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 <[email protected]>
> ---
> 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 <[email protected]>
With regards,
Daniel
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