In software engineering, busy-waiting or spinning is a technique in which a process repeatedly checks to see if a condition is true, such as whether keyboard input is available, or if a lock is available. Spinning can also be used to generate an arbitrary time delay, a technique that was necessary on systems that lacked a method of waiting a specific length of time. On modern computers with widely differing processor speeds, spinning as a time delay technique often produces unpredictable results unless code is implemented to determine how quickly the processor can execute a "do nothing" loop.
Spinning can be a valid strategy in certain circumstances, most notably in the implementation of spinlocks within operating systems designed to run on SMP systems. In general, however, spinning is considered an anti-pattern and should be avoided, as processor time that could be used to execute a different task is instead wasted on useless activity.
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The following C code examples illustrate two threads that share a global integer i. The first thread uses busy-waiting to check for a change in the value of i:
#include <stdio.h> #include <pthread.h> #include <unistd.h> #include <stdlib.h> volatile int i = 0; /* i is global, so it is visible to all functions. It's also marked volatile, because it may change in a way which is not predictable by the compiler, here from a different thread. */ /* f1 uses a spinlock to wait for i to change from 0. */ static void *f1(void *p) { while (i==0) { /* do nothing - just keep checking over and over */ } printf("i's value has changed to %d.\n", i); return NULL; } static void *f2(void *p) { sleep(60); /* sleep for 60 seconds */ i = 99; printf("t2 has changed the value of i to %d.\n", i); return NULL; } int main() { int rc; pthread_t t1, t2; rc = pthread_create(&t1, NULL, f1, NULL); if (rc != 0) { fprintf(stderr,"pthread f1 failed\n"); return EXIT_FAILURE; } rc = pthread_create(&t2, NULL, f2, NULL); if (rc != 0) { fprintf(stderr,"pthread f2 failed\n"); return EXIT_FAILURE; } pthread_join(t1, NULL); pthread_join(t2, NULL); puts("All pthreads finished."); return 0; }
Most operating systems and threading libraries provide a variety of system calls that will block the process on an event, such as lock acquisition, timer changes, I/O availability or signals. Using such calls generally produces the simplest, most efficient, fair, and race-free result. A single call checks, informs the scheduler of the event it is waiting for, inserts a memory barrier where applicable, and may perform a requested I/O operation before returning. Other processes can use the CPU while the caller is blocked. The scheduler is given the information needed to implement priority inheritance or other mechanisms to avoid starvation.
Busy-waiting itself can be made much less wasteful by using a delay function (e.g., sleep()) found in most operating systems. This puts a thread to sleep for a specified time, during which the thread will waste no CPU time. If the loop is checking something simple then it will spend most of its time asleep and will waste very little CPU time.
In low-level programming, busy-waits may actually be desirable. It may not be desirable or practical to implement interrupt-driven processing for every hardware device, particularly those that are seldom accessed. Sometimes it is necessary to write some sort of control data to hardware and then fetch device status resulting from the write operation, status that may not become valid until a number of machine cycles have elapsed following the write. The programmer could call an operating system delay function, but doing so may consume more time than would be expended in spinning for a few clock cycles waiting for the device to return its status.