I'm trying to increase OpenVPN throughput by optimizing tun manipulations, too. Right now I have more questions than answers.
I get about 800 Mbit/s speeds via OpenVPN with authentication and encryption disabled on a local machine with OpenVPN server and client running in a different network namespaces, which use veth for networking, with 1500 MTU on a TUN interface. This is rather limiting. Low-end devices like SOHO routers could only achieve 15-20 Mbit/s via OpenVPN with encryption with a 560 MHz CPU. Increasing MTU reduces overhead. You can get > 5GBit/s if you set 16000 MTU on a TUN interface. That's not only OpenVPN related. All the tunneling software I tried can't achieve gigabit speeds without encryption on my machine with MTU 1500. Didn't test tinc though. TUN supports various offloading techniques: GSO, TSO, UFO, just as hardware NICs. From what I understand, if we use GSO/GRO for TUN, we would be able to receive send small packets combined in a huge one with one send/recv call with MTU 1500 on a TUN interface, and the performance should increase and be just as it now with increased MTU. But there is a very little information of how to use offloading with TUN. I've found an old example code which creates TUN interface with GSO support (TUN_VNET_HDR), does NAT and echoes TUN data to stdout, and a script to run two instances of this software connected with a pipe. But it doesn't work for me, I never see any combined frames (gso_type is always 0 in a virtio_net_hdr header). Probably I did something wrong, but I'm not sure what exactly is wrong. Here's said application: http://ovrload.ru/f/68996_tun.tar.gz The questions are as follows: 1. Do I understand correctly that GSO/GRO would have the same effect as increasing MTU on TUN interface? 2. How GRO/GSO is different from TSO, UFO? 3. Can we get and send combined frames directly from/to NIC with offloading support? 4. How to implement GRO/GSO, TSO, UFO? What should be the logic behind it? Any reply is greatly appreciated. P.S. this could be helpful: https://ldpreload.com/p/tuntap-notes.txt > I'm trying to reduce system call overhead when reading/writing to/from a > tun device in userspace. For sockets, one can use sendmmsg()/recvmmsg(), > but a tun fd is not a socket fd, so this doesn't work. I'm see several > options to allow userspace to read/write multiple packets with one > syscall: > > - Implement a TX/RX ring buffer that is mmap()ed, like with AF_PACKET > sockets. > > - Implement a ioctl() to emulate sendmmsg()/recvmmsg(). > > - Add a flag that can be set using TUNSETIFF that makes regular > read()/write() calls handle multiple packets in one go. > > - Expose a socket fd to userspace, so regular sendmmsg()/recvmmsg() can > be used. There is tun_get_socket() which is used internally in the > kernel, but this is not exposed to userspace, and doesn't look trivial > to do either. > > What would be the right way to do this? > > -- > Met vriendelijke groet / with kind regards, > Guus Sliepen <g...@tinc-vpn.org>
