[PATCH V1] raidd5:Only move IO_THRESHOLD stripes from delay_list to hold_list once.

[majianpeng <majianpeng@gmail.com>]

To improve write perfomance by decreasing the preread stripe,only move
IO_THRESHOLD stripes from delay_list to hold_list once.

Using the follow command:
dd if=/dev/zero of=/dev/md0 bs=2M count=52100.

At default condition: speed is 95MB/s.
At the condition of preread_bypass_threshold was equal zero:speed is 105MB/s.
Using this patch:speed is 123MB/s.

If preread_bypass_threshold was zero,the performance will be better,but
not better than this patch.
I think maybe two reason:
1:If bio is REQ_SYNC
2:In function __get_priority_stripe():
>> } else if (!list_empty(&conf->hold_list) &&
>>		   ((conf->bypass_threshold &&
>>		     conf->bypass_count > conf->bypass_threshold) ||
>>		    atomic_read(&conf->pending_full_writes) == 0)) {
Preread_bypass_threshold is one condition of getting stripe from
hold_list.So only control the number of hold_list can get better
performance.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
---
 drivers/md/raid5.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 04348d7..a6749bb 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3662,6 +3662,7 @@ finish:
 
 static void raid5_activate_delayed(struct r5conf *conf)
 {
+	int count = 0;
 	if (atomic_read(&conf->preread_active_stripes) < IO_THRESHOLD) {
 		while (!list_empty(&conf->delayed_list)) {
 			struct list_head *l = conf->delayed_list.next;
@@ -3672,6 +3673,8 @@ static void raid5_activate_delayed(struct r5conf *conf)
 			if (!test_and_set_bit(STRIPE_PREREAD_ACTIVE, &sh->state))
 				atomic_inc(&conf->preread_active_stripes);
 			list_add_tail(&sh->lru, &conf->hold_list);
+			if (++count >= IO_THRESHOLD)
+				break;
 		}
 	}
 }
-- 
1.7.5.4