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block: loop: support DIO & AIO

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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: Ming Lei <ming.lei@canonical.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>
This commit is contained in:
Ming Lei 2015-08-17 10:31:51 +08:00 committed by Jens Axboe
parent ab1cb278bc
commit bc07c10a36
2 changed files with 97 additions and 3 deletions
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98
drivers/block/loop.c
View file

@ -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;
@ -1589,7 +1679,9 @@ static void loop_handle_cmd(struct loop_cmd *cmd)
failed:
if (ret)
cmd->rq->errors = -EIO;
blk_mq_complete_request(cmd->rq);
/* complete non-aio request */
if (!cmd->use_aio || ret)
blk_mq_complete_request(cmd->rq);
}
static void loop_queue_work(struct kthread_work *work)

2
drivers/block/loop.h
View file

@ -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 */