2016-10-28 22:52:52

by Shaohua Li

Subject: [PATCH 1/2] block: immediately dispatch big size request

Currently block plug holds up to 16 non-mergeable requests. This makes
sense if the request size is small, eg, reduce lock contention. But if
request size is big enough, we don't need to worry about lock
contention. Holding such request makes no sense and it lows the disk
utilization.

In practice, this improves 10% throughput for my raid5 sequential write
workload.

The size (128k) is arbitrary right now, but it makes sure lock
contention is small. This probably could be more intelligent, eg, check
average request size holded. Since this is mainly for sequential IO,
probably not worthy.

Signed-off-by: Shaohua Li <[email protected]>
---
block/blk-core.c | 4 +++-
include/linux/blkdev.h | 1 +
2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 14d7c07..0a396e9 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1763,7 +1763,9 @@ static blk_qc_t blk_queue_bio(struct request_queue *q, struct bio *bio)
if (!request_count)
trace_block_plug(q);
else {
- if (request_count >= BLK_MAX_REQUEST_COUNT) {
+ struct request *first = list_entry_rq(plug->list.next);
+ if (request_count >= BLK_MAX_REQUEST_COUNT ||
+ blk_rq_bytes(first) >= BLK_PLUG_FLUSH_SIZE) {
blk_flush_plug_list(plug, false);
trace_block_plug(q);
}
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index c47c358..72fa505 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1078,6 +1078,7 @@ struct blk_plug {
struct list_head cb_list; /* md requires an unplug callback */
};
#define BLK_MAX_REQUEST_COUNT 16
+#define BLK_PLUG_FLUSH_SIZE (128 * 1024)

struct blk_plug_cb;
typedef void (*blk_plug_cb_fn)(struct blk_plug_cb *, bool);
--
2.9.3

2016-10-28 22:52:42

by Shaohua Li

Subject: [PATCH 2/2] blk-mq: immediately dispatch big size request

This is corresponding part for blk-mq. Disk with multiple hardware
queues doesn't need this as we only hold 1 request at most.

Signed-off-by: Shaohua Li <[email protected]>
---
block/blk-mq.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index f3d27a6..9612306 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1401,13 +1401,18 @@ static blk_qc_t blk_sq_make_request(struct request_queue *q, struct bio *bio)
*/
plug = current->plug;
if (plug) {
+ struct request *first = NULL;
+
blk_mq_bio_to_request(rq, bio);
if (!request_count)
trace_block_plug(q);
+ else
+ first = list_entry_rq(plug->mq_list.next);

blk_mq_put_ctx(data.ctx);

- if (request_count >= BLK_MAX_REQUEST_COUNT) {
+ if (request_count >= BLK_MAX_REQUEST_COUNT || (first &&
+ blk_rq_bytes(first) >= BLK_PLUG_FLUSH_SIZE)) {
blk_flush_plug_list(plug, false);
trace_block_plug(q);
}
--
2.9.3