325 lines
9.0 KiB
C
325 lines
9.0 KiB
C
/*-------------------------------------------------------------------------
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*
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* s_lock.c
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* Hardware-dependent implementation of spinlocks.
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*
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* When waiting for a contended spinlock we loop tightly for awhile, then
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* delay using pg_usleep() and try again. Preferably, "awhile" should be a
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* small multiple of the maximum time we expect a spinlock to be held. 100
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* iterations seems about right as an initial guess. However, on a
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* uniprocessor the loop is a waste of cycles, while in a multi-CPU scenario
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* it's usually better to spin a bit longer than to call the kernel, so we try
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* to adapt the spin loop count depending on whether we seem to be in a
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* uniprocessor or multiprocessor.
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*
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* Note: you might think MIN_SPINS_PER_DELAY should be just 1, but you'd
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* be wrong; there are platforms where that can result in a "stuck
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* spinlock" failure. This has been seen particularly on Alphas; it seems
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* that the first TAS after returning from kernel space will always fail
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* on that hardware.
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*
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* Once we do decide to block, we use randomly increasing pg_usleep()
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* delays. The first delay is 1 msec, then the delay randomly increases to
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* about one second, after which we reset to 1 msec and start again. The
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* idea here is that in the presence of heavy contention we need to
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* increase the delay, else the spinlock holder may never get to run and
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* release the lock. (Consider situation where spinlock holder has been
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* nice'd down in priority by the scheduler --- it will not get scheduled
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* until all would-be acquirers are sleeping, so if we always use a 1-msec
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* sleep, there is a real possibility of starvation.) But we can't just
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* clamp the delay to an upper bound, else it would take a long time to
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* make a reasonable number of tries.
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*
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* We time out and declare error after NUM_DELAYS delays (thus, exactly
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* that many tries). With the given settings, this will usually take 2 or
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* so minutes. It seems better to fix the total number of tries (and thus
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* the probability of unintended failure) than to fix the total time
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* spent.
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*
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* Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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*
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* IDENTIFICATION
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* src/backend/storage/lmgr/s_lock.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "postgres.h"
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#include <time.h>
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#include <unistd.h>
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#include "common/pg_prng.h"
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#include "port/atomics.h"
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#include "storage/s_lock.h"
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#include "utils/wait_event.h"
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#define MIN_SPINS_PER_DELAY 10
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#define MAX_SPINS_PER_DELAY 1000
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#define NUM_DELAYS 1000
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#define MIN_DELAY_USEC 1000L
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#define MAX_DELAY_USEC 1000000L
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slock_t dummy_spinlock;
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static int spins_per_delay = DEFAULT_SPINS_PER_DELAY;
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/*
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* s_lock_stuck() - complain about a stuck spinlock
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*/
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static void
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s_lock_stuck(const char *file, int line, const char *func)
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{
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if (!func)
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func = "(unknown)";
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#if defined(S_LOCK_TEST)
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fprintf(stderr,
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"\nStuck spinlock detected at %s, %s:%d.\n",
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func, file, line);
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exit(1);
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#else
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elog(PANIC, "stuck spinlock detected at %s, %s:%d",
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func, file, line);
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#endif
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}
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/*
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* s_lock(lock) - platform-independent portion of waiting for a spinlock.
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*/
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int
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s_lock(volatile slock_t *lock, const char *file, int line, const char *func)
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{
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SpinDelayStatus delayStatus;
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init_spin_delay(&delayStatus, file, line, func);
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while (TAS_SPIN(lock))
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{
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perform_spin_delay(&delayStatus);
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}
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finish_spin_delay(&delayStatus);
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return delayStatus.delays;
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}
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#ifdef USE_DEFAULT_S_UNLOCK
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void
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s_unlock(volatile slock_t *lock)
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{
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#ifdef TAS_ACTIVE_WORD
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/* HP's PA-RISC */
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*TAS_ACTIVE_WORD(lock) = -1;
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#else
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*lock = 0;
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#endif
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}
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#endif
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/*
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* Wait while spinning on a contended spinlock.
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*/
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void
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perform_spin_delay(SpinDelayStatus *status)
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{
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/* CPU-specific delay each time through the loop */
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SPIN_DELAY();
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/* Block the process every spins_per_delay tries */
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if (++(status->spins) >= spins_per_delay)
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{
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if (++(status->delays) > NUM_DELAYS)
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s_lock_stuck(status->file, status->line, status->func);
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if (status->cur_delay == 0) /* first time to delay? */
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status->cur_delay = MIN_DELAY_USEC;
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/*
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* Once we start sleeping, the overhead of reporting a wait event is
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* justified. Actively spinning easily stands out in profilers, but
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* sleeping with an exponential backoff is harder to spot...
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*
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* We might want to report something more granular at some point, but
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* this is better than nothing.
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*/
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pgstat_report_wait_start(WAIT_EVENT_SPIN_DELAY);
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pg_usleep(status->cur_delay);
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pgstat_report_wait_end();
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#if defined(S_LOCK_TEST)
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fprintf(stdout, "*");
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fflush(stdout);
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#endif
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/* increase delay by a random fraction between 1X and 2X */
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status->cur_delay += (int) (status->cur_delay *
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pg_prng_double(&pg_global_prng_state) + 0.5);
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/* wrap back to minimum delay when max is exceeded */
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if (status->cur_delay > MAX_DELAY_USEC)
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status->cur_delay = MIN_DELAY_USEC;
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status->spins = 0;
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}
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}
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/*
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* After acquiring a spinlock, update estimates about how long to loop.
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*
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* If we were able to acquire the lock without delaying, it's a good
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* indication we are in a multiprocessor. If we had to delay, it's a sign
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* (but not a sure thing) that we are in a uniprocessor. Hence, we
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* decrement spins_per_delay slowly when we had to delay, and increase it
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* rapidly when we didn't. It's expected that spins_per_delay will
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* converge to the minimum value on a uniprocessor and to the maximum
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* value on a multiprocessor.
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*
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* Note: spins_per_delay is local within our current process. We want to
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* average these observations across multiple backends, since it's
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* relatively rare for this function to even get entered, and so a single
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* backend might not live long enough to converge on a good value. That
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* is handled by the two routines below.
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*/
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void
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finish_spin_delay(SpinDelayStatus *status)
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{
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if (status->cur_delay == 0)
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{
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/* we never had to delay */
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if (spins_per_delay < MAX_SPINS_PER_DELAY)
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spins_per_delay = Min(spins_per_delay + 100, MAX_SPINS_PER_DELAY);
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}
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else
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{
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if (spins_per_delay > MIN_SPINS_PER_DELAY)
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spins_per_delay = Max(spins_per_delay - 1, MIN_SPINS_PER_DELAY);
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}
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}
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/*
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* Set local copy of spins_per_delay during backend startup.
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*
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* NB: this has to be pretty fast as it is called while holding a spinlock
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*/
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void
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set_spins_per_delay(int shared_spins_per_delay)
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{
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spins_per_delay = shared_spins_per_delay;
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}
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/*
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* Update shared estimate of spins_per_delay during backend exit.
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*
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* NB: this has to be pretty fast as it is called while holding a spinlock
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*/
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int
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update_spins_per_delay(int shared_spins_per_delay)
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{
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/*
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* We use an exponential moving average with a relatively slow adaption
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* rate, so that noise in any one backend's result won't affect the shared
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* value too much. As long as both inputs are within the allowed range,
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* the result must be too, so we need not worry about clamping the result.
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*
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* We deliberately truncate rather than rounding; this is so that single
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* adjustments inside a backend can affect the shared estimate (see the
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* asymmetric adjustment rules above).
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*/
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return (shared_spins_per_delay * 15 + spins_per_delay) / 16;
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}
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/*****************************************************************************/
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#if defined(S_LOCK_TEST)
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/*
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* test program for verifying a port's spinlock support.
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*/
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struct test_lock_struct
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{
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char pad1;
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slock_t lock;
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char pad2;
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};
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volatile struct test_lock_struct test_lock;
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int
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main()
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{
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pg_prng_seed(&pg_global_prng_state, (uint64) time(NULL));
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test_lock.pad1 = test_lock.pad2 = 0x44;
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S_INIT_LOCK(&test_lock.lock);
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if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
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{
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printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
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return 1;
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}
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if (!S_LOCK_FREE(&test_lock.lock))
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{
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printf("S_LOCK_TEST: failed, lock not initialized\n");
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return 1;
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}
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S_LOCK(&test_lock.lock);
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if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
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{
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printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
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return 1;
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}
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if (S_LOCK_FREE(&test_lock.lock))
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{
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printf("S_LOCK_TEST: failed, lock not locked\n");
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return 1;
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}
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S_UNLOCK(&test_lock.lock);
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if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
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{
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printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
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return 1;
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}
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if (!S_LOCK_FREE(&test_lock.lock))
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{
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printf("S_LOCK_TEST: failed, lock not unlocked\n");
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return 1;
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}
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S_LOCK(&test_lock.lock);
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if (test_lock.pad1 != 0x44 || test_lock.pad2 != 0x44)
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{
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printf("S_LOCK_TEST: failed, declared datatype is wrong size\n");
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return 1;
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}
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if (S_LOCK_FREE(&test_lock.lock))
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{
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printf("S_LOCK_TEST: failed, lock not re-locked\n");
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return 1;
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}
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printf("S_LOCK_TEST: this will print %d stars and then\n", NUM_DELAYS);
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printf(" exit with a 'stuck spinlock' message\n");
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printf(" if S_LOCK() and TAS() are working.\n");
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fflush(stdout);
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s_lock(&test_lock.lock, __FILE__, __LINE__, __func__);
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printf("S_LOCK_TEST: failed, lock not locked\n");
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return 1;
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}
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#endif /* S_LOCK_TEST */
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