From 55c022bbddb2c056b5dff1bd1b1758d31b6d64c9 Mon Sep 17 00:00:00 2001 From: Shaohua Li Date: Fri, 8 Jul 2011 08:19:20 +0200 Subject: 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 --- include/linux/blkdev.h | 3 +++ 1 file changed, 3 insertions(+) (limited to 'include/linux/blkdev.h') diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index 36f2e2b99ae..92edb960124 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 *); -- cgit v1.2.3