Load balancing

1. Load balancing Presented by 張肇烜

2. Outline • Load Balancing • Static Load Balancing • Dynamic Load Balancing

3. Load Balancing • Use to distribute computations fairly across processors in order to obtain the highest possible execution speeds.

4. Load Balancing (cont.)

5. Static Load Balancing • Before the execution of any process. • Some potential static load balancing: • Randomized algorithm • Round robin algorithm • Weight round robin algorithm

6. Static Load Balancing (cont.) • Randomized algorithms: • Selects processes at random to take tasks.

7. Static Load Balancing (cont.) • Round robin algorithms: • Passes out tasks in sequential order of processes coming back to the first when all processes have been given a task.

8. Static Load Balancing (cont.) • Weighted round robin algorithms: • To improve imbalance problem of RR.

9. Static Load Balancing (cont.) • Several flaws: • Very difficult to estimate accurately the execution times of various parts of a program without actually executing the parts. • Communication delays that vary under different circumstances.

10. Dynamic Load Balancing • Vary load during the execution of the processes. • Does incur an additional overhead during execution, but it is much more effective than static load balancing.

11. Dynamic Load Balancing (cont.) • Can be classified as: • Centralized • Decentralized

12. Dynamic Load Balancing (cont.) • Centralized DLB: • Tasks handed out from a centralized location. Master-slave structure.

13. Dynamic Load Balancing (cont.) • Decentralized DLB: • A worker process may receive tasks from other worker processes and may send tasks to other worker processes.

14. Dynamic Load Balancing (cont.) • Task transfer mechanisms: • Receiver-Initiated Method • Sender-Initiated Method

15. Dynamic Load Balancing (cont.) • Receiver-Initiated Method: • A process requests tasks from other processes it selects. • Typically, a process would request tasks from other processes when it has few or no tasks to perform. • It has been shown to work well at high system load.

16. Dynamic Load Balancing (cont.) • Sender-Initiated Method: • A process sends tasks to other processes it selects. • Typically, a process with a heavy load passes out some of its tasks to others that are willing to accept them. • It has been shown to work well for light over system load.

17. Dynamic Load Balancing (cont.) • Some potential Dynamic load balancing : • Least-Connection Algorithm (LC) • Weighted Least-Connection Algorithm (WLC)

18. Dynamic Load Balancing (cont.) • Least-Connection Algorithm : • The requests are distributed to the server based on the number of established connections. • Least-connection algorithm performs well when the loads of requests vary frequently.

19. Dynamic Load Balancing (cont.) • Weighted Least-Connection Algorithm : • Similar to weighted round robin algorithm, weights can be combined to least-connection algorithm for heterogeneous environment.

20. Dynamic Load Balancing (cont.) • Weighted Least-Connection Algorithm : • 其算法流程如下，假設有一組伺服器S = {S0, S1, ..., Sn-1}，W(Si)表示伺服器Si的權重值，C(Si)表示伺服器Si的目前連接數。而所有伺服器目前連接數的總和為CSUM = ΣC(Si) (i=0, 1, .. , n-1)。當前的新連接請求會被發送伺服器Sm，而且僅當伺服器Sm滿足以下條件，(C(Sm) / CSUM)/ W(Sm) = min { (C(Si) / CSUM) / W(Si)} (i=0, 1, . , n-1)，其中W(Si)不得為零。