Warning:
JavaScript is turned OFF. None of the links on this page will work until it is reactivated.
If you need help turning JavaScript On, click here.
This Concept Map, created with IHMC CmapTools, has information related to: ch11 time clock, Clocks, events and process states has Coordinated Universal Time (UTC) International Atomic Time is based on very accurate physical clocks (drift rate 10-13) UTC is an international standard for time keeping It is based on atomic time, but occasionally adjusted to astronomical time It is broadcast from radio stations on land and satellite (e.g. GPS) Computers with receivers can synchronize their clocks with these timing signals Signals from land-based stations are accurate to about 0.1-10 millisecond Signals from GPS are accurate to about 1 microsecond, Clocks, events and process states has Computer clocks are not generally in perfect agreement Skew: the difference between the times on two clocks (at any instant) Computer clocks are subject to clock drift (they count time at different rates) Clock drift rate: the difference per unit of time from some ideal reference clock Ordinary quartz clocks drift by about 1 sec in 11-12 days. (10-6 secs/sec). High precision quartz clocks drift rate is about 10-7 or 10-8 secs/sec, Clocks, events and process states has Each computer in a DS has its own internal clock used by local processes to obtain the value of the current time processes on different computers can timestamp their events but clocks on different computers may give different times computer clocks drift from perfect time and their drift rates differ from one another. clock drift rate: the relative amount that a computer clock differs from a perfect clock, Clocks, events and process states has We have seen how to order events (happened before) To timestamp events, use the computer’s clock At real time, t, the OS reads the time on the computer’s hardware clock Hi(t) It calculates the time on its software clock, Clocks, events and process states has Even if clocks on all computers in a DS are set to the same time, their clocks will eventually vary quite significantly unless corrections are applied, Clocks, events and process states has A distributed system is defined as a collection P of N processes pi, i = 1,2,… N Each process pi has a state si consisting of its variables (which it transforms as it executes) Processes communicate only by messages (via a network) Actions of processes: Send, Receive, change own state Event: the occurrence of a single action that a process carries out as it executes e.g. Send, Receive, change state Events at a single process pi, can be placed in a total ordering denoted by the relationship between the events. i.e. A history of process pi: is a series of events ordered by history(pi)= hi = <ei0, ei1, ei2, …>