block: avoid building too big plug list

When I test fio script with big I/O depth, I found the total throughput drops
compared to some relative small I/O depth. The reason is the thread accumulates
big requests in its plug list and causes some delays (surely this depends
on CPU speed).
I thought we'd better have a threshold for requests. When a threshold reaches,
this means there is no request merge and queue lock contention isn't severe
when pushing per-task requests to queue, so the main advantages of blk plug
don't exist. We can force a plug list flush in this case.
With this, my test throughput actually increases and almost equals to small
I/O depth. Another side effect is irq off time decreases in blk_flush_plug_list()
for big I/O depth.
The BLK_MAX_REQUEST_COUNT is choosen arbitarily, but 16 is efficiently to
reduce lock contention to me. But I'm open here, 32 is ok in my test too.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/block/blk-core.c b/block/blk-core.c
index d2f8f4049abd..a56485292062 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1302,7 +1302,10 @@ get_rq:
                                 plug->should_sort = 1;
                 }
                 list_add_tail(&req->queuelist, &plug->list);
+                plug->count++;
                 drive_stat_acct(req, 1);
+                if (plug->count >= BLK_MAX_REQUEST_COUNT)
+                        blk_flush_plug_list(plug, false);
         } else {
                 spin_lock_irq(q->queue_lock);
                 add_acct_request(q, req, where);
@@ -2626,6 +2629,7 @@ void blk_start_plug(struct blk_plug *plug)
         INIT_LIST_HEAD(&plug->list);
         INIT_LIST_HEAD(&plug->cb_list);
         plug->should_sort = 0;
+        plug->count = 0;
 
         /*
          * If this is a nested plug, don't actually assign it. It will be
@@ -2709,6 +2713,7 @@ void blk_flush_plug_list(struct blk_plug *plug, bool from_schedule)
                 return;
 
         list_splice_init(&plug->list, &list);
+        plug->count = 0;
 
         if (plug->should_sort) {
                 list_sort(NULL, &list, plug_rq_cmp);