Synchronisation

Synchronisation


In computer science, synchronization refers to one of two distinct but related concepts: synchronization of processes, and synchronization of data. Process synchronization refers to the idea that multiple processes are to join up or handshake at a certain point, in order to reach an agreement or commit to a certain sequence of action.Data synchronization refers to the idea of keeping multiple copies of a dataset in coherence with one another, or to maintaindata integrity. Process synchronization primitives are commonly used to implement data synchronization.

The need for synchronization

Thread or process synchronizationEdit

Figure 1: Three processes accessing a shared resource (critical section) simultaneously.
Thread synchronization is defined as a mechanism which ensures that two or more concurrent processes or threads do not simultaneously execute some particular program segment known as critical section. Processes' access to critical section is controlled by using synchronization techniques. When one thread starts executing the critical section (serialized segment of the program) the other thread should wait until the first thread finishes. If proper synchronization techniques[1] are not applied, it may cause a race condition where the values of variables may be unpredictable and vary depending on the timings of context switches of the processes or threads.
For example, suppose that there are three processes, namely 1, 2, and 3. All three of them are concurrently executing, and they need to share a common resource (critical section) as shown in Figure 1. Synchronization should be used here to avoid any conflicts for accessing this shared resource. Hence, when Process 1 and 2 both try to access that resource, it should be assigned to only one process at a time. If it is assigned to Process 1, the other process (Process 2) needs to wait until Process 1 frees that resource (as shown in Figure 2).
Figure 2: A process accessing a shared resource if available, based on some synchronization technique.
Another synchronization requirement which needs to be considered is the order in which particular processes or threads should be executed. For example, we cannot board a plane until we buy a ticket. Similarly, we cannot check e-mails without validating our credentials (i.e., user name and password). In the same way, an ATM will not provide any service until we provide it with a correct PIN.
Other than mutual exclusion, synchronization also deals with the following:
  • deadlock, which occurs when many processes are waiting for a shared resource (critical section) which is being held by some other process. In this case, the processes just keep waiting and execute no further;
  • starvation, which occurs when a process is waiting to enter the critical section, but other processes monopolize the critical section, and the first process is forced to wait indefinitely;
  • priority inversion, which occurs when a high-priority process is in the critical section, and it is interrupted by a medium-priority process. This violation of priority rules can happen under certain circumstances and may lead to serious consequences in real-time systems;
  • busy waiting, which occurs when a process frequently polls to determine if it has access to a critical section. This frequent polling robs processing time from other processes.



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