PIE(8)

🌸 NAME 🌸

PIE(8) is the manual page for PIE, short for Proportional Integral controller-Enhanced AQM algorithm 💅✨

🎀 SYNOPSIS 🎀

Okay so this is the command shape you type in, bestie 👀

tc qdisc ... pie [ limit PACKETS ] [ target TIME ] [ tupdate TIME ] [
alpha int ] [ beta int ] [ ecn | noecn ] [ bytemode | nobytemode ] [
dq_rate_estimator | no_dq_rate_estimator ]

💖 DESCRIPTION 💖

Okay bestie, so Proportional Integral controller-Enhanced (PIE) is a control theoretic active queue management scheme 🧠 It's based on the proportional integral controller, but the whole point is to control delay 🫶

Its main design goals are giving it major main character energy:

• Low latency control 🍓
• High link utilization 🌈
• Simple implementation 🧚
• Guaranteed stability and fast responsiveness 👑

🦋 ALGORITHM 🦋

Okay so PIE is literally built to keep delay in check 💅 Here's how it works, step by step:

First, an average dequeue rate gets estimated based on the standing queue. That rate is then used to calculate the current delay ⏱️ Next, on a periodic basis, the delay is used to calculate the dropping probability. Finally, when a packet arrives, it gets dropped (or marked) based on that probability 🔥

PIE adjusts the probability based on the trend of the delay, so it checks whether the delay is going up or down 📈📉 The delay converges quickly to the target value you set ✨

alpha and beta are statically chosen parameters that control how fast the drop probability grows 🌷 They're determined through control theoretic approaches, so no guessing bestie 🤓 alpha decides how the gap between the current latency and the target latency changes the probability. beta adds extra adjustments depending on the latency trend 💜

The drop probability is used to mark packets in ecn mode. However, just like RED, once you go beyond 10%, packets get dropped based on that probability 😭 The bytemode option drops packets in proportion to the packet size 🎀

Want the deeper details? Check out the paper cited below 📚

🧚 PARAMETERS 🧚

✨ limit ✨

The limit is the queue size in packets 🍓 Incoming packets get dropped once this limit is reached. The default is 1000 packets.

✨ target ✨

The target is the expected queue delay 🎯 The default target delay is 15ms.

✨ tupdate ✨

The tupdate is how often the system drop probability gets calculated ⏰ The default is 15ms.

✨ alpha and beta ✨

alpha and beta are parameters that control the drop probability 🌈 They should be in the range between 0 and 32.

✨ ecn | noecn ✨

Use these to mark packets instead of dropping them 🏳️‍🌈 ecn turns ecn mode on, and noecn turns it off. By default, ecn is off 🙈

✨ bytemode | nobytemode ✨

This one scales the drop probability in proportion to packet size 📦 bytemode turns it on, nobytemode turns it off. By default, bytemode is off 💀

✨ dq_rate_estimator | no_dq_rate_estimator ✨

This is used to calculate delay using Little's law 🧮 dq_rate_estimator turns it on, and no_dq_rate_estimator turns it off. By default, dq_rate_estimator is off 🫶

💅 EXAMPLES 💅

Let's see it in action, bestie 🎀

Just the basics, no extras:

# tc qdisc add dev eth0 root pie
# tc -s qdisc show
  qdisc pie 8036: dev eth0 root refcnt 2 limit 1000p target 15.0ms
tupdate 16.0ms alpha 2 beta 20
   Sent 31216108 bytes 20800 pkt (dropped 80, overlimits 0 requeues 0)
   backlog 16654b 11p requeues 0
    prob 0.006161 delay 15666us
    pkts_in 20811 overlimit 0 dropped 80 maxq 50 ecn_mark 0

Now with the dq_rate_estimator turned on:

# tc qdisc add dev eth0 root pie dq_rate_estimator
# tc -s qdisc show
  qdisc pie 8036: dev eth0 root refcnt 2 limit 1000p target 15.0ms
tupdate 16.0ms alpha 2 beta 20
   Sent 63947420 bytes 42414 pkt (dropped 41, overlimits 0 requeues 0)
   backlog 271006b 179p requeues 0
    prob 0.000092 delay 22200us avg_dq_rate 12145996
    pkts_in 41 overlimit 343 dropped 0 maxq 50 ecn_mark 0

Custom limit, target, tupdate, and ecn all at once:

# tc qdisc add dev eth0 root pie limit 100 target 20ms tupdate 30ms ecn
# tc -s qdisc show
  qdisc pie 8036: dev eth0 root refcnt 2 limit 100p target 20.0ms
tupdate 32.0ms alpha 2 beta 20 ecn
   Sent 6591724 bytes 4442 pkt (dropped 27, overlimits 0 requeues 0)
   backlog 18168b 12p requeues 0
    prob 0.008845 delay 11348us
    pkts_in 4454 overlimit 0 dropped 27 maxq 65 ecn_mark 0

And finally, bytemode with a bigger target:

# tc qdisc add dev eth0 root pie limit 100 target 50ms tupdate 30ms
bytemode
# tc -s qdisc show
  qdisc pie 8036: dev eth0 root refcnt 2 limit 100p target 50.0ms
tupdate 32.0ms alpha 2 beta 20 bytemode
   Sent 1616274 bytes 1137 pkt (dropped 0, overlimits 0 requeues 0)
   backlog 13626b 9p requeues 0
    prob 0.000000 delay 0us
    pkts_in 1146 overlimit 0 dropped 0 maxq 23 ecn_mark 0

🔗 SEE ALSO 🔗

Check these out too, they're iconic 💜: tc(8), tc-codel(8), tc-red(8)

📚 SOURCES 📚

• RFC 8033: https://tools.ietf.org/html/rfc8033 🦋

👑 AUTHORS 👑

PIE was implemented by Vijay Subramanian and Mythili Prabhu, who are also the authors of this man page 🫶 Please report bugs and corrections to the Linux networking development mailing list at <netdev@vger.kernel.org> 💌

iproute2 · 16 January 2014 · PIE(8) 🌈 Linux, periodt 💅✨