Hi, David, Eric, Neal et al. On čtvrtek 15. února 2018 21:42:26 CET Oleksandr Natalenko wrote: > I've faced an issue with a limited TCP bandwidth between my laptop and a > server in my 1 Gbps LAN while using BBR as a congestion control mechanism. > To verify my observations, I've set up 2 KVM VMs with the following > parameters: > > 1) Linux v4.15.3 > 2) virtio NICs > 3) 128 MiB of RAM > 4) 2 vCPUs > 5) tested on both non-PREEMPT/100 Hz and PREEMPT/1000 Hz > > The VMs are interconnected via host bridge (-netdev bridge). I was running > iperf3 in the default and reverse mode. Here are the results: > > 1) BBR on both VMs > > upload: 3.42 Gbits/sec, cwnd ~ 320 KBytes > download: 3.39 Gbits/sec, cwnd ~ 320 KBytes > > 2) Reno on both VMs > > upload: 5.50 Gbits/sec, cwnd = 976 KBytes (constant) > download: 5.22 Gbits/sec, cwnd = 1.20 MBytes (constant) > > 3) Reno on client, BBR on server > > upload: 5.29 Gbits/sec, cwnd = 952 KBytes (constant) > download: 3.45 Gbits/sec, cwnd ~ 320 KBytes > > 4) BBR on client, Reno on server > > upload: 3.36 Gbits/sec, cwnd ~ 370 KBytes > download: 5.21 Gbits/sec, cwnd = 887 KBytes (constant) > > So, as you may see, when BBR is in use, upload rate is bad and cwnd is low. > If using real HW (1 Gbps LAN, laptop and server), BBR limits the throughput > to ~100 Mbps (verifiable not only by iperf3, but also by scp while > transferring some files between hosts). > > Also, I've tried to use YeAH instead of Reno, and it gives me the same > results as Reno (IOW, YeAH works fine too). > > Questions: > > 1) is this expected? > 2) or am I missing some extra BBR tuneable? > 3) if it is not a regression (I don't have any previous data to compare > with), how can I fix this? > 4) if it is a bug in BBR, what else should I provide or check for a proper > investigation?
I've played with BBR a little bit more and managed to narrow the issue down to the changes between v4.12 and v4.13. Here are my observations: v4.12 + BBR + fq_codel == OK v4.12 + BBR + fq == OK v4.13 + BBR + fq_codel == Not OK v4.13 + BBR + fq == OK I think this has something to do with an internal TCP implementation for pacing, that was introduced in v4.13 (commit 218af599fa63) specifically to allow using BBR together with non-fq qdiscs. Once BBR relies on fq, the throughput is high and saturates the link, but if another qdisc is in use, for instance, fq_codel, the throughput drops. Just to be sure, I've also tried pfifo_fast instead of fq_codel with the same outcome resulting in the low throughput. Unfortunately, I do not know if this is something expected or should be considered as a regression. Thus, asking for an advice. Ideas? Thanks. Regards, Oleksandr
