__blk_queue_split() adds significant overhead for small I/O operations.
Add a shortcut to avoid it for cases where we know we never need to
split.
Based on a patch from Ming Lei.
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
block/blk-merge.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 48e6725b32ee..06eb38357b41 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -293,7 +293,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
void __blk_queue_split(struct request_queue *q, struct bio **bio,
unsigned int *nr_segs)
{
- struct bio *split;
+ struct bio *split = NULL;
switch (bio_op(*bio)) {
case REQ_OP_DISCARD:
@@ -309,6 +309,19 @@ void __blk_queue_split(struct request_queue *q, struct bio **bio,
nr_segs);
break;
default:
+ /*
+ * All drivers must accept single-segments bios that are <=
+ * PAGE_SIZE. This is a quick and dirty check that relies on
+ * the fact that bi_io_vec[0] is always valid if a bio has data.
+ * The check might lead to occasional false negatives when bios
+ * are cloned, but compared to the performance impact of cloned
+ * bios themselves the loop below doesn't matter anyway.
+ */
+ if ((*bio)->bi_vcnt == 1 &&
+ (*bio)->bi_io_vec[0].bv_len <= PAGE_SIZE) {
+ *nr_segs = 1;
+ break;
+ }
split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);
break;
}
--
2.20.1
