On Mon, 4 Apr 2016 16:35:13 +0300 ValdikSS <i...@valdikss.org.ru> wrote:
> 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> The first step to getting better performance through GRO would be modifying TUN device to use NAPI when receiving. I tried this once, and it got more complex than I had patience for because TUN device write is obviously in userspace context.
