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+# Troubleshooting Sidekiq
+
+Sidekiq is the background job processor GitLab uses to asynchronously run
+tasks. When things go wrong it can be difficult to troubleshoot. These
+situations also tend to be high-pressure because a production system job queue
+may be filling up. Users will notice when this happens because new branches
+may not show up and merge requests may not be updated. The following are some
+troubleshooting steps that will help you diagnose the bottleneck.
+
+> **Note:** GitLab administrators/users should consider working through these
+debug steps with GitLab Support so the backtraces can be analyzed by our team.
+It may reveal a bug or necessary improvement in GitLab.
+
+> **Note:** In any of the backtraces, be weary of suspecting cases where every
+  thread appears to be waiting in the database, Redis, or waiting to acquire
+  a mutex. This **may** mean there's contention in the database, for example,
+  but look for one thread that is different than the rest. This other thread
+  may be using all available CPU, or have a Ruby Global Interpreter Lock,
+  preventing other threads from continuing.
+
+## Thread dump
+
+Send the Sidekiq process ID the `TTIN` signal and it will output thread
+backtraces in the log file.
+
+```
+kill -TTIN <sidekiq_pid>
+```
+
+Check in `/var/log/gitlab/sidekiq/current` or `$GITLAB_HOME/log/sidekiq.log` for
+the backtrace output. The backtraces will be lengthy and generally start with
+several `WARN` level messages. Here's an example of a single thread's backtrace:
+
+```
+2016-04-13T06:21:20.022Z 31517 TID-orn4urby0 WARN: ActiveRecord::RecordNotFound: Couldn't find Note with 'id'=3375386
+2016-04-13T06:21:20.022Z 31517 TID-orn4urby0 WARN: /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/activerecord-4.2.5.2/lib/active_record/core.rb:155:in `find'
+/opt/gitlab/embedded/service/gitlab-rails/app/workers/new_note_worker.rb:7:in `perform'
+/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:150:in `execute_job'
+/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/processor.rb:132:in `block (2 levels) in process'
+/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:127:in `block in invoke'
+/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/sidekiq_middleware/memory_killer.rb:17:in `call'
+/opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/sidekiq-4.0.1/lib/sidekiq/middleware/chain.rb:129:in `block in invoke'
+/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/sidekiq_middleware/arguments_logger.rb:6:in `call'
+...
+```
+
+In some cases Sidekiq may be hung and unable to respond to the `TTIN` signal.
+Move on to other troubleshooting methods if this happens.
+
+## Process profiling with `perf`
+
+Linux has a process profiling tool called `perf` that is helpful when a certain
+process is eating up a lot of CPU. If you see high CPU usage and Sidekiq won't
+respond to the `TTIN` signal, this is a good next step.
+
+If `perf` is not installed on your system, install it with `apt-get` or `yum`:
+
+```
+# Debian
+sudo apt-get install linux-tools
+
+# Ubuntu (may require these additional Kernel packages)
+sudo apt-get install linux-tools-common linux-tools-generic linux-tools-`uname -r`
+
+# Red Hat/CentOS
+sudo yum install perf
+```
+
+Run perf against the Sidekiq PID:
+
+```
+sudo perf record -p <sidekiq_pid>
+```
+
+Let this run for 30-60 seconds and then press Ctrl-C. Then view the perf report:
+
+```
+sudo perf report
+
+# Sample output
+Samples: 348K of event 'cycles', Event count (approx.): 280908431073
+ 97.69%            ruby  nokogiri.so         [.] xmlXPathNodeSetMergeAndClear
+  0.18%            ruby  libruby.so.2.1.0    [.] objspace_malloc_increase
+  0.12%            ruby  libc-2.12.so        [.] _int_malloc
+  0.10%            ruby  libc-2.12.so        [.] _int_free
+```
+
+Above you see sample output from a perf report. It shows that 97% of the CPU is
+being spent inside Nokogiri and `xmlXPathNodeSetMergeAndClear`. For something
+this obvious you should then go investigate what job in GitLab would use
+Nokogiri and XPath. Combine with `TTIN` or `gdb` output to show the
+corresponding Ruby code where this is happening.
+
+## The GNU Project Debugger (gdb)
+
+`gdb` can be another effective tool for debugging Sidekiq. It gives you a little
+more interactive way to look at each thread and see what's causing problems.
+
+> **Note:** Attaching to a process with `gdb` will suspends the normal operation
+  of the process (Sidekiq will not process jobs while `gdb` is attached).
+
+Start by attaching to the Sidekiq PID:
+
+```
+gdb -p <sidekiq_pid>
+```
+
+Then gather information on all the threads:
+
+```
+info threads
+
+# Example output
+30 Thread 0x7fe5fbd63700 (LWP 26060) 0x0000003f7cadf113 in poll () from /lib64/libc.so.6
+29 Thread 0x7fe5f2b3b700 (LWP 26533) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+28 Thread 0x7fe5f2a3a700 (LWP 26534) 0x0000003f7ce0ba5e in pthread_cond_timedwait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+27 Thread 0x7fe5f2939700 (LWP 26535) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+26 Thread 0x7fe5f2838700 (LWP 26537) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+25 Thread 0x7fe5f2737700 (LWP 26538) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+24 Thread 0x7fe5f2535700 (LWP 26540) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+23 Thread 0x7fe5f2434700 (LWP 26541) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+22 Thread 0x7fe5f2232700 (LWP 26543) 0x0000003f7ce0b68c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
+21 Thread 0x7fe5f2131700 (LWP 26544) 0x00007fe5f7b570f0 in xmlXPathNodeSetMergeAndClear ()
+from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
+...
+```
+
+If you see a suspicious thread, like the Nokogiri one above, you may want
+to get more information:
+
+```
+thread 21
+bt
+
+# Example output
+#0  0x00007ff0d6afe111 in xmlXPathNodeSetMergeAndClear () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
+#1  0x00007ff0d6b0b836 in xmlXPathNodeCollectAndTest () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
+#2  0x00007ff0d6b09037 in xmlXPathCompOpEval () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
+#3  0x00007ff0d6b09017 in xmlXPathCompOpEval () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
+#4  0x00007ff0d6b092e0 in xmlXPathCompOpEval () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
+#5  0x00007ff0d6b0bc37 in xmlXPathRunEval () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
+#6  0x00007ff0d6b0be5f in xmlXPathEvalExpression () from /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/nokogiri-1.6.7.2/lib/nokogiri/nokogiri.so
+#7  0x00007ff0d6a97dc3 in evaluate (argc=2, argv=0x1022d058, self=<value optimized out>) at xml_xpath_context.c:221
+#8  0x00007ff0daeab0ea in vm_call_cfunc_with_frame (th=0x1022a4f0, reg_cfp=0x1032b810, ci=<value optimized out>) at vm_insnhelper.c:1510
+```
+
+To output a backtrace from all threads at once:
+
+```
+apply all thread bt
+```
+
+## Check for blocking queries
+
+Sometimes the speed at which Sidekiq processes jobs can be so fast that it can
+cause database contention. Check for blocking queries when backtraces above
+show that many threads are stuck in the database adapter.
+
+The PostgreSQL wiki has details on the query you can run to see blocking
+queries. The query is different based on PostgreSQL version. See
+[Lock Monitoring](https://wiki.postgresql.org/wiki/Lock_Monitoring) for
+the query details.