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Commit 55c022bb authored by Shaohua Li's avatar Shaohua Li Committed by Jens Axboe
Browse files

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: default avatarShaohua Li <shaohua.li@intel.com>
Signed-off-by: default avatarJens Axboe <jaxboe@fusionio.com>
parent 719c0c59

block/blk-core.c

+5 −0
Original line number Diff line number Diff line
@@ -1302,7 +1302,10 @@ static int __make_request(struct request_queue *q, struct bio *bio) 
				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);

include/linux/blkdev.h

+3 −0
Original line number Diff line number Diff line
@@ -862,7 +862,10 @@ struct blk_plug { 
	struct list_head list;

	struct list_head cb_list;

	unsigned int should_sort;

	unsigned int count;

};

#define BLK_MAX_REQUEST_COUNT 16



struct blk_plug_cb {

	struct list_head list;

	void (*callback)(struct blk_plug_cb *);