Change 'load' balancing mode algorithm:

- Instead of measuring last request execution time for each drive and
choosing one with smallest time, use averaged number of requests, running
on each drive. This information is more accurate and timely. It allows to
distribute load between drives in more even and predictable way.
- For each drive track offset of the last submitted request. If new request
offset matches previous one or close for some drive, prefer that drive.
It allows to significantly speedup simultaneous sequential reads.

PR:		kern/113885
Reviewed by:	sobomax

sys/geom/mirror/g_mirror.c

@@ -451,9 +451,6 @@ g_mirror_init_disk(struct g_mirror_softc *sc, struct g_provider *pp,
 	disk->d_id = md->md_did;
 	disk->d_state = G_MIRROR_DISK_STATE_NONE;
 	disk->d_priority = md->md_priority;
-	disk->d_delay.sec = 0;
-	disk->d_delay.frac = 0;
-	binuptime(&disk->d_last_used);
 	disk->d_flags = md->md_dflags;
 	if (md->md_provider[0] != '\0')
 		disk->d_flags |= G_MIRROR_DISK_FLAG_HARDCODED;
@@ -862,16 +859,6 @@ bintime_cmp(struct bintime *bt1, struct bintime *bt2)
 	return (0);
 }
 
-static void
-g_mirror_update_delay(struct g_mirror_disk *disk, struct bio *bp)
-{
-
-	if (disk->d_softc->sc_balance != G_MIRROR_BALANCE_LOAD)
-		return;
-	binuptime(&disk->d_delay);
-	bintime_sub(&disk->d_delay, &bp->bio_t0);
-}
-
 static void
 g_mirror_done(struct bio *bp)
 {
@@ -904,8 +891,6 @@ g_mirror_regular_request(struct bio *bp)
 		g_topology_lock();
 		g_mirror_kill_consumer(sc, bp->bio_from);
 		g_topology_unlock();
-	} else {
-		g_mirror_update_delay(disk, bp);
 	}
 
 	pbp->bio_inbed++;
@@ -1465,30 +1450,35 @@ g_mirror_request_round_robin(struct g_mirror_softc *sc, struct bio *bp)
 	g_io_request(cbp, cp);
 }
 
+#define TRACK_SIZE  (1 * 1024 * 1024)
+#define LOAD_SCALE	256
+#define ABS(x)		(((x) >= 0) ? (x) : (-(x)))
+
 static void
 g_mirror_request_load(struct g_mirror_softc *sc, struct bio *bp)
 {
 	struct g_mirror_disk *disk, *dp;
 	struct g_consumer *cp;
 	struct bio *cbp;
-	struct bintime curtime;
+	int prio, best;
 
-	binuptime(&curtime);
+	/* Find a disk with the smallest load. */
-	/*
-	 * Find a disk which the smallest load.
-	 */
 	disk = NULL;
+	best = INT_MAX;
 	LIST_FOREACH(dp, &sc->sc_disks, d_next) {
 		if (dp->d_state != G_MIRROR_DISK_STATE_ACTIVE)
 			continue;
-		/* If disk wasn't used for more than 2 sec, use it. */
+		prio = dp->load;
-		if (curtime.sec - dp->d_last_used.sec >= 2) {
+		/* If disk head is precisely in position - highly prefer it. */
-			disk = dp;
+		if (dp->d_last_offset == bp->bio_offset)
-			break;
+			prio -= 2 * LOAD_SCALE;
-		}
+		else
-		if (disk == NULL ||
+		/* If disk head is close to position - prefer it. */
-		    bintime_cmp(&dp->d_delay, &disk->d_delay) < 0) {
+		if (ABS(dp->d_last_offset - bp->bio_offset) < TRACK_SIZE)
+			prio -= 1 * LOAD_SCALE;
+		if (prio <= best) {
 			disk = dp;
+			best = prio;
 		}
 	}
 	KASSERT(disk != NULL, ("NULL disk for %s.", sc->sc_name));
@@ -1505,12 +1495,18 @@ g_mirror_request_load(struct g_mirror_softc *sc, struct bio *bp)
 	cp = disk->d_consumer;
 	cbp->bio_done = g_mirror_done;
 	cbp->bio_to = cp->provider;
-	binuptime(&disk->d_last_used);
 	G_MIRROR_LOGREQ(3, cbp, "Sending request.");
 	KASSERT(cp->acr >= 1 && cp->acw >= 1 && cp->ace >= 1,
 	    ("Consumer %s not opened (r%dw%de%d).", cp->provider->name, cp->acr,
 	    cp->acw, cp->ace));
 	cp->index++;
+	/* Remember last head position */
+	disk->d_last_offset = bp->bio_offset + bp->bio_length;
+	/* Update loads. */
+	LIST_FOREACH(dp, &sc->sc_disks, d_next) {
+		dp->load = (dp->d_consumer->index * LOAD_SCALE +
+		    dp->load * 7) / 8;
+	}
 	g_io_request(cbp, cp);
 }
 

sys/geom/mirror/g_mirror.h

@@ -133,8 +133,8 @@ struct g_mirror_disk {
 	struct g_mirror_softc	*d_softc; /* Back-pointer to softc. */
 	int		 d_state;	/* Disk state. */
 	u_int		 d_priority;	/* Disk priority. */
-	struct bintime	 d_delay;	/* Disk delay. */
+	u_int		 load;		/* Averaged queue length */
-	struct bintime	 d_last_used;	/* When disk was last used. */
+	off_t		 d_last_offset;	/* Last read offset */
 	uint64_t	 d_flags;	/* Additional flags. */
 	u_int		 d_genid;	/* Disk's generation ID. */
 	struct g_mirror_disk_sync d_sync;/* Sync information. */