Data serialization

Any storage element that can be read or written by more than one thread may change while the program is running.

This is generally true of multiprogramming environments as well as multiprocessing environments, but the advent of multiprocessors adds to the scope and importance of this consideration in two ways:

Multiprocessors and thread support make it attractive and easier to write applications that share data among threads.
The kernel can no longer solve the serialization problem simply by disabling interrupts.

Note: To avoid serious problems, programs that share data must arrange to access that data serially, rather than in parallel. Before a program updates a shared data item, it must ensure that no other program (including another copy of itself running on another thread) will change the item. Reads can usually be done in parallel.

The primary mechanism that is used to keep programs from interfering with one another is the lock. A lock is an abstraction that represents permission to access one or more data items. Lock and unlock requests are atomic; that is, they are implemented in such a way that neither interrupts nor multiprocessor access affect the outcome. All programs that access a shared data item must obtain the lock that corresponds to that data item before manipulating it. If the lock is already held by another program (or another thread running the same program), the requesting program must defer its access until the lock becomes available.

Besides the time spent waiting for the lock, serialization adds to the number of times a thread becomes nondispatchable. While the thread is nondispatchable, other threads are probably causing the nondispatchable thread's cache lines to be replaced, which results in increased memory-latency costs when the thread finally gets the lock and is dispatched.

The operating system's kernel contains many shared data items, so it must perform serialization internally. Serialization delays can therefore occur even in an application program that does not share data with other programs, because the kernel services used by the program have to serialize shared kernel data.