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 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 fix is patch 8. I also provided a convenient benchmark for comparing the performance improvements by using the 9pfs "synth" driver (see patch 6 for instructions how to run the benchmark), so no guest OS installation is required to peform this benchmark A/B comparison. With patch 8 I achieved a performance improvement of factor 40 on my test machine. ** NOTE: ** These patches are not heavily tested yet, nor thouroughly reviewed for potential security issues yet. I decided to post them already though, because I won't have the time in the next few weeks for polishing them. The benchmark results should demonstrate though that it is worth the hassle. So any testing/reviews/fixes appreciated! Christian Schoenebeck (9): tests/virtio-9p: v9fs_string_read() didn't terminate string 9pfs: validate count sent by client with T_readdir hw/9pfs/9p-synth: added directory for readdir test tests/virtio-9p: added READDIR test tests/virtio-9p: check file names of READDIR response 9pfs: READDIR benchmark hw/9pfs/9p-synth: avoid n-square issue in synth_readdir() 9pfs: T_readdir latency optimization hw/9pfs/9p.c: benchmark time on T_readdir request hw/9pfs/9p-synth.c | 46 ++++++++++- hw/9pfs/9p-synth.h | 5 ++ hw/9pfs/9p.c | 150 ++++++++++++++++++--------------- hw/9pfs/9p.h | 23 ++++++ hw/9pfs/codir.c | 183 ++++++++++++++++++++++++++++++++++++++--- hw/9pfs/coth.h | 3 + tests/virtio-9p-test.c | 182 +++++++++++++++++++++++++++++++++++++++- 7 files changed, 509 insertions(+), 83 deletions(-) -- 2.20.1
