On Sat, May 17, 2025 at 11:11:51PM +0300, Nir Soffer wrote: > On both macOS and Linux, the default send buffer size is too small causing > poor > performance when reading and writing to qemu-nbd. A simple way to experience > this is to compare TCP and unix sockets, showing that TCP socket is much > faster. Programs like nbdcopy partly mitigate this by using multiple NBD > connections. > > On macOS the default send buffer size is 8192 bytes. Increasing the send > buffer > size to 2 MiB shows up to *12.6 times higher throughput* and lower cpu usage. > > On Linux the default and maximum buffer size is 212992 bytes. Increasing the > send buffer size to 2 MiB shows up to *2.7 times higher throughput* and lower > cpu usage. On older machine we see very little improvement, up to 1.03 times > higher throughput. > > We likely have the same issue on other platforms. It should be easy to enable > this change for more platform by defining UNIX_STREAM_SOCKET_SEND_BUFFER_SIZE.
Thanks; I'm staging this through my NBD tree. -- Eric Blake, Principal Software Engineer Red Hat, Inc. Virtualization: qemu.org | libguestfs.org
