[PATCH v5 0/6] 9pfs: readdir optimization

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

As previously mentioned, I was investigating performance issues with 9pfs.
Raw file read/write of 9pfs is actually quite good, provided that client
picked a reasonable high msize (maximum message size). I would recommend
to log a warning on 9p server side if a client attached with a small msize
that would cause performance issues for that reason.

However there are other aspects where 9pfs currently performs suboptimally,
especially readdir handling of 9pfs is extremely slow, a simple readdir
request of a guest typically blocks for several hundred milliseconds or
even several seconds, no matter how powerful the underlying hardware is.
The reason for this performance issue: latency.
Currently 9pfs is heavily dispatching a T_readdir request numerous times
between main I/O thread and a background I/O thread back and forth; in fact
it is actually hopping between threads even multiple times for every single
directory entry during T_readdir request handling which leads in total to
huge latencies for a single T_readdir request.

This patch series aims to address this severe performance issue of 9pfs
T_readdir request handling. The actual performance optimization is patch 5.

v4->v5:

  * Rebased to master (SHA-1 762fa6d7).

  * Dropped benchmark patches (see v4 if you want to run a benchmark on v5).

  * Divided split-readdir test into 3 individual tests, which also fixes the
    previously discussed transport error [patch 1].

  * Fixed English spelling for 'split' [patch 1].

  * Rename max_count -> maxsize [NEW patch 2].

  * Divided previous huge readdir optimization patch into individual patches
    [patch 3], [patch 4], [patch 5].

  * Added comment on v9fs_readdir_response_size() [patch 3].

  * Renamed v9fs_co_readdir_lowlat() -> v9fs_co_readdir_many() [patch 4].

  * Adjusted comment on v9fs_co_readdir_many() [patch 4].

  * Added comment on v9fs_co_run_in_worker() [NEW patch 6].

  * Adjusted commit log message of several patches.

Message-ID of previous version (v4):
  cover.1579567019.git.qemu_oss@crudebyte.com

Christian Schoenebeck (6):
  tests/virtio-9p: added split readdir tests
  9pfs readdir: rename max_count -> maxsize
  9pfs: make v9fs_readdir_response_size() public
  9pfs: add new function v9fs_co_readdir_many()
  9pfs: T_readdir latency optimization
  9pfs: clarify latency of v9fs_co_run_in_worker()

 hw/9pfs/9p.c                 | 148 ++++++++++++++--------------
 hw/9pfs/9p.h                 |  23 +++++
 hw/9pfs/codir.c              | 181 ++++++++++++++++++++++++++++++++---
 hw/9pfs/coth.h               |  15 ++-
 tests/qtest/virtio-9p-test.c | 108 +++++++++++++++++++++
 5 files changed, 386 insertions(+), 89 deletions(-)

-- 
2.20.1