Let's talk concurrency

For a long time, the de facto way of doing multi-core concurrency was using threads. However, the complexity of manipulating threads and state affected negatively how developers perceive concurrency. Fortunately, languages like Clojure and Erlang implement new paradigms that aim to make concurrency easier.

In this talk, José Valim is going to discuss the role state play in concurrency and introduce different paradigms for multi-core concurrency, like Actors and Software Transactional Memory, explaining their trade-offs and why they matter to us developers.

Transcript

1. A tale by @josevalim from @plataformatec
   CONCURRENCY
   LET’S TALK
   Friday, September 21, 2012
   

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2. Friday, September 21, 2012
   

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3. Core Team Member
   Friday, September 21, 2012
   

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4. Friday, September 21, 2012
   

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5. CONCURRENCY
   LET’S TALK
   Friday, September 21, 2012
   

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6. “Execute several
   tasks simultaneously”
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7. Server Server Server Server
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8. Server
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9. MULTI
   CORE
   SINGLE
   PROCESS
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10. state
    concurrency
    off
    off
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11. The Declarative Model
    Because mathematics is awesome!
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12. •no mutation
    •no concurrency
    •just functions
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13. factorial = lambda do |x|
    case x
    when 0
    1
    else
    x * factorial.(x-1)
    end
    end
    print factorial.(10)
    # => 3628800
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14. Determinism
    Same input = > same output
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15. •no i/o
    •no random
    •no side-effects
    •always returns the same result
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16. lambda do |a, b|
    c = expensive_function.(a)
    d = also_expensive.(b)
    c + d
    end
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17. lambda do |a, b|
    c = expensive_function.(a)
    d = also_expensive.(b)
    c + d
    end
    a = 1
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18. lambda do |1, b|
    c = 42
    d = also_expensive.(b)
    c + d
    end
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19. lambda do |a, b|
    c = expensive_function.(a)
    d = also_expensive.(b)
    c + d
    end
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20. lambda do |a, b|
    d = also_expensive.(b)
    c = expensive_function.(a)
    c + d
    end
    Friday, September 21, 2012
    

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21. Haskell
    Using determinism for
    performance and expressiveness
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22. l = lambda { |a,b,c| a + b + c }
    l.(1, 2, 3) #=> 6
    l.curry #=> #
    l.curry.(1) #=> #
    l.curry.(1).(2).(3) #=> 6
    Currying
    Friday, September 21, 2012
    

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23. l = lambda { |a,b| a * b }
    double = l.curry.(2)
    triple = l.curry.(3)
    Currying
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24. sum a b c = a + b + c
    (sum 1 2 3)
    (((sum 1) 2) 3)
    Currying
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25. x = expensive(1)
    y = expensive(3)
    z = (sum x)
    ((z 2) y)
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26. x = expensive(1)
    y = expensive(3)
    z = (sum x)
    ((z 2) y)
    Compiler
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27. x = expensive(1)
    y = expensive(3)
    z = (sum x)
    ((z 2) y)
    Compiler
    (sum expensive(1) 2 expensive(3))
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28. CONCURRENCY ON
    LET’S TURN
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29. state
    concurrency
    off
    on
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30. Dataflow variables
    Painless concurrency!
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31. lambda do |a, b|
    c = expensive_function.(a)
    d = also_expensive.(b)
    c + d
    end
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32. lambda do |a, b|
    thread {
    c = expensive_function.(a)
    }
    thread {
    d = also_expensive.(b)
    }
    c + d
    end
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33. main
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34. main
    spawn thread 1
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35. main
    spawn thread 1
    spawn thread 2
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36. main
    spawn thread 1
    spawn thread 2
    unbound c
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37. main
    spawn thread 1
    spawn thread 2
    unbound c
    defines c
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38. main
    spawn thread 1
    spawn thread 2
    unbound c
    defines c
    unbound d
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39. main
    spawn thread 1
    spawn thread 2
    unbound c
    defines c
    unbound d
    defines d
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40. main
    spawn thread 1
    spawn thread 2
    unbound c
    defines c
    unbound d
    defines d
    c + d
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41. STATE ON
    LET’S TURN
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42. state
    concurrency
    on
    on
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43. Friday, September 21, 2012
    

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44. PROBLEM
    THE
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45. class Counter
    mattr_accessor :i
    self.i = 0
    end
    thread {
    Counter.i = Counter.i + 1
    }
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46. thread 1 Counter.i thread 2
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47. thread 1 Counter.i thread 2
    0
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48. thread 1 Counter.i thread 2
    0
    0
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49. thread 1 Counter.i thread 2
    0
    0
    1
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50. thread 1 Counter.i thread 2
    0
    0
    1 1
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51. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
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52. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
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53. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
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54. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    1
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55. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    1
    2
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56. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    1
    2 2
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57. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    1
    2 2
    2
    Friday, September 21, 2012
    

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58. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    1
    2 2
    2
    2
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59. •shared-memory concurrent model
    •locks
    •transactional memory
    •message-passing concurrent model
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60. •shared-memory concurrent model
    •locks
    •transactional memory
    •message-passing concurrent model
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61. class Counter
    mattr_accessor :i
    self.i = 0
    end
    thread {
    Counter.i = Counter.i + 1
    }
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62. class Counter
    mattr_accessor :i
    self.i = 0
    end
    thread {
    synchronize {
    Counter.i = Counter.i + 1
    }
    }
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63. thread 1 Counter.i thread 2
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64. thread 1 Counter.i thread 2
    0
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65. thread 1 Counter.i thread 2
    0
    0
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66. thread 1 Counter.i thread 2
    0
    0
    1
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67. thread 1 Counter.i thread 2
    0
    0
    1 1
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68. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
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69. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    synchronize {
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70. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    2
    1
    synchronize {
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71. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    2
    1
    synchronize {
    2
    }
    Friday, September 21, 2012
    

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72. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    2
    2
    1
    synchronize {
    2
    }
    Friday, September 21, 2012
    

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73. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    2
    2
    2
    1
    synchronize {
    2
    }
    Friday, September 21, 2012
    

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74. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    2
    2
    3
    2
    1
    synchronize {
    2
    }
    Friday, September 21, 2012
    

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75. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    2
    2
    3 3
    2
    1
    synchronize {
    2
    }
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76. +most popular approach
    +explicit control over the lock
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77. - explicit control over the lock
    - pessimistic approach
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78. •shared-memory concurrent model
    •locks
    •transactional memory
    •message-passing concurrent model
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79. class Counter
    mattr_accessor :i
    self.i = ref { 0 }
    end
    thread {
    atomic {
    Counter.i = Counter.i + 1
    }
    }
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80. thread 1 Counter.i thread 2
    Friday, September 21, 2012
    

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81. thread 1 Counter.i thread 2
    0
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82. thread 1 Counter.i thread 2
    0
    0
    Friday, September 21, 2012
    

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83. thread 1 Counter.i thread 2
    0
    0
    1
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84. thread 1 Counter.i thread 2
    0
    0
    1 1
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85. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    Friday, September 21, 2012
    

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86. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    atomic {
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87. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    atomic {
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88. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    1
    atomic {
    Friday, September 21, 2012
    

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89. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    2
    1
    atomic {
    Friday, September 21, 2012
    

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90. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    2 2
    1
    atomic {
    Friday, September 21, 2012
    

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91. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    2 2
    2
    1
    atomic {
    Friday, September 21, 2012
    

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92. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    2 2
    2
    1
    atomic {
    }
    Friday, September 21, 2012
    

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93. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    2 2
    2
    1
    atomic {
    }
    Friday, September 21, 2012
    

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94. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    2 2
    2
    1
    atomic {
    }
    Friday, September 21, 2012
    

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95. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    2 2
    2
    2
    1
    atomic {
    }
    Friday, September 21, 2012
    

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96. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    2 2
    2
    2 2
    1
    atomic {
    }
    Friday, September 21, 2012
    

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97. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    2 2
    2
    2 2
    3
    1
    atomic {
    }
    Friday, September 21, 2012
    

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98. thread 1 Counter.i thread 2
    0
    1
    0
    1 1
    1
    1
    2 2
    2
    2 2
    3
    3
    1
    atomic {
    }
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99. + optimistic approach
    + no deadlock or race conditions
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100. - potential unnecessary retries
     - transaction overhead
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101. •shared-memory concurrent model
     •locks
     •transactional memory
     •message-passing concurrent model
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102. server = lambda do |i|
     receive
     when :increment
     server.(i+1)
     when :check
     client <- i
     server.(i)
     else
     warn "unknown message"
     server.(i)
     end
     end
     server.(0)
     Friday, September 21, 2012
     

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103. thread {
     server <- :increment
     }
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104. thread 1 server thread 2
     Friday, September 21, 2012
     

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105. thread 1 server thread 2
     0
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106. thread 1 server thread 2
     0
     :increment
     Friday, September 21, 2012
     

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107. thread 1 server thread 2
     0
     1
     :increment
     Friday, September 21, 2012
     

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108. thread 1 server thread 2
     0
     1
     :increment
     :increment
     Friday, September 21, 2012
     

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109. thread 1 server thread 2
     0
     2
     1
     :increment
     :increment
     Friday, September 21, 2012
     

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110. thread 1 server thread 2
     0
     2
     1
     :increment
     :increment
     :increment
     Friday, September 21, 2012
     

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111. thread 1 server thread 2
     0
     2
     1
     3
     :increment
     :increment
     :increment
     Friday, September 21, 2012
     

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112. thread 1 server thread 2
     0
     2
     1
     3
     :increment
     :increment
     :increment
     :check
     Friday, September 21, 2012
     

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113. thread 1 server thread 2
     0
     2
     1
     3
     3
     :increment
     :increment
     :increment
     :check
     Friday, September 21, 2012
     

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114. thread 1 server thread 2
     0
     2
     1
     3
     3
     :increment
     :increment
     :increment
     :check
     Friday, September 21, 2012
     

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115. thread 1 server thread 2
     0
     2
     1
     3
     3
     3
     :increment
     :increment
     :increment
     :check
     Friday, September 21, 2012
     

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116. “Do not communicate by
     sharing memory;
     instead, share memory
     by communicating;”
     Effective Go
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117. +no need for synchronization
     +easy to distribute (location
     transparency)
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118. - coordination may be tricky
     - non-conventional approach
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119. UP
     SUMMING
     Friday, September 21, 2012
     

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120. message-passing
     locks
     stm
     go, erlang
     ruby
     clojure
     dataflow oz
     Friday, September 21, 2012
     

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121. github.com/celluloid
     Friday, September 21, 2012
     

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122. @elixirlang / elixir-lang.org
     Friday, September 21, 2012
     

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123. MATTER
     DEFAULTS
     Friday, September 21, 2012
     

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124. Erlang and Clojure
     defaults to immutable
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125. MAGIC BULLET
     THERE IS NO
     Friday, September 21, 2012
     

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126. REFERENCES
     Seven languages in seven weeks
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127. REFERENCES
     Concepts, Techniques and Models
     of Computer Programming
     Friday, September 21, 2012
     

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128. REFERENCES
     Software Transactional Memory
     http://java.ociweb.com/mark/stm/article.html
     Persistent Data Structures
     http://www.infoq.com/presentations/Value-Identity-State-Rich-Hickey
     Friday, September 21, 2012
     

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129. ?
     QUESTIONS
     @josevalim
     Friday, September 21, 2012
     

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