Load balancing Presented by 張肇烜

Outline Load Balancing Static Load Balancing Dynamic Load Balancing

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

Load Balancing (cont.)

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

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

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.

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

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.

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.

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

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

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

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

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.

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.

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

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.

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.

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。