connected to p. Anything left is blocked, and risks deadlock. However, it is considered completely reducible if there are no processes left…
