cam iosched: Bettar account IOPS for smoother performance

Prevent cam_iosched_iops_tick() from discarding 'unspent' ios unless
it's a new accounting interval.

Previously ios that weren't used between ticks were lost, as a result
the iops limiter could enforce a limit below the configured maximum.

Obtained from: ElectroBSD
Submitted by: Fabian Keil
PR: 221974

1 files changed, 17 insertions, 6 deletions

diff --git a/sys/cam/cam_iosched.c b/sys/cam/cam_iosched.c
index 63d774f0a0cf..b733978016f5 100644
--- a/sys/cam/cam_iosched.c
+++ b/sys/cam/cam_iosched.c
@@ -423,20 +423,31 @@ cam_iosched_iops_init(struct iop_stats *ios)
 static int
 cam_iosched_iops_tick(struct iop_stats *ios)
 {
+	int new_ios;
 
-	if ((ios->softc->total_ticks % ios->softc->quanta) == 0)
-		ios->l_value2 = 0;
-
-	ios->l_value1 = (int)((ios->current * (uint64_t)ios->softc->this_frac) >> 16);
 	/*
 	 * Allow at least one IO per tick until all
 	 * the IOs for this interval have been spent.
 	 */
-	if (ios->l_value1 <= 0 && ios->l_value2 < ios->current) {
-		ios->l_value1 = 1;
+	new_ios = (int)((ios->current * (uint64_t)ios->softc->this_frac) >> 16);
+	if (new_ios < 1 && ios->l_value2 < ios->current) {
+		new_ios = 1;
 		ios->l_value2++;
 	}
 
+	/*
+	 * If this a new accounting interval, discard any "unspent" ios
+	 * granted in the previous interval.  Otherwise add the new ios to
+	 * the previously granted ones that haven't been spent yet.
+	 */
+	if ((ios->softc->total_ticks % ios->softc->quanta) == 0) {
+		ios->l_value1 = new_ios;
+		ios->l_value2 = 1;
+	} else {
+		ios->l_value1 += new_ios;
+	}
+
+
 	return 0;
 }