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Commit bc07c10a authored by Ming Lei's avatar Ming Lei Committed by Jens Axboe
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block: loop: support DIO & AIO 

There are at least 3 advantages to use direct I/O and AIO on
read/write loop's backing file:

1) double cache can be avoided, then memory usage gets
decreased a lot

2) not like user space direct I/O, there isn't cost of
pinning pages

3) avoid context switch for obtaining good throughput
- in buffered file read, random I/O top throughput is often obtained
only if they are submitted concurrently from lots of tasks; but for
sequential I/O, most of times they can be hit from page cache, so
concurrent submissions often introduce unnecessary context switch
and can't improve throughput much. There was such discussion[1]
to use non-blocking I/O to improve the problem for application.
- with direct I/O and AIO, concurrent submissions can be
avoided and random read throughput can't be affected meantime

xfstests(-g auto, ext4) is basically passed when running with
direct I/O(aio), one exception is generic/232, but it failed in
loop buffered I/O(4.2-rc6-next-20150814) too.

Follows the fio test result for performance purpose:
	4 jobs fio test inside ext4 file system over loop block

1) How to run
	- KVM: 4 VCPUs, 2G RAM
	- linux kernel: 4.2-rc6-next-20150814(base) with the patchset
	- the loop block is over one image on SSD.
	- linux psync, 4 jobs, size 1500M, ext4 over loop block
	- test result: IOPS from fio output

2) Throughput(IOPS) becomes a bit better with direct I/O(aio)
        -------------------------------------------------------------
        test cases          |randread   |read   |randwrite  |write  |
        -------------------------------------------------------------
        base                |8015       |113811 |67442      |106978
        -------------------------------------------------------------
        base+loop aio       |8136       |125040 |67811      |111376
        -------------------------------------------------------------

- somehow, it should be caused by more page cache avaiable for
application or one extra page copy is avoided in case of direct I/O

3) context switch
        - context switch decreased by ~50% with loop direct I/O(aio)
	compared with loop buffered I/O(4.2-rc6-next-20150814)

4) memory usage from /proc/meminfo
        -------------------------------------------------------------
                                   | Buffers       | Cached
        -------------------------------------------------------------
        base                       | > 760MB       | ~950MB
        -------------------------------------------------------------
        base+loop direct I/O(aio)  | < 5MB         | ~1.6GB
        -------------------------------------------------------------

- so there are much more page caches available for application with
direct I/O

[1] https://lwn.net/Articles/612483/



Signed-off-by: default avatarMing Lei <ming.lei@canonical.com>
Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
Signed-off-by: default avatarJens Axboe <axboe@fb.com>
parent ab1cb278

drivers/block/loop.c

+95 −3
Original line number Diff line number Diff line
@@ -445,6 +445,90 @@ static int lo_req_flush(struct loop_device *lo, struct request *rq) 
	return ret;

}



static inline void handle_partial_read(struct loop_cmd *cmd, long bytes)

{

	if (bytes < 0 || (cmd->rq->cmd_flags & REQ_WRITE))

		return;



	if (unlikely(bytes < blk_rq_bytes(cmd->rq))) {

		struct bio *bio = cmd->rq->bio;



		bio_advance(bio, bytes);

		zero_fill_bio(bio);

	}

}



static void lo_rw_aio_complete(struct kiocb *iocb, long ret, long ret2)

{

	struct loop_cmd *cmd = container_of(iocb, struct loop_cmd, iocb);

	struct request *rq = cmd->rq;



	handle_partial_read(cmd, ret);



	if (ret > 0)

		ret = 0;

	else if (ret < 0)

		ret = -EIO;



	rq->errors = ret;

	blk_mq_complete_request(rq);

}



static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,

		     loff_t pos, bool rw)

{

	struct iov_iter iter;

	struct bio_vec *bvec;

	struct bio *bio = cmd->rq->bio;

	struct file *file = lo->lo_backing_file;

	int ret;



	/* nomerge for loop request queue */

	WARN_ON(cmd->rq->bio != cmd->rq->biotail);



	bvec = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);

	iov_iter_bvec(&iter, ITER_BVEC | rw, bvec,

		      bio_segments(bio), blk_rq_bytes(cmd->rq));



	cmd->iocb.ki_pos = pos;

	cmd->iocb.ki_filp = file;

	cmd->iocb.ki_complete = lo_rw_aio_complete;

	cmd->iocb.ki_flags = IOCB_DIRECT;



	if (rw == WRITE)

		ret = file->f_op->write_iter(&cmd->iocb, &iter);

	else

		ret = file->f_op->read_iter(&cmd->iocb, &iter);



	if (ret != -EIOCBQUEUED)

		cmd->iocb.ki_complete(&cmd->iocb, ret, 0);

	return 0;

}





static inline int lo_rw_simple(struct loop_device *lo,

		struct request *rq, loff_t pos, bool rw)

{

	struct loop_cmd *cmd = blk_mq_rq_to_pdu(rq);



	if (cmd->use_aio)

		return lo_rw_aio(lo, cmd, pos, rw);



	/*

	 * lo_write_simple and lo_read_simple should have been covered

	 * by io submit style function like lo_rw_aio(), one blocker

	 * is that lo_read_simple() need to call flush_dcache_page after

	 * the page is written from kernel, and it isn't easy to handle

	 * this in io submit style function which submits all segments

	 * of the req at one time. And direct read IO doesn't need to

	 * run flush_dcache_page().

	 */

	if (rw == WRITE)

		return lo_write_simple(lo, rq, pos);

	else

		return lo_read_simple(lo, rq, pos);

}



static int do_req_filebacked(struct loop_device *lo, struct request *rq)

{

	loff_t pos;
@@ -460,13 +544,13 @@ static int do_req_filebacked(struct loop_device *lo, struct request *rq) 
		else if (lo->transfer)

			ret = lo_write_transfer(lo, rq, pos);

		else

			ret = lo_write_simple(lo, rq, pos);

			ret = lo_rw_simple(lo, rq, pos, WRITE);



	} else {

		if (lo->transfer)

			ret = lo_read_transfer(lo, rq, pos);

		else

			ret = lo_read_simple(lo, rq, pos);

			ret = lo_rw_simple(lo, rq, pos, READ);

	}



	return ret;
@@ -1570,6 +1654,12 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx, 
	if (lo->lo_state != Lo_bound)

		return -EIO;



	if (lo->use_dio && !(cmd->rq->cmd_flags & (REQ_FLUSH |

					REQ_DISCARD)))

		cmd->use_aio = true;

	else

		cmd->use_aio = false;



	queue_kthread_work(&lo->worker, &cmd->work);



	return BLK_MQ_RQ_QUEUE_OK;
@@ -1589,6 +1679,8 @@ static void loop_handle_cmd(struct loop_cmd *cmd) 
 failed:

	if (ret)

		cmd->rq->errors = -EIO;

	/* complete non-aio request */

	if (!cmd->use_aio || ret)

		blk_mq_complete_request(cmd->rq);

}

drivers/block/loop.h

+2 −0
Original line number Diff line number Diff line
@@ -69,6 +69,8 @@ struct loop_cmd { 
	struct kthread_work work;

	struct request *rq;

	struct list_head list;

	bool use_aio;           /* use AIO interface to handle I/O */

	struct kiocb iocb;

};



/* Support for loadable transfer modules */