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  How to find Global Deadlocks in Lock management pragmatics

How to find Global Deadlocks:

The searching of local deadlocks has already been described. To search global deadlocks a distinguish task called the global deadlock detector is started in some distinguished node. This task is in session with all local deadlock detectors as well as coordinates the activities of the local deadlock detectors. This global deadlock detector is able to run in any node however probably should be located to minimize its communication distance to the lock managers.

Every local deadlock Detector needs to find all potential global deadlock paths in his node. In the previous section it was revealed that a global deadlock cycle has the form:

lockwait ->... -> sessionwait ->lockwait ->...-> sessionwait ->

Consequently each local deadlock detector periodically enumerates all

session -> lockwait ->...-> sessionwait

paths in his node by working backwards from processes that are in session-wait (as opposed to console wait, disk wait, processor wait,…) Starting at such a process it observes if some local process is lock waiting for this process. If therefore the deadlock detector searches backwards looking for some process which has a session in progress.

When such a path is found the subsequent information is sent to the global deadlock detector:

•    Sessions and transactions at endpoints of the path as well as their local pre-emption costs.
•    The minimum cost transaction in the path as well as his local pre-emption cost.

(It may perhaps make sense to batch this information to the global detector.) Intermittently the global deadlock detector

•    collects these messages,
•    pastes all these paths together by matching up sessions and
•    Enumerates cycles as well as selects victims just as in the local deadlock detector case.

One tricky point is that the cost of a distributed transaction is the summation of the costs of its cohorts. The global deadlock detector estimated this cost by summing the costs of the cohorts of the transaction known to it (not every cohorts of a deadlocked transaction will be in known to the global deadlock detector.)

When a victim is selected the lock administrator of the node the victim is waiting in is informed of the deadlock. The local lock administrator in turn informs the victim with a deadlock return.

The utilization of periodic deadlock detection (as opposed to detection every time anyone waits) is even more significant for a distributed system than for a centralized system, the cost of detection is greatly higher in a distributed system. This will modify the intersection of the cost of detection and cost of detecting late curves.

If the network is actually large the deadlock detector can be staged. That is we are able to look for deadlock among four nodes then among sixteen nodes and so on.

If one node crashes afterwards its partition of the system is unavailable.

In this situation its cohorts in other nodes can wait for it to recover or they can abort. If the down node takes place to house the global lock manager then no global deadlocks will be detected until the node recovers. If this isn’t good then the lock managers can nominate a new global lock manager whenever the current one crashes. The new manager is able to run in any node that can be in session with all other nodes.

The new global lock administrator collects the local graphs and goes about gluing them together, finding cycles, and picking victims.

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