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Re: [PATCH 0/9] 9pfs: readdir optimization
From: |
Greg Kurz |
Subject: |
Re: [PATCH 0/9] 9pfs: readdir optimization |
Date: |
Wed, 18 Dec 2019 08:59:37 +0100 |
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 <address@hidden> 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(-)
>