Evaluation of the HTB hysteresis patches on a real-life production system.

   For context see [1]netdev email discussion, titled: "HTB scheduler
   HTB_HYSTERESIS modifications".
   [2][PATCH 1/2] HTB scheduler, change default hysteresis mode to off.
   [3][PATCH 2/2] Change HTB_HYSTERESIS to a runtime parameter
   htb_hysteresis.

   We saw an increased number of interrupts on the test system, when
   enabling/disabling htb_hysteresis.
   This didn't show up on the production system.
# Enable:
echo 1 > /sys/module/sch_htb/parameters/htb_hysteresis

# Disable:
echo 0 > /sys/module/sch_htb/parameters/htb_hysteresis

   I believe that the increased intr (on test system) is an artifact of
   the TCP download got less bursty and more smooth.

  Kernel on Production host

   A new kernel 2.6.25.4 with my HTB hysteresis patches, and default
   htb_hysteresis off, were booted (tuesdag 10/6-2008) on the production
   host (internal name BCU24). Previous kernel 2.6.15. The production
   host is power full machine, a HP DL380 dual Xeon (3Ghz, 2Mb Cache,
   ht).

  Interrupts

   There is no real change in the interrupt load, see interrupts image:
   [bcu24_interrupts_week.png]

   The machine has a simple HTB setup on eth0 and eth1 each with 610 HTB
   class leafs under the root node.
   It also has quite a lot of iptables rules (11405 rules and 3015
   chains).

   When changing htb_hysteresis runtime, there where not measurable
   changes in interrupts. 

  CPU usage

   Surprisingly the CPU usage decreased significanly, when upgrading to
   kernel 2.6.25.4 (from kernel 2.6.15).
   I have not Oprofiled, but from previous experiences, I believe that
   its primarily caused by Patrick McHardys RCU patches to netfilter
   conntrack, Thanks!!! :-).
   [bcu24_cpuload_week.png]

   Only looking at the system and softirq load:
   [bcu24_softirq_week.png]

   When changing htb_hysteresis runtime, there where not measurable
   changes in CUP usage. 

  Traffic Load

   The traffic and packets/sec load didn't changed significantly over the
   period.
   [bcu24_traffic_week.png]
   [bcu24_packets_week.png]

  Author of this document

   [4]http://people.netfilter.org/hawk/patches/htb_nohyst/production_test
   _results.html

   Name: Jesper Dangaard Brouer
   Email: hawk (at) diku (dot) dk
   Author of [5]http://adsl-optimizer.dk
   LinkedIn: [6]http://www.linkedin.com/in/brouer

References

   1. http://thread.gmane.org/gmane.linux.network/95592
   2. http://people.netfilter.org/hawk/patches/htb_nohyst/0001-HTB-scheduler-change-default-hysteresis-mode-to-off.patch
   3. http://people.netfilter.org/hawk/patches/htb_nohyst/0002-Change-HTB_HYSTERESIS-to-a-runtime-parameter-htb_hys.patch
   4. http://people.netfilter.org/hawk/patches/htb_nohyst/production_test_results.html
   5. http://www.adsl-optimizer.dk/
   6. http://www.linkedin.com/in/brouer