[PATCH v9 0/6] block: loop: improve loop with AIO

Ming Lei
2015-08-06T08:42:55Z

Hi Guys,

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 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

So this patchset trys to improve loop via AIO, and about 45% memory
usage can be decreased, see detailed data in commit log of patch4,
also IO throughput isn't affected too.

Following losetup changes for supporting loop dio/aio:
- losetup git tree:
http://kernel.ubuntu.com/git/ming/util-linux.git/log/?h=loop-dio-v7
- how to set direct io
losetup --direct-io 1 /dev/loopN
- how to clear direct io
losetup --direct-io 0 /dev/loopN
- how to show if direct io is used for accessing backing file
losetup -l

V9:
- only 4/6 and 5/6 updated
- update __loop_update_dio()
- serialize direct I/O with buffered I/O for avoiding
possible race

V8:
- only 4/6 and 5/6 updated
- support direct io when backing device's sector size is
512, 1024, 2048 and 4096(only 512-byte sector was supported
in V7)
V7:
- only 4/6 and 5/6 updated
- update direct io after lo->offset is changed(4/6)
- fix updating LO_FLAGS_DIRECT_IO in __loop_update_dio()(4/6)
- introduce ioctl cmd of LOOP_SET_DIRECT_IO(5/6) for case of
'mount -o loop'

V6:
- only patch 4 and patch 5 get updated
- check lo->lo_offset to decide if direct IO can be supported(4/5)
- introduce one flag for userspace(losetup) to keep updated
if using direct I/O to access backing file(4/5)
- implement patches for util-linux(losetup) so that losetup can
enable direct I/O feature:(4/5)

http://kernel.ubuntu.com/git/ming/util-linux.git/log/?h=losetup-dio

- remove the direct IO control interface from sysfs(4/5)
- handle partial read in case of direct read (5/5)
- add more comments for direct IO (5/5)
V5:
- don't introduce IOCB_DONT_DIRTY_PAGE and bypass dirtying
for ITER_KVEC and ITER_BVEC direct IO(read), as required by
Christoph

V4:
- add detailed commit log for 'use kthread_work'
- allow userspace(sysfs, losetup) to decide if dio/aio is
used as suggested by Christoph and Dave Chinner
- only use dio if the backing block device's min io size
is 512 as pointed by Dave Chinner & Christoph
V3:
- based on Al's iov_iter work and Christoph's kiocb changes
- use kthread_work
- introduce IOCB_DONT_DIRTY_PAGE flag
- set QUEUE_FLAG_NOMERGES for loop's request queue
V2:
- remove 'extra' parameter to aio_kernel_alloc()
- try to avoid memory allcation inside queue req callback
- introduce 'use_mq' sysfs file for enabling kernel aio or disabling it
V1:
- link:
http://marc.info/?t=140803157700004&r=1&w=2
- improve failure path in aio_kernel_submit()

drivers/block/loop.c | 312 ++++++++++++++++++++++++++++++++++++++++++++++++++-----------
drivers/block/loop.h | 17 ++--
fs/direct-io.c | 9 +-
include/uapi/linux/loop.h | 2 +
4 files changed, 274 insertions(+), 66 deletions(-)

Thanks,
Ming