From 891852cc12ca101acfeab16c20df1a0b2a043646 Mon Sep 17 00:00:00 2001
From: Alexander Motin <mav@FreeBSD.org>
Date: Thu, 3 Dec 2009 21:47:51 +0000
Subject: [PATCH] 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 | 52 ++++++++++++++++++--------------------
 sys/geom/mirror/g_mirror.h |  4 +--
 2 files changed, 26 insertions(+), 30 deletions(-)

diff --git a/sys/geom/mirror/g_mirror.c b/sys/geom/mirror/g_mirror.c
index 13ff6eaabb60..973e10b9b2e2 100644
--- a/sys/geom/mirror/g_mirror.c
+++ b/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 which the smallest load.
-	 */
+	/* Find a disk with 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. */
-		if (curtime.sec - dp->d_last_used.sec >= 2) {
-			disk = dp;
-			break;
-		}
-		if (disk == NULL ||
-		    bintime_cmp(&dp->d_delay, &disk->d_delay) < 0) {
+		prio = dp->load;
+		/* If disk head is precisely in position - highly prefer it. */
+		if (dp->d_last_offset == bp->bio_offset)
+			prio -= 2 * LOAD_SCALE;
+		else
+		/* If disk head is close to position - prefer it. */
+		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);
 }
 
diff --git a/sys/geom/mirror/g_mirror.h b/sys/geom/mirror/g_mirror.h
index eb67b6e08ba7..8cd9a691e40c 100644
--- a/sys/geom/mirror/g_mirror.h
+++ b/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. */
-	struct bintime	 d_last_used;	/* When disk was last used. */
+	u_int		 load;		/* Averaged queue length */
+	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. */