Top 10 Lists: Week 05
1.Virtual Memory
Virtual memory is a “magical” common feature of a computer. The way it works is by making a page file, consist of an unused RAM copied to the hard disk for later usage. This page file works a bit slower than the actual RAM, probably depends on what hard disk used by the computer. Windows 98 is one of the first operating systems that include this feature.
2.Thrashing
Thrashing occurs when the virtual memory of a computer is rapidly exchanging data for data on hard disk, to the exclusion of most application-level processing. Thrashing can lead to hard disk collapsing.
3.Demand Paging
How demand paging works is similar with page request which need swapping, but the pages needed won’t be on physical memory until needed.
4.Demand Paging Performance
The performance of demand paging is often measured in terms of the effective access time. Effective access time is the amount of time it takes to access memory, if the cost of page faults are amortized over all memory accesses. In some sense it is an average or expected access time.
5.Non-uniform Memory Access (NUMA)
NUMA is a method for configure a cluster of microprocessors in a multiprocessing system so that they can share memory locally, which improve a system performance and adding the ability of the system to be expanded. NUMA can be thought of as a “cluster in a box” typically consists of four microprocessors interconnected on a local bus to shared memory on a single motherboard.
6.Page Replacement Algorithm
This algorithm will helps decide which page to be replaced. Some of the algorithm used is FIFO (First in First out, which the oldest page will be removed), LRU (Least Recent Used, which page has not been used for a long time will be removed), and Optimum Algorithm (which page will not be used in the future will be removed).
7.Copy On Write (CoW)
CoW is a technique to copy data resource from a system. If a unit of data is copied but not modified, the “copy” can exist as a reference to the original data. Only when the copied data is modified is a copy created, and new bytes are actually written. Copy-on-write is closely related to data deduplication. Whereas data deduplication analyzes chunks or blocks of data, copy-on-write applies to entire files or allocated units of memory.
8.Kernel
For operating systems, kernel is a program with authority of everything inside a system. Before any operating systems start, kernel will start first to “wake up” the systems and handle it until the operating system shuts. The Kernel is responsible for low-level tasks such as disk management, memory management, task management, etc. It provides an interface between the user and the hardware components of the system. When a process makes a request to the Kernel, then it is called System Call.
For food, kernel is that yellow bit we eat from a corn cob, when dried we can cook it to be a popcorn.
9.Global vs Local Allocation
Global replacement allows a process to choose a replacement frame from the set of all frames, even if that frame is currently allocated to some other process. Local replacement requires that each process selects from only its own set of allocated frames.
10.Allocation of Frames
When a page fault happens, there is a free frame available to store a new page into a frame. While the page swap is taking place, a page replacement can be selected, which is written to the disk as the user process continues to execute. The operating system allocates all its buffer and tablespace from the free-frame list for the new page.