11.3: Starvation

Concept of Starvation

Starvation is usually caused by an overly simplistic scheduling algorithm. For example, if a system always switches between the first two tasks while a third never gets to run, then the third task is being starved of CPU time. The scheduling algorithm, which is part of the kernel, is supposed to allocate resources equally among all processes; that is, the algorithm should allocate resources so that no process continually is blocked from accessing the resources it needs to execute to completion.

Many operating system schedulers employ the concept of process priority. Each process gets a priority - usually the lower the number the higher the priority - making a priority of zero the highest priority a process can have. A high priority process A will run before a low priority process B. If the high priority process (process A) blocks and never gives up control of the processor, the low priority process (B) will (in some systems) never be scheduled—it will experience starvation. If there is an even higher priority process X, which is dependent on a result from process B, then process X might never finish, even though it is the most important process in the system. This condition is called a priority inversion. Modern scheduling algorithms normally contain code to guarantee that all processes will receive a minimum amount of each important resource (most often CPU time) in order to prevent any process from being subjected to starvation.

Starvation is normally caused by a deadlock that causes a process to freeze waiting for resources. Two or more processes become deadlocked when each of them is doing nothing while waiting for a resource occupied by another program in the same set, the two (or more) processes that are waiting can starve while waiting on the one process that has control of the resource. On the other hand, a process is in starvation when it is waiting for a resource that is continuously given to other processes because it can never complete without access to the necessary resource. Starvation-freedom is a stronger guarantee than the absence of deadlock: a mutual exclusion algorithm that must choose to allow one of two processes into a critical section and picks one arbitrarily is deadlock-free, but not starvation-free.

A possible solution to starvation is to use a scheduling algorithm with priority queue that also uses the aging technique. Aging is a technique of gradually increasing the priority of processes that wait in the system for a long time. For example, if a process X has a priority of 100, it would probably be near the bottom of the priority list, it would get very little processing time on a busy system. Using the concept of aging, over some set period of time, process X's priority would decrease, to say 50. If process X still did not get enough resources or processing time, after another period of time the priority would again decrease, to say 25. Eventually process X would get to a high enough priority (low number) that it would be scheduled for access to resources/processor and would complete in a proper fashion.

Adapted from:
"Starvation (computer science)" by Multiple Contributors, Wikipedia is licensed under CC BY-SA 3.0