clock

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The clock algorithm approximates the ideal virtual memory paging scenario where the least recently used physical pages are paged out first.

The name refers to its most common illustration. Imagine the physical pages of memory spread out in a circle. (Likewise, an algorithm would perhaps use a circular buffer to track the physical pages.) The MMU of the computer will set the reference bit of each physical page whenever it is addressed by a program. Here's where the clock idea comes in: imagine in your illustration a single clock hand rotating around, aiming at each physical page sequentially. When your paging algorithm needs to page something out to free up a chunk of physical memory, it moves this clock hand analyzing each page sequentially. When a page is examined, if its reference bit is set then that bit is cleared and the clock hand moves on to the next page in its search. Only when a physical page with an already clear reference bit is found can the clock stop and that page can be paged out to free up a chunk of memory. The theory is that the least recently used pages will be the first unreferenced pages encountered by this rotating clock hand, and thus will be the ones selected.

There are some variations, such as two handed clocks where the gap between the clock hands define a more narrow range of physical pages that can be scanned during each run of the paging algorithm. This is useful to avoid thrashing your VM.

clock

1. n 1. [techspeak] The master oscillator that steps a CPU or other digital circuit through its paces. This has nothing to do with the time of day, although the software counter that keeps track of the latter may be derived from the former. 2. vt. To run a CPU or other digital circuit at a particular rate. "If you clock it at 100MHz, it gets warm.". See overclock. 3. vt. To force a digital circuit from one state to the next by applying a single clock pulse. "The data must be stable 10ns before you clock the latch."

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

Our first glimpse of time-keeping tools (i.e., not estimating based on the position of the sun) comes from The Babylonian Empire. Using a large amount of observations on the path the Sun traversed through the sky, they succeeded in creating the first clock. A stone pillar was stuck in the ground with furrows cut into the surrounding land. By measuring on which furrow the ]shadow] of the pillar landed, the Babylonians had made the first non-human time-keeping device.

In the Asterix series of comics, a tall pointed menhir is often seen during their travels to Egypt. This was not a menhir at all -- it was a sundial. These huge structures were named 'Cleopatra's Needles' by the Romans. The Egyptian day was divided into 12 parts, each being (obviously) about two modern hours.

The Egyptians also succeeded in building a portable time keeping device as they realised that it would be somewhat inconvenient to lug around a stone pillar to tell the time. They built a small sundial on a peice of metal or wood known as a 'style'. This had the advantage of portability. There was an inherent problem with sundials, however -- they only worked in the sun. This meant that on cloudy days, or at night, a sundial was completely useless.

500 years, later, the Egyptians, always innovators, found a solution to this problem -- the water clock. Two jugs were kept, one directly on top of the other. The top one wa siflled with water and had a hole drilled in the bottom. As the water level decreased, people could check the time by seeing how far the level had dropped. But this also had a problem -- during cold weather (which I suppose was not too common in Egypt) the water clock would freeze.

Around 250 B.C the hourglass was invented. It was a modified version of the water clock (it used sand), but if the humidity was high, the sand got stuck. Another problem. Also around this time, the candle was used as a timekeeper. The rate at which the candle melted was measured and used to tell the time.

The mechanical clock was invented during the Middle Ages. These clocks used chains, weights, gears and springs to turn a pointer on a clock face. Most were huge affairs, put up on cathedral towers so that the whole town could see the time. These mechanical clocks finally became portable in the 16th century when German watchmaker Peter Henlein created the mainspring. This tightly coiled spring unwound slowly, powering the clock as it did so. Many modern clocks use a mainspring -- these are the clocks that must be wound up once or twice a day.

Around this time, the term 'watch' was introduced. The origin of the word is due to the fact that the town guards, or watches carried portable watches on straps, These clocks became known as watches also.

In 1656, Christian Huygnes, a Dutchman, used Galileo's work on the regular motion of pendulums to design the first pendulum clock the world had ever seen.

An interesting watch was that belonging to Mary, Queen of Scots. She wore her watch as a necklace with a small skull at the end. To see the time, she would turn over the skull and the jaw would open revealing a dial. Since then, clocks have progressed hugely. Today, we have TV watches, calculator watches, xclock and countless others.

Clock (?), n. [AS. clucge bell; akin to D. klok clock, bell, G. glocke, Dan. klokke, Sw. klocka, Icel. klukka bell, LL. clocca, cloca (whence F. cloche); al perh. of Celtic origin; cf. Ir. & Gael. clog bell, clock, W. cloch bell. Cf. Cloak.]

1.

A machine for measuring time, indicating the hour and other divisions by means of hands moving on a dial plate. Its works are moved by a weight or a spring, and it is often so constructed as to tell the hour by the stroke of a hammer on a bell. It is not adapted, like the watch, to be carried on the person.

2.

A watch, esp. one that strikes.

Walton.

3.

The striking of a clock.

Dryden.

4.

A figure or figured work on the ankle or side of a stocking.

Swift.

The phrases what o'clock? it is nine o'clock, etc., are contracted from what of the clock? it is nine of the clock, etc.

Alarm clock. See under Alarm. -- Astronomical clock. (a) A clock of superior construction, with a compensating pendulum, etc., to measure time with great accuracy, for use in astronomical observatories; -- called a regulator when used by watchmakers as a standard for regulating timepieces. (b) A clock with mechanism for indicating certain astronomical phenomena, as the phases of the moon, position of the sun in the ecliptic, equation of time, etc. -- Electric clock. (a) A clock moved or regulated by electricity or electro-magnetism. (b) A clock connected with an electro-magnetic recording apparatus. -- Ship's clock Naut., a clock arranged to strike from one to eight strokes, at half hourly intervals, marking the divisions of the ship's watches. -- Sidereal clock, an astronomical clock regulated to keep sidereal time.

© Webster 1913.

Clock (?), v. t.

To ornament with figured work, as the side of a stocking.

© Webster 1913.

Clock, v. t. & i.

To call, as a hen. See Cluck.

© Webster 1913.

Clock, n. Zool.

A large beetle, esp. the European dung beetle (Scarabaeus stercorarius).

© Webster 1913.