/* * Wait while spinning on a contended spinlock. */ void perform_spin_delay(SpinDelayStatus *status) { /* CPU-specific delay each time through the loop */ SPIN_DELAY(); /* Block the process every spins_per_delay tries */ if (++(status->spins) >= spins_per_delay) { if (++(status->delays) > NUM_DELAYS) s_lock_stuck(status->file, status->line, status->func); if (status->cur_delay == 0) /* first time to delay? */ status->cur_delay = MIN_DELAY_USEC; pg_usleep(status->cur_delay); #if defined(S_LOCK_TEST) fprintf(stdout, "*"); fflush(stdout); #endif /* increase delay by a random fraction between 1X and 2X */ status->cur_delay += (int) (status->cur_delay * ((double) random() / (double) MAX_RANDOM_VALUE) + 0.5); /* wrap back to minimum delay when max is exceeded */ if (status->cur_delay > MAX_DELAY_USEC) status->cur_delay = MIN_DELAY_USEC; status->spins = 0; } }
/* * s_lock(lock) - platform-independent portion of waiting for a spinlock. */ void s_lock(volatile slock_t *lock, const char *file, int line TSRMLS_DC) { /* * We loop tightly for awhile, then delay using pg_usleep() and try again. * Preferably, "awhile" should be a small multiple of the maximum time we * expect a spinlock to be held. 100 iterations seems about right as an * initial guess. However, on a uniprocessor the loop is a waste of * cycles, while in a multi-CPU scenario it's usually better to spin a bit * longer than to call the kernel, so we try to adapt the spin loop count * depending on whether we seem to be in a uniprocessor or multiprocessor. * * Note: you might think MIN_SPINS_PER_DELAY should be just 1, but you'd * be wrong; there are platforms where that can result in a "stuck * spinlock" failure. This has been seen particularly on Alphas; it seems * that the first TAS after returning from kernel space will always fail * on that hardware. * * Once we do decide to block, we use randomly increasing pg_usleep() * delays. The first delay is 1 msec, then the delay randomly increases to * about one second, after which we reset to 1 msec and start again. The * idea here is that in the presence of heavy contention we need to * increase the delay, else the spinlock holder may never get to run and * release the lock. (Consider situation where spinlock holder has been * nice'd down in priority by the scheduler --- it will not get scheduled * until all would-be acquirers are sleeping, so if we always use a 1-msec * sleep, there is a real possibility of starvation.) But we can't just * clamp the delay to an upper bound, else it would take a long time to * make a reasonable number of tries. * * We time out and declare error after NUM_DELAYS delays (thus, exactly * that many tries). With the given settings, this will usually take 2 or * so minutes. It seems better to fix the total number of tries (and thus * the probability of unintended failure) than to fix the total time * spent. * * The pg_usleep() delays are measured in milliseconds because 1 msec is a * common resolution limit at the OS level for newer platforms. On older * platforms the resolution limit is usually 10 msec, in which case the * total delay before timeout will be a bit more. */ #define MIN_SPINS_PER_DELAY 10 #define MAX_SPINS_PER_DELAY 1000 #define NUM_DELAYS 1000 #define MIN_DELAY_MSEC 1 #define MAX_DELAY_MSEC 1000 int spins = 0; int delays = 0; int cur_delay = 0; while (TAS(lock)) { /* CPU-specific delay each time through the loop */ SPIN_DELAY(); /* Block the process every spins_per_delay tries */ if (++spins >= spins_per_delay) { if (++delays > NUM_DELAYS) s_lock_stuck(lock, file, line TSRMLS_CC); if (cur_delay == 0) /* first time to delay? */ cur_delay = MIN_DELAY_MSEC; pg_usleep(cur_delay * 1000L); #if defined(S_LOCK_TEST) fprintf(stdout, "*"); fflush(stdout); #endif /* increase delay by a random fraction between 1X and 2X */ cur_delay += (int) (cur_delay * ((double) rand() / (double) MAX_RANDOM_VALUE) + 0.5); /* wrap back to minimum delay when max is exceeded */ if (cur_delay > MAX_DELAY_MSEC) cur_delay = MIN_DELAY_MSEC; spins = 0; } } /* * If we were able to acquire the lock without delaying, it's a good * indication we are in a multiprocessor. If we had to delay, it's a sign * (but not a sure thing) that we are in a uniprocessor. Hence, we * decrement spins_per_delay slowly when we had to delay, and increase it * rapidly when we didn't. It's expected that spins_per_delay will * converge to the minimum value on a uniprocessor and to the maximum * value on a multiprocessor. * * Note: spins_per_delay is local within our current process. We want to * average these observations across multiple backends, since it's * relatively rare for this function to even get entered, and so a single * backend might not live long enough to converge on a good value. That * is handled by the two routines below. */ if (cur_delay == 0) { /* we never had to delay */ if (spins_per_delay < MAX_SPINS_PER_DELAY) spins_per_delay = Min(spins_per_delay + 100, MAX_SPINS_PER_DELAY); } else { if (spins_per_delay > MIN_SPINS_PER_DELAY) spins_per_delay = Max(spins_per_delay - 1, MIN_SPINS_PER_DELAY); } }