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 -- You received this bug notification because you are a member of Kernel 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 copy_net_ns<-created_new_namespaces or unshare(clone_newnet). 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