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Subject[PATCH v4 5/5] block: loop: support DIO & AIO
Date
There are about 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

Follows my fio test result:

1. 16 jobs fio test inside ext4 file system over loop block
1) How to run
- linux kernel: 4.1.0-rc2-next-20150506 with the patchset
- the loop block is over one image on HDD.
- linux psync, 16 jobs, size 400M, ext4 over loop block
- test result: IOPS from fio output

2) Throughput result:
-------------------------------------------------------------
test cases |randread |read |randwrite |write |
-------------------------------------------------------------
base |240 |8705 |3763 |20914
-------------------------------------------------------------
base+loop aio |242 |9258 |4577 |21451
-------------------------------------------------------------

3) context switch
- context switch decreased by ~16% with loop aio for randread,
and decreased by ~33% for read

4) memory usage
- After these four tests with loop aio: ~10% memory becomes used
- After these four tests without loop aio: more than 55% memory
becomes used

2. single job fio test inside ext4 file system over loop block(for Maxim Patlasov)
1) How to run
- linux kernel: 4.1.0-rc2-next-20150506 with the patchset
- the loop block is over one image on HDD.
- linux psync, 1 job, size 4000M, ext4 over loop block
- test result: IOPS from fio output

2) Throughput result:
-------------------------------------------------------------
test cases |randread |read |randwrite |write |
-------------------------------------------------------------
base |109 |21180 |4192 |22782
-------------------------------------------------------------
base+loop aio |114 |21018 |5404 |22670
-------------------------------------------------------------

3) context switch
- context switch decreased by ~10% with loop aio for randread,
and decreased by ~50% for read

4) memory usage
- After these four tests with loop aio: ~10% memory becomes used
- After these four tests without loop aio: more than 55% memory
becomes used

Both 'context switch' and 'memory usage' data are got from sar.

[1] https://lwn.net/Articles/612483/
[2] sar graph when running fio over loop without the patchset
http://kernel.ubuntu.com/~ming/block/loop-aio/v3/lo-nonaio.pdf

[3] sar graph when running fio over loop with the patchset
http://kernel.ubuntu.com/~ming/block/loop-aio/v3/lo-aio.pdf

[4] sar graph when running fio over loop without the patchset
http://kernel.ubuntu.com/~ming/block/loop-aio/v3/lo-nonaio-1job.pdf

[5] sar graph when running fio over loop with the patchset
http://kernel.ubuntu.com/~ming/block/loop-aio/v3/lo-aio-1job.pdf

Signed-off-by: Ming Lei <ming.lei@canonical.com>
---
drivers/block/loop.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++++---
drivers/block/loop.h | 2 ++
2 files changed, 71 insertions(+), 3 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 0b1ee2b..df0f712 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -388,6 +388,65 @@ static int lo_req_flush(struct loop_device *lo, struct request *rq)

return ret;
}
+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;
+
+ 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_DONT_DIRTY_PAGE | 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);
+
+ 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)
{
@@ -404,13 +463,13 @@ 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;
@@ -1550,6 +1609,12 @@ static int loop_queue_rq(struct blk_mq_hw_ctx *hctx,
if (lo->lo_state != Lo_bound)
return -EIO;

+ if (lo->use_dio && !lo->transfer &&
+ !(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;
@@ -1569,7 +1634,8 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
failed:
if (ret)
cmd->rq->errors = -EIO;
- blk_mq_complete_request(cmd->rq);
+ if (!cmd->use_aio || ret)
+ blk_mq_complete_request(cmd->rq);
}

static void loop_queue_work(struct kthread_work *work)
diff --git a/drivers/block/loop.h b/drivers/block/loop.h
index d1de221..fb2237c 100644
--- a/drivers/block/loop.h
+++ b/drivers/block/loop.h
@@ -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 */
--
1.9.1


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