In
computer scienceComputer science is the study of the theoretical foundations of information and computation, and of practical techniques for their implementation and application in computer systems. It is frequently described as the systematic study of algorithmic processes that create, describe and transform...
, the
fetch-and-add CPUThe Central Processing Unit or processor is the portion of a computer system that carries out the instructions of a computer program, and is the primary element carrying out the computer's functions. This term has been in use in the computer industry at least since the early 1960s...
instruction is a special instruction that atomically modifies the contents of a memory location. It is used to implement
mutual exclusionMutual exclusion algorithms are used in concurrent programming to avoid the simultaneous use of a common resource, such as a global variable, by pieces of computer code called critical sections. A critical section is a piece of code where a process or thread accesses a common resource...
and concurrent algorithms in multiprocessor systems, a generalization of semaphores.
In
uniprocessorA uniprocessor system is a computer system with a single central processing unit. As more and more computers employ multiprocessing architectures, such as SMP and MPP, the term is used to refer to systems that still have only one CPU...
systems, it is sufficient to disable
interruptIn computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
s before accessing a critical region.
However, in multiprocessor systems, it is impossible and undesirable to disable
interruptIn computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
s on all processors at the same time; and even with interrupts disabled two or more processors could be attempting to access the same memory at the same time.
In
computer scienceComputer science is the study of the theoretical foundations of information and computation, and of practical techniques for their implementation and application in computer systems. It is frequently described as the systematic study of algorithmic processes that create, describe and transform...
, the
fetch-and-add CPUThe Central Processing Unit or processor is the portion of a computer system that carries out the instructions of a computer program, and is the primary element carrying out the computer's functions. This term has been in use in the computer industry at least since the early 1960s...
instruction is a special instruction that atomically modifies the contents of a memory location. It is used to implement
mutual exclusionMutual exclusion algorithms are used in concurrent programming to avoid the simultaneous use of a common resource, such as a global variable, by pieces of computer code called critical sections. A critical section is a piece of code where a process or thread accesses a common resource...
and concurrent algorithms in multiprocessor systems, a generalization of semaphores.
In
uniprocessorA uniprocessor system is a computer system with a single central processing unit. As more and more computers employ multiprocessing architectures, such as SMP and MPP, the term is used to refer to systems that still have only one CPU...
systems, it is sufficient to disable
interruptIn computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
s before accessing a critical region.
However, in multiprocessor systems, it is impossible and undesirable to disable
interruptIn computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
s on all processors at the same time; and even with interrupts disabled two or more processors could be attempting to access the same memory at the same time. The fetch-and-add instruction allows any processor to atomically increment a value in memory location, preventing such multiple processor collisions.
Maurice HerlihyMaurice Herlihy is a computer scientist active in the field of multiprocessor synchronization, including work on the use of Compare And Swap machine code instructions to implement synchronization primitives...
(1991) proved that fetch-and-add has a finite consensus number, in contrast to the
compare-and-swapIn computer science, the compare-and-swap CPU instruction is a special instruction that atomically compares the contents of a memory location to a given value and, if they are the same, modifies the contents of that memory location to a given new value...
operation. The fetch-and-add operation can solve the wait-free consensus problem for no more than two concurrent processes.
Implementation
The standard fetch and add -instruction behaves like the following function. Crucially the entire function is executed atomically: no process can interrupt the function mid-execution and hence see a state that only exists during the execution of the function. This code only serves to help explain the behaviour of fetch-and-add; atomicity requires explicit hardware support and hence can not be implemented as a simple high level function.
<< atomic >>
function FetchAndAdd(
address location) {
int value := *location
*location := value + 1
return value
}
With fetch-and-add primitive a mutual exclusion lock can be implemented as:
record locktype {
int ticketnumber
int turn
}
procedure LockInit(
locktype* lock ) {
lock.ticketnumber := 0
lock.turn := 0
}
procedure Lock(
locktype* lock ) {
int myturn := FetchAndAdd( &lock.ticketnumber )
while lock.turn ≠ myturn
skip //
spin until lock is acquired
}
procedure UnLock(
locktype* lock) {
FetchAndAdd( &lock.turn )
}
These routines provide a mutual-exclusion lock when following conditions are met:
- Locktype data structure is initialized with function LockInit before use
- Number of tasks waiting for the lock does not exceed INT_MAX at any time
- Integer datatype used in lock values can 'wrap around' when continuously incremented
x86 implementation
In the x86 architecture, the instruction ADD with the destination operand specifying a memory location is a fetch-and-add instruction that has been there since the 8086 (it just wasn't called that then), and with the LOCK prefix, is atomic across multiple processors. However, it could not return the original value of the memory location (though it returned some flags) until the 486 introduced the XADD instruction.
See also
- Test-and-set
In computer science, the test-and-set instruction is an instruction used to both test and write to a memory location as part of a single atomic operation. This means setting a value, but first performing some test . The value is set dependent on the result of the test...
- Test and Test-and-set
In computer science, the test-and-set CPU instruction is used to implementmutual exclusion in multiprocessor environments. Although a correct lock can be implemented with test-and-set, it can lead to memory contention in busy lock .To lower the overhead a more elaborate locking protocol...
- Compare-and-swap
In computer science, the compare-and-swap CPU instruction is a special instruction that atomically compares the contents of a memory location to a given value and, if they are the same, modifies the contents of that memory location to a given new value...
- Load-Link/Store-Conditional
In computer science, load-link and store-conditional are a pair of instructions that together implement a lock-free atomic read-modify-write operation....