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Singhal s dynamic information-structure algorithm in .NET framework Maker Code 128C in .NET framework Singhal s dynamic information-structure algorithm barcode for VB.NET




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9.5 Singhal s dynamic information-structure algorithm use none none creator toproduce none with nonevb.net + print to barcode scanner An explanation of the algorithm iPad At high level, Si acqu ires permission to execute the CS from all sites in its request set Ri and it releases the CS by sending a REPLY message to all sites in its inform set Ii . If site Si , which itself is requesting the CS, receives a higher priority REQUEST message from a site Sj , then Si takes the following actions: (i) Si immediately sends a REPLY message to Sj , (ii) if Sj is not in Ri , then1 Si also sends a REQUEST message to Sj , and (iii) Si places an entry for Sj in Ri . Otherwise (i.

e., if the request of Si has priority over the request of Sj ), Si places an entry for Sj into Ii so that Sj can be sent a REPLY message when Si finishes with the execution of the CS. If Si receives a REQUEST message from Sj when it is executing the CS, then it simply puts Sj in Ii so that Sj can be sent a REPLY message when Si finishes with the execution of the CS.

If Si receives a REQUEST message from Sj when it is neither requesting nor executing the CS, then it places an entry for Sj in Ri and sends Sj a REPLY message. Rules for information exchange and updating request and inform sets are such that the staircase pattern is preserved in the system even after the sites have executed the CS any number of times. However, the positions of sites in the staircase pattern change as the system evolves.

(For a proof of this, see [28].) The site to execute CS last positions itself at the right end of the staircase pattern..

9.5.2 Correctness We informally discuss why the algorithm achieves mutual exclusion and why it is free from deadlocks. For a formal proof, the readers are referred to [27]..

Achieving mutual exclusion Note that the initial state of the information-structure satisfies the following condition: for every Si and Sj , either Sj Ri or Si Rj . Therefore, if two sites request CS, one of them will always ask for the permission of the another. However, this is not sufficient for mutual exclusion [28].

Whenever there is a conflict between two sites (i.e., they concurrently invoke mutual exclusion), the sites dynamically adjust their request sets such that both request permission of each other satisfying the condition for mutual exclusion.

This is a nice feature of the algorithm because if the informationstructures of the sites satisfy the condition for mutual exclusion all the. Absence of Sj from Ri none none implies that Si has not previously sent a REQUEST message to Sj . This is the reason why Si also sends a REQUEST message to Sj when it receives a REQUEST message from Sj . This step is also required to preserve the staircase pattern of the informationstructure of the system.

. Distributed mutual exclusion algorithms time, the sites will e none for none xchange more messages. Instead, it is more desirable to dynamically adjust the request set of the sites as and when needed to insure mutual exclusion because it optimizes the number of messages exchanged..

Freedom from deadlocks In the algorithm, each none none request is assigned a globally unique timestamp which determines its priority. The algorithm is free from deadlocks because sites use timestamp ordering (which is unique system wide) to decide request priority and a request is blocked only by higher priority requests. Example Consider a system with five sites S1 S5 .

Suppose S2 and S3 want to enter the CS concurrently, and they both send appropriate request messages. S3 sends a request message to sites in its Request set {S1 , S2 }, and S2 sends a request message to the only site in its Request set {S1 }. There are three possible scenarios: 1.

If timestamp of S3 s request is smaller, then on receiving S3 s request, S2 sends a REPLY message to S3 . S2 also adds S3 to its Request set and sends S3 a REQUEST message. On receiving a REPLY message from S2 , S3 removes S2 from its Request set.

S1 sends a REPLY to both S2 and S3 because it is neither requesting to enter the CS nor executing the CS. S1 adds S2 and S3 to its Request set because any one of these sites could possibly be in the CS when S1 requests for an entry into CS in the future. On receiving S1 s REPLY message, S3 removes S1 from its Request set and, since it has REPLY messages from all sites in its (initial) Request set, it enters the CS.

2. If timestamp of S3 is larger, then on receiving S3 s request, S2 adds S3 to its Inform set. When S2 gets a REPLY from S1 , it enters the CS.

When S2 relinquishes the CS, it informs S3 (the i.d. of S3 is present in S2 s Inform set) about its consent to enter the CS.

Then, S2 removes S3 from its Inform set and add S3 to its Request set. This is logical because S3 could be executing in CS when S2 requests a CS entry permission in the future. 3.

If S2 receives a REPLY from S1 and starts executing CS before S3 s REQUEST reaches S2 , S2 simply adds S3 to its Inform set, and sends S3 a REPLY after exiting the CS..
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