block: avoid building too big plug list (55c022bb) · Commits · 戴 / test

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>

block/blk-core.c

+5 −0

Original line number	Diff line number	Diff line
		@@ -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);

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 );

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