Linux 6.17 Network Tuning: DualPI2 + BBR to Eliminate Bufferbloat on 50Mbps Bandwidth

I am not a native English speaker; this article was translated by Gemini.

Background

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When deploying a VPN relay service on a 50Mbps cloud host, I encountered the classic Bufferbloat problem: whenever a large download saturated the bandwidth, SSH latency spiked from 50ms to over 200ms, severely degrading the interactive experience.

The traditional fq + bbr combo has limited effectiveness on narrow bandwidth links because fq only ensures fair queuing—it doesn’t actively control queuing delay.

The Solution: DualPI2 + BBR

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Linux 6.17 introduced the sch_dualpi2 scheduler, designed specifically for low latency:

Configuration Steps

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1. Requirements

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Linux Kernel 6.17 or higher (includes the sch_dualpi2 module).

2. sysctl Configuration

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Edit /etc/sysctl.conf:

# --- Core Scheduling Scheme ---
net.core.default_qdisc = dualpi2
net.ipv4.tcp_congestion_control = bbr

# --- Latency Optimization ---
net.ipv4.tcp_ecn = 1              # Essential for dualpi2
net.ipv4.tcp_fastopen = 3
net.ipv4.tcp_recovery = 1         # RACK for fast reordering fixes
net.ipv4.tcp_notsent_lowat = 8192 # Lower BBR kernel buffering

# --- Buffer Settings (Preventing Bloat on 50Mbps) ---
net.core.rmem_max = 4194304
net.core.wmem_max = 4194304
net.ipv4.tcp_rmem = 4096 16384 4194304
net.ipv4.tcp_wmem = 4096 16384 4194304

# --- Stability Optimization ---
net.core.somaxconn = 2048
net.ipv4.tcp_max_syn_backlog = 2048
net.mptcp.enabled = 1

3. Auto-load Module

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echo "sch_dualpi2" | sudo tee /etc/modules-load.d/dualpi2.conf

4. Persistence (udev Rule)

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Create /etc/udev/rules.d/99-ens3-dualpi2.rules:

ACTION=="add", SUBSYSTEM=="net", NAME=="ens3", RUN+="/sbin/tc qdisc replace dev ens3 root dualpi2"

Replace ens3 with your network interface name.

5. Apply Configuration

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sudo sysctl -p
sudo tc qdisc replace dev ens3 root dualpi2

Verification Methods

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Check DualPI2 Status

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tc -s qdisc show dev ens3
# Observe the target value, default is 5ms

Stress Testing (Solution for ICMP-Disabled Servers)

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Traditional ping fails on servers with ICMP blocked. Use kernel RTT statistics instead:

# Client: Saturate the bandwidth
iperf3 -c <server> -t 60

# Server: Observe Kernel RTT
ss -tni
# Focus on rtt:X/Y — X is mean RTT, Y is jitter (mdev)

Results

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Under full 50Mbps load:

SSH latency remained only 6.4ms above the physical limit during full downloads, with 2ms jitter. Bufferbloat is completely eliminated.