[PATCH v3 00/12] fuse: optimize scatter-gather direct IO

[Maxim Patlasov <mpatlasov@parallels.com>]

Hi,

Existing fuse implementation processes scatter-gather direct IO in suboptimal
way: fuse_direct_IO passes iovec[] to fuse_loop_dio and the latter calls
fuse_direct_read/write for each iovec from iovec[] array. Thus we have as many
submitted fuse-requests as the number of elements in iovec[] array. This is
pure waste of resources and affects performance negatively especially for the
case of many small chunks (e.g. page-size) packed in one iovec[] array.

The patch-set amends situation in a natural way: let's simply pack as
many iovec[] segments to every fuse-request as possible.

To estimate performance improvement I used slightly modified fusexmp over
tmpfs (clearing O_DIRECT bit from fi->flags in xmp_open). The test opened
a file with O_DIRECT, then called readv/writev in a loop. An iovec[] for
readv/writev consisted of 32 segments of 4K each. The throughput on some
commodity (rather feeble) server was (in MB/sec):

        original / patched
writev: ~107     / ~480
readv:  ~114     / ~569

We're exploring possibility to use fuse for our own distributed storage
implementation and big iovec[] arrays of many page-size chunks is typical
use-case for device virtualization thread performing i/o on behalf of
virtual-machine it serves.

Changed in v2:
 - inline array of page pointers req->pages[] is replaced with dynamically
   allocated one; the number of elements is calculated a bit more
   intelligently than being equal to FUSE_MAX_PAGES_PER_REQ; this is done
   for the sake of memory economy.
 - a dynamically allocated array of so-called 'page descriptors' - an offset
   in page plus the length of fragment - is added to fuse_req; this is done
   to simplify processing fuse requests covering several iov-s.

Changed in v3:
 - used iov_iter in fuse_get_user_pages() and __fuse_direct_io()
 - zeroed req->pages[] array on allocation
 - a bunch of minor cleanup changes:
    - used unsigned for npages
    - freed req->pages[] and req->page_descs[] together
    - renamed fuse_get_ua() to fuse_get_user_addr()
    - renamed fuse_get_fr_sz() to fuse_get_user_size()
    - simplified loop in fuse_page_descs_length_init()
 - rebased on v3.7-rc2

Thanks,
Maxim

---

Maxim Patlasov (12):
      fuse: general infrastructure for pages[] of variable size
      fuse: categorize fuse_get_req()
      fuse: rework fuse_retrieve()
      fuse: rework fuse_readpages()
      fuse: rework fuse_perform_write()
      fuse: rework fuse_do_ioctl()
      fuse: add per-page descriptor <offset, length> to fuse_req
      fuse: use req->page_descs[] for argpages cases
      mm: minor cleanup of iov_iter_single_seg_count()
      fuse: pass iov[] to fuse_get_user_pages()
      fuse: optimize fuse_get_user_pages()
      fuse: optimize __fuse_direct_io()


 fs/fuse/cuse.c     |    3 -
 fs/fuse/dev.c      |   98 +++++++++++++++------
 fs/fuse/dir.c      |   39 ++++----
 fs/fuse/file.c     |  242 ++++++++++++++++++++++++++++++++++------------------
 fs/fuse/fuse_i.h   |   47 ++++++++--
 fs/fuse/inode.c    |    6 +
 include/linux/fs.h |    2 
 mm/filemap.c       |    2 
 8 files changed, 292 insertions(+), 147 deletions(-)

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