[PATCH] maximize dispatching in block throttle

[Hillf Danton]

When dispatching bio, the quantum is divided into read/write budgets,
and dispatching for write could not exceed the write budget even if
the read budget is not exhausted, either dispatching for read.

It is changed to exhaust the quantum, if possible, in this work for
dispatching bio.

Though it is hard to understand that 50/50 division is not selected,
the difference between divisions could impact little on dispatching as
much as quantum allows then.

Signed-off-by: Hillf Danton <dhillf@xxxxxxxxx>
---

--- a/block/blk-throttle.c	2010-11-01 19:54:12.000000000 +0800
+++ b/block/blk-throttle.c	2010-11-26 21:49:00.000000000 +0800
@@ -647,11 +647,16 @@ static int throtl_dispatch_tg(struct thr
 	unsigned int max_nr_reads = throtl_grp_quantum*3/4;
 	unsigned int max_nr_writes = throtl_grp_quantum - nr_reads;
 	struct bio *bio;
+	int read_throttled = 0, write_throttled = 0;

 	/* Try to dispatch 75% READS and 25% WRITES */
-
+ try_read:
 	while ((bio = bio_list_peek(&tg->bio_lists[READ]))
-		&& tg_may_dispatch(td, tg, bio, NULL)) {
+		&& ! read_throttled) {
+		if (! tg_may_dispatch(td, tg, bio, NULL)) {
+			read_throttled = 1;
+			break;
+		}

 		tg_dispatch_one_bio(td, tg, bio_data_dir(bio), bl);
 		nr_reads++;
@@ -659,9 +664,15 @@ static int throtl_dispatch_tg(struct thr
 		if (nr_reads >= max_nr_reads)
 			break;
 	}
-
+	if (! bio)
+		read_throttled = 1;
+ try_write:
 	while ((bio = bio_list_peek(&tg->bio_lists[WRITE]))
-		&& tg_may_dispatch(td, tg, bio, NULL)) {
+		&& ! write_throttled) {
+		if (! tg_may_dispatch(td, tg, bio, NULL)) {
+			write_throttled = 1;
+			break;
+		}

 		tg_dispatch_one_bio(td, tg, bio_data_dir(bio), bl);
 		nr_writes++;
@@ -669,7 +680,23 @@ static int throtl_dispatch_tg(struct thr
 		if (nr_writes >= max_nr_writes)
 			break;
 	}
+	if (! bio)
+		write_throttled = 1;
+
+	if (write_throttled && read_throttled)
+		goto out;

+	if (! (throtl_grp_quantum > nr_writes + nr_reads))
+		goto out;
+		
+	if (read_throttled) {
+		max_nr_writes = throtl_grp_quantum - nr_reads;
+		goto try_write;
+	} else {
+		max_nr_reads = throtl_grp_quantum - nr_writes;
+		goto try_read;
+	}
+ out:
 	return nr_reads + nr_writes;
 }
--
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