Re: [PATCH 0/9] 9pfs: readdir optimization

[Greg Kurz <groug@kaod.org>]

Hi Christian,

It seems that there was an issue with the posting of these series. Threading
is inexistant. All the emails appear scattered and unsorted in my mailbox,
between 12/16 and 12/18... which is a bit painful. I'll try to find some time
to have a look anyway, but this greatly lowers the odds for these series to get
multiple reviews, which seems problematic given the ** NOTE: ** section you've
added to the cover. Please fix this.

Cheers,

--
Greg

On Wed, 18 Dec 2019 00:11:10 +0100
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> 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(-)
>