Re: [PATCH v2] qemu-nbd: Use SOMAXCONN for socket listen() backlog

[Nir Soffer]

On Fri, Feb 5, 2021 at 8:57 PM Eric Blake <eblake@redhat.com> wrote:
>
> Our default of a backlog of 1 connection is rather puny, particularly
> for scenarios where we expect multiple listeners to connect (such as
> qemu-nbd -e X).  This is especially important for Unix sockets, as a
> definite benefit to clients: at least on Linux, a client trying to
> connect to a Unix socket with a backlog gets an EAGAIN failure with no
> way to poll() for when the backlog is no longer present short of
> sleeping an arbitrary amount of time before retrying.
>
> See https://bugzilla.redhat.com/1925045 for a demonstration of where
> our low backlog prevents libnbd from connecting as many parallel
> clients as it wants.
>
> Reported-by: Richard W.M. Jones <rjones@redhat.com>
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>
> v2: target the correct API used by qemu-nbd, rather than an unrelated
> legacy wrapper [Dan]
>
>  qemu-nbd.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/qemu-nbd.c b/qemu-nbd.c
> index 608c63e82a25..cd20ee73be19 100644
> --- a/qemu-nbd.c
> +++ b/qemu-nbd.c
> @@ -965,7 +965,8 @@ int main(int argc, char **argv)
>      server = qio_net_listener_new();
>      if (socket_activation == 0) {
>          saddr = nbd_build_socket_address(sockpath, bindto, port);
> -        if (qio_net_listener_open_sync(server, saddr, 1, &local_err) < 0) {
> +        if (qio_net_listener_open_sync(server, saddr, SOMAXCONN,

Shouldn't we use value based on --shared=N?

Using maximum value makes sense for generic server expecting to handle many
connections from different clients. qemu-nbd is typically used by one
client, and we
need to make it possible to connect a known number of connections quickly.

> +                                       &local_err) < 0) {
>              object_unref(OBJECT(server));
>              error_report_err(local_err);
>              exit(EXIT_FAILURE);
> --
> 2.30.0
>
>