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[PATCH v3 00/12] fuse: optimize scatter-gather direct IO [message #48641] Fri, 26 October 2012 15:47 Go to previous message
Maxim Patlasov is currently offline  Maxim Patlasov
Messages: 45
Registered: July 2012
From: *

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



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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