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

From: Eric Blake
Subject: Re: [PATCH v2] qemu-nbd: Use SOMAXCONN for socket listen() backlog
Date: Mon, 8 Feb 2021 10:49:42 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

On 2/5/21 1:55 PM, Nir Soffer wrote:
> 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?

That's a possibility.  I don't know how many resources we tie up if we
allow more clients to try to connect() than what we will ultimately
accept when --shared is small.

But there's also the issue that for qemu-nbd, we default to 1 client
unless you pass -e/--shared on the command line, whereas starting an NBD
server in qemu via QMP (including via qemu-storage-daemon) defaults
max-connections to 0 meaning unlimited.

> 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.

At any rate, when max-connections is specified to something smaller than
SOMAXCONN, listen()ing to a smaller number of connections seems
reasonable, so I'll play with that for v3.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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