I wonder if this fix fell out or is somehow different now?
The below script, on 4.11rc7:
1cpu: 0m1.379s
12cpu: 1m36.556s
72cpu: 2m20.118s
This is a *huge* impact for neutron L3 agent on my OpenStack system.
# cd /tmp
# ip netns add foo
# ip netns add bar
# for i in `seq 0 1000` ; do echo -e 'netns exec foo echo\nnetns exec bar echo'
>> ipnetns.batch ; done
# time ip -b ipnetns.batch > /dev/null
--
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Packages, which is subscribed to linux in Ubuntu.
https://bugs.launchpad.net/bugs/1328088
Title:
Kernel network namespace performance regression during rcu development
on kernels above 3.8
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Trusty:
Fix Released
Status in linux source package in Utopic:
Fix Released
Bug description:
SRU Justification:
Impact: network namespace creation has performance regression since v3.5.
Fix: my analysis, lklm discussion, upstream patch
Testcase:
http://people.canonical.com/~inaddy/lp1328088/make_fake_routers.sh
http://people.canonical.com/~inaddy/lp1328088/parse.py
http://people.canonical.com/~inaddy/lp1328088/charts/250.html
http://people.canonical.com/~inaddy/lp1328088/charts/250-tag.html
Running make_fake_routers.sh 4000 and using parse.py you can check if
"fake routers" are being created in a good rate /sec (and you can
compare with all generated charts).
Original Description:
Please, follow this in:
http://people.canonical.com/~inaddy/lp1328088/. Same description on
daily-basis updated text.
--
It was brought to my attention that network namespace creation scalability
was affected during kernel development.
The following script was used for all the tests and charts generation:
http://people.canonical.com/~inaddy/lp1328088/make_fake_routers.sh
http://people.canonical.com/~inaddy/lp1328088/parse.py
I measured how many "fake routers" (above script) could be added per
second from 0 to 4000 created routers mark. Using this script and a
git bisect on kernel tree I was led to one specific commit causing
regression: #911af50 "rcu: Provide compile-time control for no-CBs
CPUs". Even Though this change was experimental at that point, it
introduced a performance scalability regression (explained below) that
still last and seems to be the default option for distributions
nowadays.
RCU related code looked like to be responsible for the problem. With
that, every commit from tag v3.8..master that changed any of this
files: "kernel/rcutree.c kernel/rcutree.h kernel/rcutree_plugin.h
include/trace/events/rcu.h include/linux/rcupdate.h" was tested. The
idea was to check performance regression during rcu development. In
the worst case, the regression not being related to rcu, I would still
have data to interpret the performance/scalability regression.
All text below this refer to 2 groups of charts, generated during the
study:
1) Kernel git tags from 3.8 to 3.14.
http://people.canonical.com/~inaddy/lp1328088/charts/250-tag.html
2) Kernel git commits for rcu development (111 commits).
http://people.canonical.com/~inaddy/lp1328088/charts/250.html
Since there was difference in results depending on how many cpus or
how the no-cb cpus were configured, 3 kernel config options were used
on every measure:
- CONFIG_RCU_NOCB_CPU (disabled): nocbno
- CONFIG_RCU_NOCB_CPU_ALL (enabled): nocball
- CONFIG_RCU_NOCB_CPU_NONE (enabled): nocbnone
Obs: For 1 cpu cases: nocbno, nocbnone, nocball behaves the same since
w/ only 1 cpu there is no no-cb cpu
After charts being generated it was clear that NOCB_CPU_ALL (4 cpus)
affected the "fake routers" creation process performance and this
regression continues up to upstream version. It was also clear that,
after commit #911af50, having more than 1 cpu does not improve
performance/scalability for netns, makes it worse.
#911af50
...
+#ifdef CONFIG_RCU_NOCB_CPU_ALL
+ pr_info("\tExperimental no-CBs for all CPUs\n");
+ cpumask_setall(rcu_nocb_mask);
+#endif /* #ifdef CONFIG_RCU_NOCB_CPU_ALL */
...
Comparing standing out points (see charts):
#81e5949 - good
#911af50 - bad
I was able to see that, from the script above, the following lines
causes major impact on netns scalability/performance:
1) ip netns add -> huge performance regression:
1 cpu: no regression
4 cpu: regression for NOCB_CPU_ALL
obs: regression from 250 netns/sec to 50 netns/sec
on 500 netns already created mark
2) ip netns exec -> some performance regression
1 cpu: no regression
4 cpu: regression for NOCB_CPU_ALL
obs: regression from 40 netns (+1 exec per netns
creation) to 20 netns/sec on 500 netns created
mark
# Assumption (to be confirmed)
rcu callbacks being offloaded to other cpus caused regression in
