Re: [PATCH] block: optimize for small BS IO

[Ming Lei <ming.lei@redhat.com>]

On Tue, Oct 29, 2019 at 12:19:04PM +0800, Ming Lei wrote:
> __blk_queue_split() may be a bit heavy for small BS(such as 512B, or
> 4KB) IO, so introduce one flag to decide if this bio includes multiple
> page. And only consider to try splitting this bio in case that
> the multiple page flag is set.
> 
> ~3% - 5% IOPS improvement can be observed on io_uring test over
> null_blk(MQ), and the io_uring test code is from fio/t/io_uring.c
> 
> bch_bio_map() should be the only one which doesn't use bio_add_page(),
> so force to mark bio built via bch_bio_map() as MULTI_PAGE.
> 
> RAID5 has similar usage too, however the bio is really single-page bio,
> so not necessary to handle it.
> 
> Cc: Coly Li <colyli@suse.de>
> Cc: linux-bcache@vger.kernel.org
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/bio.c               | 8 ++++++++
>  block/blk-merge.c         | 4 ++++
>  block/bounce.c            | 3 +++
>  drivers/md/bcache/util.c  | 2 ++
>  include/linux/blk_types.h | 1 +
>  5 files changed, 18 insertions(+)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 8f0ed6228fc5..c288364b7cf3 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -583,6 +583,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
>  	bio_set_flag(bio, BIO_CLONED);
>  	if (bio_flagged(bio_src, BIO_THROTTLED))
>  		bio_set_flag(bio, BIO_THROTTLED);
> +	if (bio_flagged(bio_src, BIO_MULTI_PAGE))
> +		bio_set_flag(bio, BIO_MULTI_PAGE);
>  	bio->bi_opf = bio_src->bi_opf;
>  	bio->bi_ioprio = bio_src->bi_ioprio;
>  	bio->bi_write_hint = bio_src->bi_write_hint;
> @@ -757,6 +759,9 @@ bool __bio_try_merge_page(struct bio *bio, struct page *page,
>  		if (page_is_mergeable(bv, page, len, off, same_page)) {
>  			bv->bv_len += len;
>  			bio->bi_iter.bi_size += len;
> +
> +			if (!*same_page)
> +				bio_set_flag(bio, BIO_MULTI_PAGE);
>  			return true;
>  		}
>  	}
> @@ -789,6 +794,9 @@ void __bio_add_page(struct bio *bio, struct page *page,
>  	bio->bi_iter.bi_size += len;
>  	bio->bi_vcnt++;
>  
> +	if (bio->bi_vcnt >= 2 && !bio_flagged(bio, BIO_MULTI_PAGE))
> +		bio_set_flag(bio, BIO_MULTI_PAGE);

We have users of adding multiple pages in single bio_add_page(),
will fix it in V2.


Thanks,
Ming