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TCP_NV will try to get the base RTT from a socket_ops BPF program if one is loaded. NV will then use the base RTT to bound its min RTT (its notion of the base RTT). It uses the base RTT as an upper bound and 80% of the base RTT as its lower bound. In other words, NV will consider filtered RTTs larger than base RTT as a sign of congestion. As a result, there is no minRTT inflation when there is a lot of congestion. For example, in a DC where the RTTs are less than 40us when there is no congestion, a base RTT value of 80us improves the performance of NV. The difference between the uncongested RTT and the base RTT provided represents how much queueing we are willing to have (in practice it can be higher). NV has been tunned to reduce congestion when there are many flows at the cost of one flow not achieving full bandwith utilization. When a reasonable base RTT is provided, one NV flow can now fully utilize the full bandwidth. In addition, the performance is also improved when there are many flows. In the following examples the NV results are using a kernel with this patch set (i.e. both NV results are using the new nv_loss_dec_factor). With one host sending to another host and only one flow the goodputs are: Cubic: 9.3 Gbps, NV: 5.5 Gbps, NV (baseRTT=80us): 9.2 Gbps With 2 hosts sending to one host (1 flow per host, the goodput per flow is: Cubic: 4.6 Gbps, NV: 4.5 Gbps, NV (baseRTT=80us)L 4.6 Gbps But the RTTs seen by a ping process in the sender is: Cubic: 3.3ms NV: 97us, NV (baseRTT=80us): 146us With a lot of flows things look even better for NV with baseRTT. Here we have 3 hosts sending to one host. Each sending host has 6 flows: 1 stream, 4x1MB RPC, 1x10KB RPC. Cubic, NV and NV with baseRTT all fully utilize the full available bandwidth. However, the distribution of bandwidth among the flows is very different. For the 10KB RPC flow: Cubic: 27Mbps, NV: 111Mbps, NV (baseRTT=80us): 222Mbps The 99% latencies for the 10KB flows are: Cubic: 26ms, NV: 1ms, NV (baseRTT=80us): 500us The RTT seen by a ping process at the senders: Cubic: 3.2ms NV: 720us, NV (baseRTT=80us): 330us Signed-off-by: Lawrence Brakmo <brakmo@fb.com> Acked-by: Alexei Starovoitov <ast@kernel.org> Signed-off-by: David S. Miller <davem@davemloft.net> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.