From 55c022bbddb2c056b5dff1bd1b1758d31b6d64c9 Mon Sep 17 00:00:00 2001 From: Shaohua Li Date: Fri, 8 Jul 2011 08:19:20 +0200 Subject: [PATCH] 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 Signed-off-by: Jens Axboe --- block/blk-core.c | 5 +++++ include/linux/blkdev.h | 3 +++ 2 files changed, 8 insertions(+) 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); diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index 36f2e2b99ae3..92edb9601242 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -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 *); -- 2.30.2