Re: [PATCH v4 2/3] nbd: Set unix socket send buffer on macOS

["Daniel P. Berrangé" <berrange@redhat.com>]

On Sat, May 17, 2025 at 11:11:53PM +0300, Nir Soffer wrote:
> On macOS we need to increase unix socket buffers size on the client and
> server to get good performance. We set socket buffers on macOS after
> connecting or accepting a client connection.
> 
> Testing shows that setting socket receive buffer size (SO_RCVBUF) has no
> effect on performance, so we set only the send buffer size (SO_SNDBUF).
> It seems to work like Linux but not documented.
> 
> Testing shows that optimal buffer size is 512k to 4 MiB, depending on
> the test case. The difference is very small, so I chose 2 MiB.
> 
> I tested reading from qemu-nbd and writing to qemu-nbd with qemu-img and
> computing a blkhash with nbdcopy and blksum.
> 
> To focus on NBD communication and get less noisy results, I tested
> reading and writing to null-co driver. I added a read-pattern option to
> the null-co driver to return data full of 0xff:
> 
>     NULL="json:{'driver': 'raw', 'file': {'driver': 'null-co', 'size': '10g', 'read-pattern': 255}}"
> 
> For testing buffer size I added an environment variable for setting the
> socket buffer size.
> 
> Read from qemu-nbd via qemu-img convert. In this test buffer size of 2m
> is optimal (12.6 times faster).
> 
>     qemu-nbd -r -t -e 0 -f raw -k /tmp/nbd.sock "$NULL" &
>     qemu-img convert -f raw -O raw -W -n "nbd+unix:///?socket=/tmp/nbd.sock" "$NULL"
> 
> | buffer size | time    | user    | system  |
> |-------------|---------|---------|---------|
> |     default |  13.361 |   2.653 |   5.702 |
> |       65536 |   2.283 |   0.204 |   1.318 |
> |      131072 |   1.673 |   0.062 |   1.008 |
> |      262144 |   1.592 |   0.053 |   0.952 |
> |      524288 |   1.496 |   0.049 |   0.887 |
> |     1048576 |   1.234 |   0.047 |   0.738 |
> |     2097152 |   1.060 |   0.080 |   0.602 |
> |     4194304 |   1.061 |   0.076 |   0.604 |
> 
> Write to qemu-nbd with qemu-img convert. In this test buffer size of 2m
> is optimal (9.2 times faster).
> 
>     qemu-nbd -t -e 0 -f raw -k /tmp/nbd.sock "$NULL" &
>     qemu-img convert -f raw -O raw -W -n "$NULL" "nbd+unix:///?socket=/tmp/nbd.sock"
> 
> | buffer size | time    | user    | system  |
> |-------------|---------|---------|---------|
> |     default |   8.063 |   2.522 |   4.184 |
> |       65536 |   1.472 |   0.430 |   0.867 |
> |      131072 |   1.071 |   0.297 |   0.654 |
> |      262144 |   1.012 |   0.239 |   0.587 |
> |      524288 |   0.970 |   0.201 |   0.514 |
> |     1048576 |   0.895 |   0.184 |   0.454 |
> |     2097152 |   0.877 |   0.174 |   0.440 |
> |     4194304 |   0.944 |   0.231 |   0.535 |
> 
> Compute a blkhash with nbdcopy, using 4 NBD connections and 256k request
> size. In this test buffer size of 4m is optimal (5.1 times faster).
> 
>     qemu-nbd -r -t -e 0 -f raw -k /tmp/nbd.sock "$NULL" &
>     nbdcopy --blkhash "nbd+unix:///?socket=/tmp/nbd.sock" null:
> 
> | buffer size | time    | user    | system  |
> |-------------|---------|---------|---------|
> |     default |   8.624 |   5.727 |   6.507 |
> |       65536 |   2.563 |   4.760 |   2.498 |
> |      131072 |   1.903 |   4.559 |   2.093 |
> |      262144 |   1.759 |   4.513 |   1.935 |
> |      524288 |   1.729 |   4.489 |   1.924 |
> |     1048576 |   1.696 |   4.479 |   1.884 |
> |     2097152 |   1.710 |   4.480 |   1.763 |
> |     4194304 |   1.687 |   4.479 |   1.712 |
> 
> Compute a blkhash with blksum, using 1 NBD connection and 256k read
> size. In this test buffer size of 512k is optimal (10.3 times faster).
> 
>     qemu-nbd -r -t -e 0 -f raw -k /tmp/nbd.sock "$NULL" &
>     blksum "nbd+unix:///?socket=/tmp/nbd.sock"
> 
> | buffer size | time    | user    | system  |
> |-------------|---------|---------|---------|
> |     default |  13.085 |   5.664 |   6.461 |
> |       65536 |   3.299 |   5.106 |   2.515 |
> |      131072 |   2.396 |   4.989 |   2.069 |
> |      262144 |   1.607 |   4.724 |   1.555 |
> |      524288 |   1.271 |   4.528 |   1.224 |
> |     1048576 |   1.294 |   4.565 |   1.333 |
> |     2097152 |   1.299 |   4.569 |   1.344 |
> |     4194304 |   1.291 |   4.559 |   1.327 |
> 
> Signed-off-by: Nir Soffer <nirsof@gmail.com>
> ---
>  nbd/client-connection.c |  3 +++
>  nbd/common.c            | 25 +++++++++++++++++++++++++
>  nbd/nbd-internal.h      |  5 +++++
>  nbd/server.c            |  2 ++
>  4 files changed, 35 insertions(+)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>


With regards,
Daniel
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