Re: [knot-dns-users] Kernel Connection Multiplexor

Hi, Linux 4.6 was just released, and it includes a new feature called "Kernel Connection Multiplexor": http://kernelnewbies.org/Linux_4.6#head-d86a7a8affd7cefef85fff400e39403718b… 1.5. Kernel Connection Multiplexor, a facility for accelerating application layer protocols This release adds Kernel Connection Multiplexor (KCM), a facility that provides a message-based interface over TCP for accelerating application layer protocols. The motivation for this is based on the observation that although TCP is byte stream transport protocol with no concept of message boundaries, a common use case is to implement a framed application layer protocol running over TCP. Most TCP stacks offer byte stream API for applications, which places the burden of message delineation, message I/O operation atomicity, and load balancing in the application. With KCM an application can efficiently send and receive application protocol messages over TCP using a datagram interface. The kernel provides necessary assurances that messages are sent and received atomically. This relieves much of the burden applications have in mapping a message based protocol onto the TCP stream. KCM also make application layer messages a unit of work in the kernel for the purposes of steerng and scheduling, which in turn allows a simpler networking model in multithreaded applications. In order to delineate message in a TCP stream for receive in KCM, the kernel implements a message parser based on BPF, which parses application layer messages and returns a message length. Nearly all binary application protocols are parseable in this manner, so KCM should be applicable across a wide range of applications. DNS-over-TCP is definitely amenable to this scheme, since messages are framed with a 2-byte message length value. It also sounds like it can be combined with recvmmsg(): Q: What about the problem of a connections with very slow rate of incoming data? As a result your application can get storms of very short reads. And it actually happens a lot with connection from mobile devices and it is a problem for servers handling a lot of connections. A: The storm of short reads will occur regardless of whether KCM is used or not. KCM does have one advantage in this scenario though, it will only wake up the application when a full message has been received, not for each packet that makes up part of a bigger messages. If a bunch of small messages are received, the application can receive messages in batches using recvmmsg. Maybe this could help speed up a DNS server, or even improve resistance against slowloris style TCP attacks.