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TOP 10 LIST WEEK 05

1. What is a Virtual Memory?
A computer can address more memory than the amount physically installed on the system. This extra memory is actually called virtual memory and it is a section of a hard disk that’s set up to emulate the computer’s RAM. The main visible advantage of this scheme is that programs can be larger than physical memory.

2. Demand Paging
A demand paging system is quite similar to a paging system with swapping where processes reside in secondary memory and pages are loaded only on demand, not in advance. When a context switch occurs, the operating system does not copy any of the old program’s pages out to the disk or any of the new program’s pages into the main memory. Instead, it just begins executing the new program after loading the first page and fetches that program’s pages as they are referenced.

3. Page Vault
A page fault occurs when a program attempts to access a block of memory that is not stored in the physical memory, or RAM. The fault notifies the operating system that it must locate the data in virtual memory, then transfer it from the storage device to the system RAM.

4. Copy-On-Write
Copy-on-write is an optimization strategy used in computer programming. The fundamental idea is that if multiple callers ask for resources which are initially indistinguishable, you can give them pointers to the same resource. This function can be maintained until a caller tries to modify its “copy” of the resource, at which point a true private copy is created to prevent the changes becoming visible to everyone else.

5. Allocation of Frames
Virtual memory is implemented using demand paging. Demand paging necessitates the development of a page-replacement algorithm and a frame allocation algorithm. Frame allocation algorithms are used if you have multiple processes; it helps decide how many frames to allocate to each process.

6. Global vs. Local Allocation
To allocate a fixed fraction of the memory to every process, the local algorithm correspond, whereas global algorithm dynamically allocate page frames among the runnable processes. Therefore the page frame’s number assigned to each process varies in time. Global algorithms generally works better than the local one, mainly when the size of working set can vary over the process lifetime.

• Global replacement: Process selects a replacement frame from the set of all frames; one process can take a frame from another.

• Local replacement: Each process selects from only its own set of allocated frames

7. Non-Uniform Memory Access
NUMA (non-uniform memory access) is a method of configuring a cluster of microprocessor in a multiprocessing system so that they can share memory locally, improving performance and the ability of the system to be expanded. NUMA is used in a symmetric multiprocessing (SMP) system.

8. Thrashing
We know every program divided into some pages. When a program need a page which is not in RAM that is called page fault. Whenever a page fault happens, operating system will try to fetch that page from secondary memory and try to swap it with one of the page in RAM. This is called swapping. If this page fault and then swapping happening very frequently at higher rate, then operating system has to spend more time to swap these pages. This state is called thrashing. Because of this, CPU utilization is going to be reduced.

9. Buddy System Allocator
Buddy allocation system is an algorithm in which a larger memory block is divided into small parts to satisfy the request. This algorithm is used to give best fit. The two smaller parts of block are of equal size and called as buddies. In the same manner one of the two buddies will further divide into smaller parts until the request is fulfilled.

10. Slab Allocator
Slab allocation eliminates fragmentation caused by allocations and deallocations. This method is used to retain allocated memory that contains a data object of a certain type for reuse upon subsequent allocations of objects of the same type. In slab allocation memory chunks suitable to fit data objects of certain type or size are preallocated.