netdev - Cake not doing rate limiting in a way it is expected to do

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Date:   Tue, 28 May 2019 11:41:14 +0530
From:   Akshat Kakkar <akshat.1984@...il.com>
To:     netdev <netdev@...r.kernel.org>, lartc <lartc@...r.kernel.org>
Subject: Cake not doing rate limiting in a way it is expected to do

Cake is expected to handle traffic in 2 steps :
First is on the basis of host
Second is within every host, on the basis of flow

So, if I limit traffic to 20Mbps shared across 2 host A & B,
Following are various scenarios, expectation and observations
1. If either A or B is downloading, they will be getting speed of 20Mbps
Observation: Meeting with expectation

2. If both A & B downloads (single download each), each will be
getting speed of 20Mbps
Observation: Meeting with expecation but its very jittery (around
20%), i.e. speed varies from 8Mbps to 12 Mbps. If I use fq_codel speed
is same BUT jitter is very less (around 1%).

3. Now if A starts 3 downloads, and B is still having single download,
A each download should be around 3.3 Mbps and B should be around
10Mbps
Observation: Around 5 Mbps for each download with lot of jitter, i.e.
no advantage of having CAKE!!!

Linux Kernel 4.20
For case 3, output of command : tc -s class show dev eno2

class htb 1:1 root leaf 8003: prio 1 rate 20000Kbit ceil 20000Kbit
burst 200Kb cburst 1600b
 Sent 688474645 bytes 455058 pkt (dropped 0, overlimits 381196 requeues 0)
 rate 19874Kbit 1641pps backlog 21196b 14p requeues 0
 lended: 382532 borrowed: 0 giants: 0
 tokens: 1260573 ctokens: -9427

class cake 8003:44f parent 8003:
 (dropped 3404, overlimits 0 requeues 0)
 backlog 9084b 6p requeues 0
class cake 8003:516 parent 8003:
 (dropped 3565, overlimits 0 requeues 0)
 backlog 0b 0p requeues 0
class cake 8003:590 parent 8003:
 (dropped 3023, overlimits 0 requeues 0)
 backlog 4542b 3p requeues 0
class cake 8003:605 parent 8003:
 (dropped 1772, overlimits 0 requeues 0)
 backlog 7570b 5p requeues 0