Round Robin Algo.

[WISDOM]

What is Round Robin Algorithm?

[Vision]

A scheduling algorithm in which processes are activated in a fixed cyclic order. Those which cannot proceed because they are waiting for some event (e.g. termination of a child process or an input/output operation) simply return control to the scheduler. The virtue of round-robin scheduling is its simplicity - only the processes themselves need to know what they are waiting for or how to tell if it has happened. However, if a process goes back to sleep just before the event for which it is waiting occurs then the event will not get handled until all the other processes have been activated.

[WISDOM]

Where is this algorithm used or implemented?

[Vision]

Round-Robin Scheduling Algorithm, that is used to prevent starvation. Lets say you have 4 jobs.
Job A Job B Job C and Job D
Round Robin will give each job the same amount of CPU time until they are finished.
so it will go like A B C D A B C D A B C D. for like 3 cycles each. If one of the jobs finishes it will be removed from the queue. If a job is added it will be added to the circular queue

[WISDOM]

which scheduling algorithm is used by windows XP ?

[Vision]

• Windows XP uses a quantum-based, preemptive priority scheduling algorithm
• Threads are scheduled rather than processes.
• Since the preemptive priority algorithm is implemented with multiple queues, it can also be considered a multiple feedback-queue algorithm. However, each class of thread is normally restricted to a small band of 5 priority levels, from 2 below the base priority for its process to 2 above. Each band of priorities overlaps with the band above it and the band below it.
• Preemption can occur for any of 4 reasons:
  • higher-priority thread becomes ready
  • thread terminates
  • time quantum exhausted
  • thread performs a blocking system call, such as for I/O, in which case it leaves the READY state and enters a WAITING state.
• 32 priority levels are used, where priority 31 is the highest priority and priority 0 is the lowest priority
  • memory management thread: priority 0
    • text shows idle thread with priority 1, which is higher than the memory management thread, but it does not explain how this could work
  • variable class of priorities (1-15)
  • real-time class of priorities (16-31)
• Threads in the real-time class have fixed priorities
• The running thread is always one with the highest priority level.
• If no ready thread exists, the idle thread is run.
• When a thread's time quantum runs out, its priority is lowered (by one it appears), but its priority is never lowered too far
• When a thread becomes READY after WAITING, it is given a priority boost, with the largest boost for waiting for keyboard I/O and a smaller boost for waiting for disk.
• any thread of the process associated with the window that the user is currently interacting with (the foreground process) is given a priority boost and as well has its quantum size tripled.

[vjsreevs]

nice explanation on XP vision .... thanx 4 tat ...

wisdom round robin is mostly used at places where the response time 4 all the processes need 2 b fast ....
whatever may b the completion time but the processes starts exec ... it avoids starvation ...

I want to mention one more example where round robin mechanism is used.I am right now working on a project whre we need to balance the load among the webservers.So we are using load balancer and it distributes the requests using the round robinhood mechanism so that all the servers handle requests and no single server is over burdened or starved.

[WISDOM]

Hey unregd., can u explain a lilltle more about u'r project. i am not asking anything specific to u'r project or the ideas that u r going to use but the theme of the project and using which tools are u going to achieve that........eager to hear frm u'r side.

[balanagireddy]

Basically my project is related to web server performance.Currently our website is able to handle 200 users simultaneously.If we upgrade server we can handle 100 more users.Insead we are trying to employ a cost effective method by maintaining a cluster.Where a load balancer computer distributes request to backend servers(medium configuration) which can handle 100 req/sec.So if we have 5 servers at backend we can handle 400-500 req/sec easily.And the tools are open source tools freely available.This method can be employed not only for webserver and employes for wide range of daemons.