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Subject[PATCH] block_dev: implement readpages() to optimize sequential read
Date
Sequential read from a block device is expected to be equal or faster
than from the file on a filesystem. But it is not correct due to the
lack of effective readpages() in the address space operations for
block device.

This implements readpages() operation for block device by using
mpage_readpages() which can create multipage BIOs instead of BIOs for
each page. It enables to reduce system CPU time consumption.

Install 1GB of RAM disk storage:

# modprobe scsi_debug dev_size_mb=1024 delay=0

Sequential read from file on a filesystem:

# mkfs.ext4 /dev/$DEV
# mount /dev/$DEV /mnt
# fio --name=t --size=512m --rw=read --filename=/mnt/file
...
read : io=524288KB, bw=2133.4MB/s, iops=546133, runt= 240msec

Sequential read from a block device:
# fio --name=t --size=512m --rw=read --filename=/dev/sdb
...
(Without this commit)
read : io=524288KB, bw=1700.2MB/s, iops=435455, runt= 301msec

(With this commit)
read : io=524288KB, bw=2160.4MB/s, iops=553046, runt= 237msec

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
---
fs/block_dev.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 6d72746..e2f3ad08 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -304,6 +304,12 @@ static int blkdev_readpage(struct file * file, struct page * page)
return block_read_full_page(page, blkdev_get_block);
}

+static int blkdev_readpages(struct file *file, struct address_space *mapping,
+ struct list_head *pages, unsigned nr_pages)
+{
+ return mpage_readpages(mapping, pages, nr_pages, blkdev_get_block);
+}
+
static int blkdev_write_begin(struct file *file, struct address_space *mapping,
loff_t pos, unsigned len, unsigned flags,
struct page **pagep, void **fsdata)
@@ -1622,6 +1628,7 @@ static int blkdev_releasepage(struct page *page, gfp_t wait)

static const struct address_space_operations def_blk_aops = {
.readpage = blkdev_readpage,
+ .readpages = blkdev_readpages,
.writepage = blkdev_writepage,
.write_begin = blkdev_write_begin,
.write_end = blkdev_write_end,
--
1.9.1


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