The Suzuki–Kasami algorithm[1] is a token-based algorithm for achieving mutual exclusion in distributed systems. The process holding the token is the only process able to enter its critical section.
This is a modification to Ricart–Agrawala algorithm[2] in which a REQUEST and REPLY message are used for attaining the critical section, but in this algorithm, a method was introduced in which a seniority vise and also by handing over the critical section to other node by sending a single PRIVILEGE message to other node. So, the node which has the privilege it can use the critical section and if it does not have one it cannot. If a process wants to enter its critical section and it does not have the token, it broadcasts a request message to all other processes in the system. The process that has the token, if it is not currently in a critical section, will then send the token to the requesting process. The algorithm makes use of increasing Request Numbers to allow messages to arrive out-of-order.
Algorithm description
Let be the number of processes. Each process is identified by an integer in .
Data structures
Each process maintains one data structure:
- an array (for Request Number), being the ID of the process containing this array, where stores the last Request Number received by from
The token contains two data structures:
- an array (for Last request Number), where stores the most recent Request Number of process for which the token was successfully granted
- a queue , storing the ID of processes waiting for the token
Algorithm
Requesting the critical section (CS)
When process wants to enter the CS, if it does not have the token, it:
- increments its sequence number
- sends a request message containing new sequence number to all processes in the system
Releasing the CS
When process leaves the CS, it:
- sets of the token equal to . This indicates that its request has been executed
- for every process not in the token queue , it appends to if . This indicates that process has an outstanding request
- if the token queue is not empty after this update, it pops a process ID from and sends the token to
- otherwise, it keeps the token
Receiving a request
When process receives a request from with sequence number , it:
- sets to (if , the message is outdated)
- if process has the token and is not in CS, and if (indicating an outstanding request), it sends the token to process
Executing the CS
A process enters the CS when it has acquired the token.
Performance
- Either or messages for CS invocation (no messages if process holds the token; otherwise requests and reply)
- Synchronization delay is or ( requests and reply)
Notes on the algorithm
- Only the site currently holding the token can access the CS
- All processes involved in the assignment of the CS
- Request messages sent to all nodes
- Not based on Lamport’s logical clock
- The algorithm uses sequence numbers instead
- Used to keep track of outdated requests
- They advance independently on each site
The main design issues of the algorithm:
- Telling outdated requests from current ones
- Determining which site is going to get the token next
References
- ↑ Ichiro Suzuki, Tadao Kasami, , ACM Transactions on Computer Systems, Volume 3 Issue 4, Nov. 1985 (pages 344 - 349)
- ↑ Ricart, Glenn, and Ashok K. Agrawala. "An optimal algorithm for mutual exclusion in computer networks." Communications of the ACM 24.1 (1981): 9-17.