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Reducing memory usage of Sidekiq tasks

This issue seeks to profile and discuss possible ways to reduce memory usage of Sidekiq tasks. ZenDesk issue: https://gitlab.zendesk.com/agent/tickets/28848.

Attached is the Sidekiq memory profile output using this gem after pushing 673 commits from 8.9 RC1 to RC8 with an open MR.

memoryprof.txt

High-level stats:

Total allocated: 1532771872 bytes (3674543 objects)
Total retained:  8712646 bytes (18922 objects)

allocated memory by gem
-----------------------------------
1304777487  embedded/lib
 131201999  activerecord-4.2.6
  34888039  gitlab_git-10.2.3
  18294371  activesupport-4.2.6
  11421226  arel-6.0.3
   6359276  activemodel-4.2.6
   5800902  gitlab-rails/lib
   5308953  redis-3.3.0
   4948654  nokogiri-1.6.8
   4343896  gitlab-rails/app
   1372880  other
    970972  rufus-scheduler-3.1.10
    539692  actionpack-4.2.6
    481434  redis-namespace-1.5.2
    437424  paranoia-2.1.4
    371517  rack-1.6.4
    318740  redis-store-1.1.7
    297196  connection_pool-2.2.0
    204832  redis-activesupport-4.1.5
     77430  acts-as-taggable-on-3.5.0
     60059  json-1.8.3
     55196  sidekiq-4.1.2
     55060  i18n-0.7.0
     49353  state_machines-0.4.0
     36612  sidekiq-cron-0.4.0
     15824  rugged-0.24.0
     15032  html-pipeline-1.11.0
     11128  kaminari-0.17.0
      8692  state_machines-activemodel-0.4.0
      8450  thread_safe-0.3.5
      7847  carrierwave-0.10.0
      7640  sanitize-2.1.0
      6372  state_machines-activerecord-0.4.0
      6031  pg-0.18.4
      4440  tzinfo-1.2.2
      2496  attr_encrypted-3.0.1
      1888  elasticsearch-model-0.1.8
      1824  default_value_for-3.0.1
       968  concurrent-ruby-1.0.2
        40  after_commit_queue-1.3.0

I'm not sure if I understand why the bulk of the memory seems to be taken by something in the Timeout class:

allocated memory by file
-----------------------------------
1184661200  /opt/gitlab/embedded/lib/ruby/2.1.0/timeout.rb
  45590734  /opt/gitlab/embedded/lib/ruby/2.1.0/psych/visitors/yaml_tree.rb
  39803445  /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/postgresql_adapter.rb
  36118171  /opt/gitlab/embedded/lib/ruby/2.1.0/psych/scalar_scanner.rb
  26668819  /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/gitlab_git-10.2.3/lib/gitlab_git/commit.rb
  20125587  /opt/gitlab/embedded/service/gem/ruby/2.1.0/gems/activerecord-4.2.6/lib/active_record/connection_adapters/postgresql/database_statements.rb
  14137696  /opt/gitlab/embedded/lib/ruby/2.1.0/psych/tree_builder.rb