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block: avoid building too big plug list

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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>
This commit is contained in:
Shaohua Li 2011-07-08 08:19:20 +02:00 committed by Jens Axboe
parent 719c0c5906
commit 55c022bbdd
2 changed files with 8 additions and 0 deletions
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5
block/blk-core.c
View File

@ -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);

3
include/linux/blkdev.h
View File

@ -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 *);