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QEMU main repository: Please see https://www.qemu.org/docs/master/devel/submitting-a-patch.html for how to submit changes to QEMU. Pull Requests are ignored. Please only use release tarballs from the QEMU website. http://www.qemu.org

emulation kvm qemu virtualization

History

Nir Soffer 479ec81061 nbd: Set unix socket send buffer on Linux Like macOS we have similar issue on Linux. For TCP socket the send buffer size is 2626560 bytes (~2.5 MiB) and we get good performance. However for unix socket the default and maximum buffer size is 212992 bytes (208 KiB) and we see poor performance when using one NBD connection, up to 4 times slower than macOS on the same machine. Tracing shows that for every 2 MiB payload (qemu uses 2 MiB io size), we do 1 recvmsg call with TCP socket, and 10 recvmsg calls with unix socket. Fixing this issue requires changing the maximum send buffer size (the receive buffer size is ignored). This can be done using: $ cat /etc/sysctl.d/net-mem-max.conf net.core.wmem_max = 2097152 $ sudo sysctl -p /etc/sysctl.d/net-mem-max.conf With this we can set the socket buffer size to 2 MiB. With the defaults the value requested by qemu is clipped to the maximum size and has no effect. I tested on 2 machines: - Fedora 42 VM on MacBook Pro M2 Max - Dell PowerEdge R640 (Intel(R) Xeon(R) Gold 6230 CPU @ 2.10GHz) On the older Dell machine we see very little improvement, up to 1.03 higher throughput. On the M2 machine we see up to 2.67 times higher throughput. The following results are from the M2 machine. Reading from qemu-nbd with qemu-img convert. In this test buffer size of 4m is optimal (2.28 times faster). \| buffer size \| time \| user \| system \| \|-------------\|---------\|---------\|---------\| \| default \| 4.292 \| 0.243 \| 1.604 \| \| 524288 \| 2.167 \| 0.058 \| 1.288 \| \| 1048576 \| 2.041 \| 0.060 \| 1.238 \| \| 2097152 \| 1.884 \| 0.060 \| 1.191 \| \| 4194304 \| 1.881 \| 0.054 \| 1.196 \| Writing to qemu-nbd with qemu-img convert. In this test buffer size of 1m is optimal (2.67 times faster). \| buffer size \| time \| user \| system \| \|-------------\|---------\|---------\|---------\| \| default \| 3.113 \| 0.334 \| 1.094 \| \| 524288 \| 1.173 \| 0.179 \| 0.654 \| \| 1048576 \| 1.164 \| 0.164 \| 0.670 \| \| 2097152 \| 1.227 \| 0.197 \| 0.663 \| \| 4194304 \| 1.227 \| 0.198 \| 0.666 \| Computing a blkhash with nbdcopy. In this test buffer size of 512k is optimal (1.19 times faster). \| buffer size \| time \| user \| system \| \|-------------\|---------\|---------\|---------\| \| default \| 2.140 \| 4.483 \| 2.681 \| \| 524288 \| 1.794 \| 4.467 \| 2.572 \| \| 1048576 \| 1.807 \| 4.447 \| 2.644 \| \| 2097152 \| 1.822 \| 4.461 \| 2.698 \| \| 4194304 \| 1.827 \| 4.465 \| 2.700 \| Computing a blkhash with blksum. In this test buffer size of 4m is optimal (2.65 times faster). \| buffer size \| time \| user \| system \| \|-------------\|---------\|---------\|---------\| \| default \| 3.582 \| 4.595 \| 2.392 \| \| 524288 \| 1.499 \| 4.384 \| 1.482 \| \| 1048576 \| 1.377 \| 4.381 \| 1.345 \| \| 2097152 \| 1.388 \| 4.389 \| 1.354 \| \| 4194304 \| 1.352 \| 4.395 \| 1.302 \| Signed-off-by: Nir Soffer <nirsof@gmail.com> Message-ID: <20250517201154.88456-4-nirsof@gmail.com> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> Signed-off-by: Eric Blake <eblake@redhat.com>		11 months ago
..
client-connection.c	nbd: Set unix socket send buffer on macOS	10 months ago
client.c	nbd/server: do not poll within a coroutine context	2 years ago
common.c	nbd: Set unix socket send buffer on Linux	10 months ago
meson.build	nbd: move connection code from block/nbd to nbd/client-connection	5 years ago
nbd-internal.h	nbd: Set unix socket send buffer on macOS	10 months ago
server.c	nbd: Set unix socket send buffer on macOS	10 months ago
trace-events	nbd/server: CVE-2024-7409: Drop non-negotiating clients	2 years ago
trace.h	trace: switch position of headers to what Meson requires	6 years ago