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Re: [PATCH v3 1/2] qemu-nbd: Use SOMAXCONN for socket listen() backlog

From: Eric Blake
Subject: Re: [PATCH v3 1/2] qemu-nbd: Use SOMAXCONN for socket listen() backlog
Date: Tue, 9 Feb 2021 10:12:22 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

On 2/9/21 10:08 AM, Richard W.M. Jones wrote:
> On Tue, Feb 09, 2021 at 09:27:58AM -0600, Eric Blake wrote:
>> Our default of a backlog of 1 connection is rather puny; it gets in
>> the way when we are explicitly allowing multiple clients (such as
>> qemu-nbd -e N [--shared], or nbd-server-start with its default
>> "max-connections":0 for unlimited), but is even a problem when we
>> stick to qemu-nbd's default of only 1 active client but use -t
>> [--persistent] where a second client can start using the server once
>> the first finishes.  While the effects are less noticeable on TCP
>> sockets (since the client can poll() to learn when the server is ready
>> again), it is definitely observable on Unix sockets, where on Unix, a

s/where on Unix/where on Linux/

>> client will fail with EAGAIN and no recourse but to sleep an arbitrary
>> amount of time before retrying if the server backlog is already full.
>> Since QMP nbd-server-start is always persistent, it now always
>> requests a backlog of SOMAXCONN; meanwhile, qemu-nbd will request
>> SOMAXCONN if persistent, otherwise its backlog should be based on the
>> expected number of clients.
>> 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>
>> CC: qemu-stable@nongnu.org
>> ---
>>  blockdev-nbd.c |  7 ++++++-
>>  qemu-nbd.c     | 10 +++++++++-
>>  2 files changed, 15 insertions(+), 2 deletions(-)

> Works fine here, so:
> Tested-by: Richard W.M. Jones <rjones@redhat.com>

Thanks for testing.

> Rich.

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

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