nbd: Set unix socket send buffer on macOS

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>
Message-ID: <20250517201154.88456-3-nirsof@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

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