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  Preemptive scheduling and Lock management pragmatics

Relationship in a Operating system Lock Manager:

Most operating systems provide a lock administrator to regulate access to files and other system resources. This lock manager generally supports a limited set of lock names the modes: share, exclusive as well as beware and has some form of deadlock detection. These lock managers are usually net prepared for the demands of a data management system (fast calls, lots of locks, several modes, lock classes,.,.)The basic lock manager could be extended as well as refined and in time that is what will happen.

There is a big problem regarding having two lock managers in the same host. Every may think it has no deadlock but if their graphs are glued together a “global” deadlock exists. This makes it extremely difficult to build on top of the basic lock manager.

Convoy Phenomenon: Preemptive Scheduling is bad

Lock administrator has strong interactions with the scheduler. Presume that there are certain high traffic shared system resources. Operating on these resources consists of locking them altering them as well as then unlocking them (the buffer pool and log are examples of this.) These operations are designed to be extremely fast so that the resource is almost always free. In particular the resource is never held during an I/O operation, For illustration the buffer manager latch is acquired every 1000 instructions and is held for about 50 instructions.

If the system has no pre-emptive scheduling then on a uni-processor then when a process begins the resource is free and when he completes the resource is free (because he doesn’t hold it when he does I/O or yields the processor.) On a multi-processor if the resource is busy, the process can sit in a busy wait until the resource is free for the reason that the resource is known to be held by others for only a short time.

If the basic system has a pre-emptive scheduler and if that scheduler pre-empts a process holding a critical resource (example the log latch) then terrible things happen: Each other processes waiting for the latch will dispatched prior to this process is dispatched, and because the resource is high traffic each of these processes requests and waits for the resource. Eventually the holder of the resource is re-dispatched and he almost immediately grants the latch to the next waiter. But for the reason that it is high traffic the process almost immediately re-requests the latch (that is about 1000 instructions later.) Fair scheduling requires that he wait therefore he goes on the end of the queue waiting for those ahead of him. This queue of waiters is called a procession. It is a steady phenomenon- once a convoy is established it persists for a very long time.

We (System R) have found numerous solutions to this problem. The apparent solution is to eliminate such resources. That is a good idea as well as can be achieved to some degree by refining the granularity of the lockable unit (example twenty buffer manager latches rather than just one.) Nevertheless if a convoy ever forms on any of these latches it will be stable so that is not a solution. I run off it as an exercise for the reader to find a better solution to the problem.

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