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bh=kSLklQL8x+Q9SXf0U3zJXRaUrG/FZM/PT3K4dcKAfYg=; b=DIzBwbw3eQfl0y17cohRAR9f4+il/Nb3Kc0EaMMgFYjmGXAIpP/1VlHW44+6R5VDwhAvrv cD4l3XCCcmRuYJtLThgk5adLJgjlKEGP6zUaQ4ulOdl7fCtLTP5pzPSGFTddegZ4DAdVV4 5xw6CyAydMwc3gyjT2PWyhn+Csuoveg= X-MC-Unique: WW5pnnTWOjuva08xRSYAWA-1 From: Eric Blake To: qemu-devel@nongnu.org Subject: [PULL 01/14] qemu-nbd: Use SOMAXCONN for socket listen() backlog Date: Fri, 12 Feb 2021 14:16:06 -0600 Message-Id: <20210212201619.1388255-2-eblake@redhat.com> In-Reply-To: <20210212201619.1388255-1-eblake@redhat.com> References: <20210212201619.1388255-1-eblake@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eblake@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=63.128.21.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -33 X-Spam_score: -3.4 X-Spam_bar: --- X-Spam_report: (-3.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.569, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , =?UTF-8?q?Daniel=20P=20=2E=20Berrang=C3=A9?= , "open list:Network Block Dev..." , qemu-stable@nongnu.org, "Richard W . M . Jones" , Max Reitz Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: fail (Header signature does not verify) 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 Linux, a 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 Signed-off-by: Eric Blake CC: qemu-stable@nongnu.org Message-Id: <20210209152759.209074-2-eblake@redhat.com> Tested-by: Richard W.M. Jones Reviewed-by: Daniel P. Berrang=C3=A9 Signed-off-by: Eric Blake --- blockdev-nbd.c | 7 ++++++- qemu-nbd.c | 10 +++++++++- 2 files changed, 15 insertions(+), 2 deletions(-) diff --git a/blockdev-nbd.c b/blockdev-nbd.c index d8443d235b73..b264620b98d8 100644 --- a/blockdev-nbd.c +++ b/blockdev-nbd.c @@ -134,7 +134,12 @@ void nbd_server_start(SocketAddress *addr, const char = *tls_creds, qio_net_listener_set_name(nbd_server->listener, "nbd-listener"); - if (qio_net_listener_open_sync(nbd_server->listener, addr, 1, errp) < = 0) { + /* + * Because this server is persistent, a backlog of SOMAXCONN is + * better than trying to size it to max_connections. + */ + if (qio_net_listener_open_sync(nbd_server->listener, addr, SOMAXCONN, + errp) < 0) { goto error; } diff --git a/qemu-nbd.c b/qemu-nbd.c index 608c63e82a25..1a340ea4858d 100644 --- a/qemu-nbd.c +++ b/qemu-nbd.c @@ -964,8 +964,16 @@ int main(int argc, char **argv) server =3D qio_net_listener_new(); if (socket_activation =3D=3D 0) { + int backlog; + + if (persistent) { + backlog =3D SOMAXCONN; + } else { + backlog =3D MIN(shared, SOMAXCONN); + } saddr =3D 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, backlog, + &local_err) < 0) { object_unref(OBJECT(server)); error_report_err(local_err); exit(EXIT_FAILURE); --=20 2.30.1