In concurrent computing, a deadlock is a state in which each member of a group is waiting for another member, including itself, to take action, such as sending a message or more releasing a lock. Deadlock is a common problem in multiprocessing systems, parallel computing, distributed systems, where software and hardware locks are used to arbitrate shared resources and implement process synchronization. In an operating system, a deadlock occurs when a process or thread enters a waiting state because a requested system resource is held by another waiting process, which in turn is waiting for another resource held by another waiting process. If a process is unable to change its state indefinitely because the resources requested by it are being used by another waiting process the system is said to be in a deadlock. In a communications system, deadlocks occur due to lost or corrupt signals rather than resource contention. A deadlock situation on a resource can arise if and only if all of the following conditions hold in a system: Mutual exclusion: At least one resource must be held in a non-shareable mode.

Otherwise, the processes would not be prevented from using the resource when necessary. Only one process can use the resource at any given instant of time. Hold and wait or resource holding: a process is holding at least one resource and requesting additional resources which are being held by other processes. No preemption: a resource can be released only voluntarily by the process holding it. Circular wait: each process must be waiting for a resource, being held by another process, which in turn is waiting for the first process to release the resource. In general, there is a set of waiting processes, P =, such that P1 is waiting for a resource held by P2, P2 is waiting for a resource held by P3 and so on until PN is waiting for a resource held by P1; these four conditions are known as the Coffman conditions from their first description in a 1971 article by Edward G. Coffman, Jr. Most current operating systems cannot prevent deadlocks; when a deadlock occurs, different operating systems respond to them in different non-standard manners.

Most approaches work by preventing one of the four Coffman conditions from occurring the fourth one. Major approaches are as follows. In this approach, it is assumed; this is an application of the Ostrich algorithm. This approach was used by MINIX and UNIX; this is used when the time intervals between occurrences of deadlocks are large and the data loss incurred each time is tolerable. Under the deadlock detection, deadlocks are allowed to occur; the state of the system is examined to detect that a deadlock has occurred and subsequently it is corrected. An algorithm is employed that tracks resource allocation and process states, it rolls back and restarts one or more of the processes in order to remove the detected deadlock. Detecting a deadlock that has occurred is possible since the resources that each process has locked and/or requested are known to the resource scheduler of the operating system. After a deadlock is detected, it can be corrected by using one of the following methods: Process termination: one or more processes involved in the deadlock may be aborted.

One could choose to abort all competing processes involved in the deadlock. This ensures that deadlock is resolved with speed, but the expense is high. Or, one could choose to abort one process at a time; this approach has high overhead because after each abort an algorithm must determine whether the system is still in deadlock. Several factors must be considered while choosing a candidate for termination, such as priority and age of the process. Resource preemption: resources allocated to various processes may be successively preempted and allocated to other processes until the deadlock is broken. Deadlock prevention works by preventing one of the four Coffman conditions from occurring. Removing the mutual exclusion condition means that no process will have exclusive access to a resource; this proves impossible for resources. But with spooled resources, the deadlock could still occur. Algorithms that avoid mutual exclusion are called non-blocking synchronization algorithms; the hold and wait or resource holding conditions may be removed by requiring processes to request all the resources they will need before starting up.

This advance knowledge is difficult to satisfy and, in any case, is an inefficient use of resources. Another way is to require processes to request resources only; this too is impractical. It is so because resources may remain unused for long periods. A process requiring a popular resource may have to wait indefinitely, as such a resource may always be allocated to some process, resulting in resource starvation; the no preemption condition may be difficult or impossible to avoid as a process has to be able to have a resource for a certain amount of time, or the processing outcome may be inconsistent or thrashing may occur. However, the inability to enforce preemption may interfere with a priority algorithm. Preemption of a "locked out" resource implies a rollback, is to be avoided since it is costly in overhead. Algorithms that allow preemption include lock-free and wait-free algorithms and optimistic concurrency control. If a process holding some resources and re

Peru at the FIFA World Cup

Peru participated in the World Cup during the first World Cup in 1930 and in tournaments since 1958. Peru were invited to the first FIFA World Cup in Uruguay in 1930, they finished 10th out of 13 teams. The team either withdrew or did not enter the next four tournaments, in 1934, 1938, 1950, 1954. Peru participated but did not qualify in 1958, 1962, or 1966. Peru had its best result in Mexico 1970. Peru again finished in the top eight at the World Cup in Argentina 1978, they finished first in their group during the first round of the tournament, but were eliminated after losing all their games in the second round. Peru reached the next World Cup finals in Spain 1982, although the team was eliminated in the first round after 2 draws and 1 loss. From 1986 to 2014, Peru did not advance past the CONMEBOL qualifying round. After finishing fifth in CONMEBOL qualifying, Peru reached the 2018 finals by defeating New Zealand in an intercontinental play-off. Peru did not make it past the Group Stage in 2018.

As one of the 13 national sides that accepted the invitation to the inaugural World Cup, Peru was placed in Group 3 with Romania and hosts Uruguay. With a mere 300 spectators, Peru's match against Romania holds the record of lowest attendance in a World Cup game. Peru lost in a violent game where, in the second half, a fight broke out and a Peruvian became the first player sent off in a World Cup. A few days Peru and Uruguay played the inaugural match in the Estadio Centenario in Montevideo; the Peruvians were lauded by the spectators for their defense and the impressive ability of forward José María Lavalle. Peru lost to the eventual champions, who defeated their subsequent opponents by scoring at least 4 goals per game; the Peruvian squad next qualified for the Mexico 1970 World Cup, where they were placed in Group 4 with West Germany and Morocco. Playing their first game two days after the 1970 Ancash earthquake, the team conceded two goals to Bulgaria. However, as Brian Glanville stated, "the elusive dribbling of Cubillas, the powerful breaks from the back four of Héctor Chumpitaz, the running of Sotil and Gallardo, turned the tide."

Peru won 3–2. After defeating Morocco and losing to West Germany, the team finished second in the group and advanced to the quarterfinals. Facing Rivellino, Tostão, Jairzinho and Pelé, Peru was beaten by Brazil in "a spectacular and effervescent game, a game in which both sides delighted in attack and scorned caution," and where both teams displayed "a feast of open play and goals."In the Argentina 1978 World Cup, Peru was placed in Group 4 with Scotland and the Netherlands. With a midfield identified "as the best in the world" by Argentine sports magazine El Gráfico, Peru advanced to the second round as group leaders after defeating Scotland, drawing with the Netherlands, beating Iran; the second round was divided into two groups, Peru's results placed it in Group B with Poland and Argentina. After losing to Brazil and Poland, Peru was out of contention despite having one more game to play against Argentina; this last match proved controversial because the Peruvians lost by a margin that allowed Argentina to reach the final instead of Brazil.

Rumors circulated that the match had been fixed, but nothing was proven. Peru participated in the Spain 1982 World Cup as part of Group 1 with Italy and Cameroon. Prior to the competition, Peru embarked on a practice tour that included victories against Hungary in Budapest and France in Paris, a tie with Algeria in Algiers, upon returning home, a victory over Romania. In the World Cup finals, Peru's opening match against Cameroon ended in a scoreless draw. Against Italy, Peru "put on a mix of solid defending and spectacular attacking" that resulted in a draw against the eventual champions. Peru only needed another draw to advance in the tournament, it managed to maintain a scoreless first half against Poland, but a "mixup in the midfield" and a "tired defense" during the second half led to Peru's defeat and early elimination. Teams were invited to participate at the 1930 World Cup, so there was no qualifying tournaments. Peru did not enter or withdrew from the qualifying rounds in 1934, 1938, 1950 and 1954.

Peru failed to qualify in 1958, 1962 and 1966. For the 1970 CONMEBOL qualifying tournament, Peru eliminated Argentina; the game between Peru and Bolivia in La Paz is infamous for being fixed by Argentina in favor of Bolivia. Match referee Sergio Chechelev annulled a valid goal from Peru without any justification, allowing Bolivia to win 2-1. Years Chechelev admitted that Argentina had paid him to favor Bolivia. Against Argentina in Buenos Aires, the last game of the group, Peru managed a 2-2 draw, preventing Argentina's intervention from succeeding in the end; this is the only time. Peru was eliminated by Chile in 1974. In 1978 and 1982, Peru advanced to the finals. Peru had two wins and two draws in both three-team groups, eliminating Chile and Ecuador in 1978 and Uruguay and Columbia in 1982. Peru qualified for Mexico 1986. Peru was placed in Group 1 with Argentina and Venezuela. Peru's last two matches were against Argentina. Peru beat Argentina 1-0 in Lima on 23 June 1985. In a rematch seven days in Buenos Aires, Peru led 2-1 but Ricardo Gareca inspired his team to a 2-2 draw sending Argentina into the World Cup finals which they would win.

A win would have put Peru directly into the finals, they still had a chance to qualify through the CONMEBOL playoff. Peru lost 2-5 aggregate to Chile in the first r


The Ålfotbreen glacier is located in Vestland county, Norway. The 17-square-kilometre glacier is located on the border of three municipalities in the Nordfjord region: Bremanger and Gloppen, it is located west of the village of Hyen, south of the village of Ålfoten, east of the village of Svelgen. The glacier reaches an elevation of 1,385 metres at its highest point; the area surrounding the glacier, closer to the Nordfjorden, is one of the wettest places in Norway, getting an average of 5,600 millimetres of rainfall each year. The heavy precipitation keeps the glacier healthy with an average thickness of 8 to 10 metres. On 9 January 2009, the 226-square-kilometre area surrounding the glacier, the glacier itself, the nearby Gjegnalundsbreen glacier, were preserved as a landscape protection area by the Norwegian government. Mass balance measurements have been performed at the glacier since 1963 by the Norwegian Water Resources and Energy Directorate. List of glaciers in Norway