PG参数autovacuum_naptime解析

/*
 * Main loop for the autovacuum launcher process.
 */
NON_EXEC_STATIC void
AutoVacLauncherMain(int argc, char *argv[])
{
	...
   
	/* loop until shutdown request */
	while (!ShutdownRequestPending)
	{
		struct timeval nap;
		TimestampTz current_time = 0;
		bool		can_launch;

		/*
		 * This loop is a bit different from the normal use of WaitLatch,
		 * because we'd like to sleep before the first launch of a child
		 * process.  So it's WaitLatch, then ResetLatch, then check for
		 * wakening conditions.
		 */

		launcher_determine_sleep(!dlist_is_empty(&AutoVacuumShmem->av_freeWorkers),
								 false, &nap);

		/*
		 * Wait until naptime expires or we get some type of signal (all the
		 * signal handlers will wake us by calling SetLatch).
		 */
		(void) WaitLatch(MyLatch,
						 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
						 (nap.tv_sec * 1000L) + (nap.tv_usec / 1000L),
						 WAIT_EVENT_AUTOVACUUM_MAIN);

		ResetLatch(MyLatch);

		HandleAutoVacLauncherInterrupts();
        ....
}

/*
 * Determine the time to sleep, based on the database list.
 *
 * The "canlaunch" parameter indicates whether we can start a worker right now,
 * for example due to the workers being all busy.  If this is false, we will
 * cause a long sleep, which will be interrupted when a worker exits.
 */
static void
launcher_determine_sleep(bool canlaunch, bool recursing, struct timeval *nap)
{
	/*
	 * We sleep until the next scheduled vacuum.  We trust that when the
	 * database list was built, care was taken so that no entries have times
	 * in the past; if the first entry has too close a next_worker value, or a
	 * time in the past, we will sleep a small nominal time.
	 */
	if (!canlaunch)
	{
		nap->tv_sec = autovacuum_naptime;
		nap->tv_usec = 0;
	}
	else if (!dlist_is_empty(&DatabaseList))
	{
		TimestampTz current_time = GetCurrentTimestamp();
		TimestampTz next_wakeup;
		avl_dbase  *avdb;
		long		secs;
		int			usecs;

		avdb = dlist_tail_element(avl_dbase, adl_node, &DatabaseList);

		next_wakeup = avdb->adl_next_worker;
		TimestampDifference(current_time, next_wakeup, &secs, &usecs);

		nap->tv_sec = secs;
		nap->tv_usec = usecs;
	}
	else
	{
		/* list is empty, sleep for whole autovacuum_naptime seconds  */
		nap->tv_sec = autovacuum_naptime;
		nap->tv_usec = 0;
	}

	/*
	 * If the result is exactly zero, it means a database had an entry with
	 * time in the past.  Rebuild the list so that the databases are evenly
	 * distributed again, and recalculate the time to sleep.  This can happen
	 * if there are more tables needing vacuum than workers, and they all take
	 * longer to vacuum than autovacuum_naptime.
	 *
	 * We only recurse once.  rebuild_database_list should always return times
	 * in the future, but it seems best not to trust too much on that.
	 */
	if (nap->tv_sec == 0 && nap->tv_usec == 0 && !recursing)
	{
		rebuild_database_list(InvalidOid);
		launcher_determine_sleep(canlaunch, true, nap);
		return;
	}

	/* The smallest time we'll allow the launcher to sleep. */
	if (nap->tv_sec <= 0 && nap->tv_usec <= MIN_AUTOVAC_SLEEPTIME * 1000)
	{
		nap->tv_sec = 0;
		nap->tv_usec = MIN_AUTOVAC_SLEEPTIME * 1000;
	}

	/*
	 * If the sleep time is too large, clamp it to an arbitrary maximum (plus
	 * any fractional seconds, for simplicity).  This avoids an essentially
	 * infinite sleep in strange cases like the system clock going backwards a
	 * few years.
	 */
	if (nap->tv_sec > MAX_AUTOVAC_SLEEPTIME)
		nap->tv_sec = MAX_AUTOVAC_SLEEPTIME;
}

/*
 * Build an updated DatabaseList.  It must only contain databases that appear
 * in pgstats, and must be sorted by next_worker from highest to lowest,
 * distributed regularly across the next autovacuum_naptime interval.
 *
 * Receives the Oid of the database that made this list be generated (we call
 * this the "new" database, because when the database was already present on
 * the list, we expect that this function is not called at all).  The
 * preexisting list, if any, will be used to preserve the order of the
 * databases in the autovacuum_naptime period.  The new database is put at the
 * end of the interval.  The actual values are not saved, which should not be
 * much of a problem.
 */
static void
rebuild_database_list(Oid newdb)
{
	List	   *dblist;
	ListCell   *cell;
	MemoryContext newcxt;
	MemoryContext oldcxt;
	MemoryContext tmpcxt;
	HASHCTL		hctl;
	int			score;
	int			nelems;
	HTAB	   *dbhash;
	dlist_iter	iter;

	/* use fresh stats */
	autovac_refresh_stats();

	newcxt = AllocSetContextCreate(AutovacMemCxt,
								   "AV dblist",
								   ALLOCSET_DEFAULT_SIZES);
	tmpcxt = AllocSetContextCreate(newcxt,
								   "tmp AV dblist",
								   ALLOCSET_DEFAULT_SIZES);
	oldcxt = MemoryContextSwitchTo(tmpcxt);

	/*
	 * Implementing this is not as simple as it sounds, because we need to put
	 * the new database at the end of the list; next the databases that were
	 * already on the list, and finally (at the tail of the list) all the
	 * other databases that are not on the existing list.
	 *
	 * To do this, we build an empty hash table of scored databases.  We will
	 * start with the lowest score (zero) for the new database, then
	 * increasing scores for the databases in the existing list, in order, and
	 * lastly increasing scores for all databases gotten via
	 * get_database_list() that are not already on the hash.
	 *
	 * Then we will put all the hash elements into an array, sort the array by
	 * score, and finally put the array elements into the new doubly linked
	 * list.
	 */
	hctl.keysize = sizeof(Oid);
	hctl.entrysize = sizeof(avl_dbase);
	hctl.hcxt = tmpcxt;
	dbhash = hash_create("db hash", 20, &hctl,	/* magic number here FIXME */
						 HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);

	/* start by inserting the new database */
	score = 0;
	if (OidIsValid(newdb))
	{
		avl_dbase  *db;
		PgStat_StatDBEntry *entry;

		/* only consider this database if it has a pgstat entry */
		entry = pgstat_fetch_stat_dbentry(newdb);
		if (entry != NULL)
		{
			/* we assume it isn't found because the hash was just created */
			db = hash_search(dbhash, &newdb, HASH_ENTER, NULL);

			/* hash_search already filled in the key */
			db->adl_score = score++;
			/* next_worker is filled in later */
		}
	}

	/* Now insert the databases from the existing list */
	dlist_foreach(iter, &DatabaseList)
	{
		avl_dbase  *avdb = dlist_container(avl_dbase, adl_node, iter.cur);
		avl_dbase  *db;
		bool		found;
		PgStat_StatDBEntry *entry;

		/*
		 * skip databases with no stat entries -- in particular, this gets rid
		 * of dropped databases
		 */
		entry = pgstat_fetch_stat_dbentry(avdb->adl_datid);
		if (entry == NULL)
			continue;

		db = hash_search(dbhash, &(avdb->adl_datid), HASH_ENTER, &found);

		if (!found)
		{
			/* hash_search already filled in the key */
			db->adl_score = score++;
			/* next_worker is filled in later */
		}
	}

	/* finally, insert all qualifying databases not previously inserted */
	dblist = get_database_list();
	foreach(cell, dblist)
	{
		avw_dbase  *avdb = lfirst(cell);
		avl_dbase  *db;
		bool		found;
		PgStat_StatDBEntry *entry;

		/* only consider databases with a pgstat entry */
		entry = pgstat_fetch_stat_dbentry(avdb->adw_datid);
		if (entry == NULL)
			continue;

		db = hash_search(dbhash, &(avdb->adw_datid), HASH_ENTER, &found);
		/* only update the score if the database was not already on the hash */
		if (!found)
		{
			/* hash_search already filled in the key */
			db->adl_score = score++;
			/* next_worker is filled in later */
		}
	}
	nelems = score;

	/* from here on, the allocated memory belongs to the new list */
	MemoryContextSwitchTo(newcxt);
	dlist_init(&DatabaseList);

	if (nelems > 0)
	{
		TimestampTz current_time;
		int			millis_increment;
		avl_dbase  *dbary;
		avl_dbase  *db;
		HASH_SEQ_STATUS seq;
		int			i;

		/* put all the hash elements into an array */
		dbary = palloc(nelems * sizeof(avl_dbase));

		i = 0;
		hash_seq_init(&seq, dbhash);
		while ((db = hash_seq_search(&seq)) != NULL)
			memcpy(&(dbary[i++]), db, sizeof(avl_dbase));

		/* sort the array */
		qsort(dbary, nelems, sizeof(avl_dbase), db_comparator);

		/*
		 * Determine the time interval between databases in the schedule. If
		 * we see that the configured naptime would take us to sleep times
		 * lower than our min sleep time (which launcher_determine_sleep is
		 * coded not to allow), silently use a larger naptime (but don't touch
		 * the GUC variable).
		 */
		millis_increment = 1000.0 * autovacuum_naptime / nelems;
		if (millis_increment <= MIN_AUTOVAC_SLEEPTIME)
			millis_increment = MIN_AUTOVAC_SLEEPTIME * 1.1;

		current_time = GetCurrentTimestamp();

		/*
		 * move the elements from the array into the dlist, setting the
		 * next_worker while walking the array
		 */
		for (i = 0; i < nelems; i++)
		{
			avl_dbase  *db = &(dbary[i]);

			current_time = TimestampTzPlusMilliseconds(current_time,
													   millis_increment);
			db->adl_next_worker = current_time;

			/* later elements should go closer to the head of the list */
			dlist_push_head(&DatabaseList, &db->adl_node);
		}
	}

	/* all done, clean up memory */
	if (DatabaseListCxt != NULL)
		MemoryContextDelete(DatabaseListCxt);
	MemoryContextDelete(tmpcxt);
	DatabaseListCxt = newcxt;
	MemoryContextSwitchTo(oldcxt);
}

millis_increment = 1000.0 * autovacuum_naptime / nelems;
if (millis_increment <= MIN_AUTOVAC_SLEEPTIME)
	millis_increment = MIN_AUTOVAC_SLEEPTIME * 1.1;

// 第一阶段：新数据库（参数传入）
if (OidIsValid(newdb)) {
    db->adl_score = score++;  // 初始评分=0，逐步递增
}

// 第二阶段：现有数据库列表
dlist_foreach(iter, &DatabaseList) {
    if (!found) db->adl_score = score++;  // 延续递增
}

// 第三阶段：全量数据库遍历
dblist = get_database_list();
foreach(cell, dblist) {
    if (!found) db->adl_score = score++;  // 继续递增
}

/* the minimum allowed time between two awakenings of the launcher */
#define MIN_AUTOVAC_SLEEPTIME 100.0 /* milliseconds */
#define MAX_AUTOVAC_SLEEPTIME 300	/* seconds */