Threads Aren't Evil

To skip to actual technical content go to slide 71 https://speakerdeck.com/schneems/threads-arent-evil?slide=71 the first slides make no sense without narration.

Okay, so threads are pretty evil.

But they are also useful, and given the right development patterns: not impossible to work with.

In this talk you'll look at some real Ruby libraries where threads were applied to accomplish otherwise impossible tasks. We'll look at re-writing a synchronous library to support parallel execution for performance gains. We'll also talk about the operating system internals of exactly what makes a thread a thread.

If you're not comfortable with the "T" word (threads), this talk is the perfect introduction to practical concurrent programming in Ruby.

Transcript

1. Threads
   Aren’t
   Evil
   @schneems
   

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2. Threads
   

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3. Are
   

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4. Not
   

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5. Evil
   

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6. View Slide

7. View Slide

8. “Ruby doesn’t
   have real threads”
   

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9. “Ruby doesn’t
   have real threads”
   

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10. “Ruby doesn’t
    have real threads”
    

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11. “Ruby is
    incapable of using
    threads because of
    the GIL
    

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12. “Ruby is
    incapable of using
    threads because of
    the GIL
    

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13. “Ruby is
    incapable of using
    threads because of
    the GIL
    

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14. "Rails is not able
    to run on threads
    so we can't use
    them in Ruby"
    

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15. "Rails is not able
    to run on threads
    so we can't use
    them in Ruby"
    

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16. "My ruby code is
    not thread safe
    and will not run in
    threads"
    

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17. What do you hear?
    

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18. “doesn’t”
    

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19. “incapable”
    

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20. “not able”
    

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21. “will not”
    

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22. View Slide

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25. View Slide

26. View Slide

27. “doesn’t”
    

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28. “incapable”
    

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29. “not able”
    

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30. “will not”
    

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31. View Slide

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44. цинга
    

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45. цинга
    

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46. цинга
    

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47. цинга
    

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48. View Slide

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55. View Slide

56. Problem Solved?
    

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57. View Slide

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64. View Slide

65. The Future
    

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66. BTW
    

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67. Find me
    @schneems
    

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68. https://
    www.
    schneems
    .com
    

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69. View Slide

70. View Slide

71. Who here knows
    what a thread is?
    

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72. Process
    Address Space
    

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73. Process
    Address Space
    - code
    

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74. Process
    Address Space
    - code
    - data
    

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75. Process
    Address Space
    - code
    - data
    - register
    

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76. Process
    Address Space
    - code
    - data
    - register
    - stack
    

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77. Process
    Address Space
    - code
    - data
    - register
    - stack
    

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78. #include
    int increment(int x) {
    return x + 1;
    }
    int main() {
    int i = 0;
    printf("Incremented to: %i\n", increment(i))
    }
    

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79. _main:
    100000f40: 55 pushq %rbp
    100000f41: 48 89 e5 movq %rsp, %rbp
    100000f44: 48 83 ec 10 subq $16, %rsp
    100000f48: c7 45 fc 00 00 00 00 movl $0, -4(%rbp)
    100000f4f: c7 45 f8 00 00 00 00 movl $0, -8(%rbp)
    100000f56: 83 7d f8 0a cmpl $10, -8(%rbp)
    100000f5a: 0f 8d 1f 00 00 00 jge 31
    100000f60: 48 8d 3d 43 00 00 00 leaq 67(%rip), %rdi
    100000f67: b0 00 movb $0, %al
    100000f69: e8 1a 00 00 00 callq 26
    100000f6e: 89 45 f4 movl %eax, -12(%rbp)
    100000f71: 8b 45 f8 movl -8(%rbp), %eax
    100000f74: 83 c0 01 addl $1, %eax
    100000f77: 89 45 f8 movl %eax, -8(%rbp)
    100000f7a: e9 d7 ff ff ff jmp -41
    100000f7f: 8b 45 fc movl -4(%rbp), %eax
    100000f82: 48 83 c4 10 addq $16, %rsp
    100000f86: 5d popq %rbp
    100000f87: c3 retq
    

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80. _main:
    100000f40: 55 pushq %rbp
    100000f41: 48 89 e5 movq %rsp, %rbp
    100000f44: 48 83 ec 10 subq $16, %rsp
    100000f48: c7 45 fc 00 00 00 00 movl $0, -4(%rbp)
    100000f4f: c7 45 f8 00 00 00 00 movl $0, -8(%rbp)
    100000f56: 83 7d f8 0a cmpl $10, -8(%rbp)
    100000f5a: 0f 8d 1f 00 00 00 jge 31
    100000f60: 48 8d 3d 43 00 00 00 leaq 67(%rip), %rdi
    100000f67: b0 00 movb $0, %al
    100000f69: e8 1a 00 00 00 callq 26
    100000f6e: 89 45 f4 movl %eax, -12(%rbp)
    100000f71: 8b 45 f8 movl -8(%rbp), %eax
    100000f74: 83 c0 01 addl $1, %eax
    100000f77: 89 45 f8 movl %eax, -8(%rbp)
    100000f7a: e9 d7 ff ff ff jmp -41
    100000f7f: 8b 45 fc movl -4(%rbp), %eax
    100000f82: 48 83 c4 10 addq $16, %rsp
    100000f86: 5d popq %rbp
    100000f87: c3 retq
    

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81. _main:
    100000f40: 55 pushq %rbp
    100000f41: 48 89 e5 movq %rsp, %rbp
    100000f44: 48 83 ec 10 subq $16, %rsp
    100000f48: c7 45 fc 00 00 00 00 movl $0, -4(%rbp)
    100000f4f: c7 45 f8 00 00 00 00 movl $0, -8(%rbp)
    100000f56: 83 7d f8 0a cmpl $10, -8(%rbp)
    100000f5a: 0f 8d 1f 00 00 00 jge 31
    100000f60: 48 8d 3d 43 00 00 00 leaq 67(%rip), %rdi
    100000f67: b0 00 movb $0, %al
    100000f69: e8 1a 00 00 00 callq 26
    100000f6e: 89 45 f4 movl %eax, -12(%rbp)
    100000f71: 8b 45 f8 movl -8(%rbp), %eax
    100000f74: 83 c0 01 addl $1, %eax
    100000f77: 89 45 f8 movl %eax, -8(%rbp)
    100000f7a: e9 d7 ff ff ff jmp -41
    100000f7f: 8b 45 fc movl -4(%rbp), %eax
    100000f82: 48 83 c4 10 addq $16, %rsp
    100000f86: 5d popq %rbp
    100000f87: c3 retq
    

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82. _main:
    100000f40: 55 pushq %rbp
    100000f41: 48 89 e5 movq %rsp, %rbp
    100000f44: 48 83 ec 10 subq $16, %rsp
    100000f48: c7 45 fc 00 00 00 00 movl $0, -4(%rbp)
    100000f4f: c7 45 f8 00 00 00 00 movl $0, -8(%rbp)
    100000f56: 83 7d f8 0a cmpl $10, -8(%rbp)
    100000f5a: 0f 8d 1f 00 00 00 jge 31
    100000f60: 48 8d 3d 43 00 00 00 leaq 67(%rip), %rdi
    100000f67: b0 00 movb $0, %al
    100000f69: e8 1a 00 00 00 callq 26
    100000f6e: 89 45 f4 movl %eax, -12(%rbp)
    100000f71: 8b 45 f8 movl -8(%rbp), %eax
    100000f74: 83 c0 01 addl $1, %eax
    100000f77: 89 45 f8 movl %eax, -8(%rbp)
    100000f7a: e9 d7 ff ff ff jmp -41
    100000f7f: 8b 45 fc movl -4(%rbp), %eax
    100000f82: 48 83 c4 10 addq $16, %rsp
    100000f86: 5d popq %rbp
    100000f87: c3 retq
    

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83. #include
    int increment(int x) {
    return x + 1;
    }
    int main() {
    int i = 0;
    printf("Incremented to: %i\n", increment(i))
    }
    

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84. #include
    int increment(int x) {
    return x + 1;
    }
    int main() {
    int i = 0;
    printf("Incremented to: %i\n", increment(i))
    }
    

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85. #include
    int increment(int x) {
    return x + 1;
    }
    int main() {
    int i = 0;
    printf("Incremented to: %i\n", increment(i))
    }
    

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86. #include
    int increment(int x) {
    return x + 1;
    }
    int main() {
    int i = 0;
    printf("Incremented to: %i\n", increment(i))
    }
    

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87. #include
    int increment(int x) {
    return x + 1;
    }
    int main() {
    int i = 0;
    printf("Incremented to: %i\n", increment(i))
    }
    

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88. Thanks to
    Julia Evans
    For the example code
    https://jvns.ca
    

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89. Process
    Address Space
    - code
    - data
    - register
    - stack
    

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90. Process
    Address Space
    - code
    - data
    - register
    - stack
    

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91. Process Control Block
    Process State
    Process Number
    Program Counter
    Registers
    Memory Limits
    List of open Files
    Signal Mask
    CPU Scheduling info
    (PCB)
    

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92. View Slide

93. Process
    - code
    - data
    - register
    - stack
    

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94. Process
    - code
    - data
    - register
    - stack
    CPU
    
    

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95. Process
    - code
    - data
    - register
    - stack
    
    
    
    
    Input
    Output
    (IO)
    

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96. Process
    - code
    - data
    - register
    - stack
    
    
    
    
    Input
    Output
    (IO)
    

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97. Process
    - code
    - data
    - register
    - stack
    
    
    
    
    Input
    Output
    (IO)
    

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98. View Slide

99. View Slide

100. View Slide

101. Pros?
     

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102. Better CPU
     utilization
     

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103. Cons?
     

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104. Need lots of
     processes
     

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105. Processes take
     up memory
     which is a finite
     resource
     

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106. Loading and
     unloading a PCB
     is expensive
     

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107. Sharing data
     between
     processes is
     HARD
     

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110. View Slide

111. Introducing
     

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112. Threads
     

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113. Process
     - code
     - data
     - register
     - stack
     

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114. Process
     - code
     - data
     - register
     - stack
     

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115. Process
     - code
     - data
     - register
     - stack
     

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116. Process
     register
     stack
     ⚡
     

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117. Process
     register
     stack
     ⚡ ⚡ ⚡
     register
     stack
     register
     stack
     

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118. Pros?
     

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119. Better CPU
     utilization
     

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120. Reuse existing
     process memory
     (code and data)
     

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121. Smaller Context
     Switch Time
     

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122. Cons?
     

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123. Shared code +
     shared data =
     
     

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124. Shared code +
     shared data =
     
     

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125. Threads are
     simple
     

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126. Threads are hard
     

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127. In every language
     *
     

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128. It’s possible to
     avoid threads
     

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129. Javascript
     is single threaded
     

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130. Threading is
     easier in some
     languages
     

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131. Functional
     languages (can)
     make threading
     easy
     

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132. Functional
     languages make
     side effects
     impossible
     

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133. Which makes
     coding harder
     (IMHO)
     

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134. OMSCS
     

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135. Threading sucks
     (less) in Ruby
     

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136. Are threads just
     for performance?
     

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137. Nope
     

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138. Thread.new do
     while @running
     sleep @timeout
     @reaper.reap
     end
     end
     

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139. Thread.new do
     while @running
     sleep @timeout
     @reaper.reap
     end
     end
     

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140. Was that scary?
     

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141. Are threads scary?
     

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142. (hint: no)
     

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143. Any other
     examples?
     

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144. @background_worker_thread = Thread.new do
     @background_worker.start {
     ScoutApm::Debug.instance.call_periodic_hooks
     ScoutApm::Agent.instance.process_metrics
     clean_old_percentiles
     }
     end
     

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145. What about the
     GVL?
     

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146. array = Array.new(100) { String.new }
     t1 = Thread.new do
     array.each {|x| x.prepend("hello") }
     end
     t2 = Thread.new do
     array.each {|x| x << " world" }
     end
     t1.join; t2.join
     puts array
     

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147. hello world
     hello world
     hello world
     hello world
     hello world
     hello world
     hello world
     hello world
     

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148. hello world
     hello world
     hello world
     hello world
     hello world
     hello world
     hello world
     hello world
     

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149. What did the GVL
     do?
     

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150. array = Array.new(100) { String.new }
     t1 = Thread.new do
     array.each {|x| x.prepend("hello") }
     end
     t2 = Thread.new do
     array.each {|x| x << " world" }
     end
     t1.join; t2.join
     puts array
     

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151. array = Array.new(100) { String.new }
     t1 = Thread.new do
     array.each {|x| x.prepend("hello") }
     end
     t2 = Thread.new do
     array.each {|x| x << " world" }
     end
     t1.join; t2.join
     puts array
     

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152. GVL means we
     cannot see speed
     increase just by
     using threads
     

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153. View Slide

154. GVL is released
     when we do IO!
     

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155. array = Array.new(100) { String.new }
     t1 = Thread.new do
     uri = URI.parse(“http://example.com/hello")
     hi = Net::Http.get(uri)
     array.each {|x| x.prepend(hi) }
     end
     t2 = Thread.new do
     uri = URI.parse(“http://example.com/hello")
     world = Net::Http.get(uri)
     array.each {|x| x << world }
     end
     t1.join; t2.join
     puts array
     

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156. When is the GVL
     released exactly?
     

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157. void *
     rb_thread_call_without_gvl2(void *(*func)(void *),
     void *data1, rb_unblock_function_t *ubf, void *data2)
     {
     return call_without_gvl(func, data1, ubf, data2,
     TRUE);
     }
     void *
     rb_thread_call_without_gvl(void *(*func)(void *data),
     void *data1, rb_unblock_function_t *ubf, void *data2)
     {
     return call_without_gvl(func, data1, ubf, data2,
     FALSE);
     }
     

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158. void *
     rb_thread_call_without_gvl2(void *(*func)(void *),
     void *data1, rb_unblock_function_t *ubf, void *data2)
     {
     return call_without_gvl(func, data1, ubf, data2,
     TRUE);
     }
     void *
     rb_thread_call_without_gvl(void *(*func)(void *data),
     void *data1, rb_unblock_function_t *ubf, void *data2)
     {
     return call_without_gvl(func, data1, ubf, data2,
     FALSE);
     }
     

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159. Use a debugger to
     see GVL get
     released!
     

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160. $ lldb ruby scratch.rb
     breakpoint set --name
     rb_thread_call_without_gvl
     breakpoint set --name
     rb_thread_call_without_gv2
     

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161. $ lldb ruby scratch.rb
     breakpoint set --name
     rb_thread_call_without_gvl
     breakpoint set --name
     rb_thread_call_without_gv2
     

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162. $ lldb ruby scratch.rb
     breakpoint set --name
     rb_thread_call_without_gvl
     breakpoint set --name
     rb_thread_call_without_gv2
     

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163. Now run
     

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164. $ lldb ruby scratch.rb
     breakpoint set --name
     rb_thread_call_without_gvl
     breakpoint set --name
     rb_thread_call_without_gv2
     run
     

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165. When we hit a
     pause, find the
     backtrace with
     

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166. call rb_backtrace()
     

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167. require ‘net/http'
     url = “https://www.schneems.com"
     uri = URI.parse(url)
     response = Net::HTTP.get(uri)
     puts response
     

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168. require ‘net/http'
     url = “https://www.schneems.com"
     uri = URI.parse(url)
     response = Net::HTTP.get(uri)
     puts response
     

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169. require ‘net/http'
     url = “https://www.schneems.com"
     uri = URI.parse(url)
     response = Net::HTTP.get(uri)
     puts response
     

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170. require ‘net/http'
     url = “https://www.schneems.com"
     uri = URI.parse(url)
     response = Net::HTTP.get(uri)
     puts response
     

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171. require ‘net/http'
     url = “https://www.schneems.com"
     uri = URI.parse(url)
     response = Net::HTTP.get(uri)
     puts response
     

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172. require ‘net/http'
     url = “https://www.schneems.com"
     uri = URI.parse(url)
     response = Net::HTTP.get(uri)
     puts response
     

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173. None of the
     above?
     

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174. Use the debugger!
     

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175. 1309 {
     1310 void *val = 0;
     1311
     -> 1312 rb_thread_t *th = GET_THREAD();
     1313 int saved_errno = 0;
     1314
     1315 if (ubf == RUBY_UBF_IO || ubf == RUBY_UBF_PROCESS) {
     (lldb) call rb_backtrace()
     from :4:in `'
     from :4:in `require'
     from.../rubygems.rb:1363:in `'
     from.../rubygems/specification.rb:873:in `load_defaults'
     from.../rubygems/specification.rb:821:in `each_spec'
     from.../rubygems/specification.rb:743:in `each_gemspec'
     from.../rubygems/specification.rb:743:in `each'
     from.../rubygems/specification.rb:744:in `block in each_gemspec'
     from.../rubygems/specification.rb:744:in `each'
     from.../rubygems/specification.rb:745:in `block (2 levels) in each_gemspec'
     from.../rubygems/specification.rb:822:in `block in each_spec'
     from.../rubygems/specification.rb:1161:in `load'
     from.../rubygems/specification.rb:1161:in `read'
     (lldb)
     

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176. require ‘net/http'
     url = “https://www.schneems.com"
     uri = URI.parse(url)
     response = Net::HTTP.get(uri)
     puts response
     

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177. I told a small lie
     

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178. void *
     rb_thread_call_without_gvl2(void *(*func)(void *),
     void *data1, rb_unblock_function_t *ubf, void *data2)
     {
     return call_without_gvl(func, data1, ubf, data2,
     TRUE);
     }
     void *
     rb_thread_call_without_gvl(void *(*func)(void *data),
     void *data1, rb_unblock_function_t *ubf, void *data2)
     {
     return call_without_gvl(func, data1, ubf, data2,
     FALSE);
     }
     

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179. #define GVL_UNLOCK_BEGIN() do { \
     rb_thread_t *_th_stored = GET_THREAD(); \
     RB_GC_SAVE_MACHINE_CONTEXT(_th_stored); \
     gvl_release(_th_stored->vm);
     #define GVL_UNLOCK_END() \
     gvl_acquire(_th_stored->vm, _th_stored);
     \
     rb_thread_set_current(_th_stored); \
     } while(0)
     

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180. BTW, you don’t
     need to know how
     to do this.
     

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181. I’m just showing
     you it’s not magic
     

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182. Recap: Threads
     are not…
     

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183. Recap: Threads
     are not evil
     

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184. Recap: GVL is not
     magic
     

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185. How do we start
     using threads?
     

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186. Make a Toy
     threading library!
     

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187. It’s fun I promise
     

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188. Make a worker
     thread
     

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189. require ‘thread
     my_queue = Queue.new
     my_thread = Thread.new do
     loop do
     job = my_queue.pop
     job.call
     end
     end
     

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190. require ‘thread
     my_queue = Queue.new
     my_thread = Thread.new do
     loop do
     job = my_queue.pop
     job.call
     end
     end
     

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191. require ‘thread
     my_queue = Queue.new
     my_thread = Thread.new do
     loop do
     job = my_queue.pop
     job.call
     end
     end
     

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192. require ‘thread
     my_queue = Queue.new
     my_thread = Thread.new do
     loop do
     job = my_queue.pop
     job.call
     end
     end
     

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193. Use a worker
     thread via a
     “boss” i.e. main
     

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194. 10.times {
     job = Proc.new { puts "hello" }
     my_queue.push(job)
     }
     

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195. It works!
     

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196. But there are bugs
     

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197. How can we be
     sure that all jobs
     are finished?
     

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198. Join the thread
     

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199. require 'thread'
     my_queue = Queue.new
     my_thread = Thread.new do
     loop do
     job = my_queue.pop
     job.call
     end
     end
     10.times {
     job = Proc.new { puts "hello" }
     my_queue.push(job)
     }
     my_thread.join
     

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200. Infinite loop
     

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201. Why?
     

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202. require ‘thread
     my_queue = Queue.new
     my_thread = Thread.new do
     loop do
     job = my_queue.pop
     job.call
     end
     end
     

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203. Poison our queue
     

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204. my_queue = Queue.new
     POISON = Object.new
     my_thread = Thread.new do
     loop do
     job = my_queue.pop
     break if job == POISON
     job.call
     end
     end
     

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205. require 'thread'
     my_queue = Queue.new
     POISON = Object.new
     my_thread = Thread.new do
     loop do
     job = my_queue.pop
     break if job == POISON
     job.call
     end
     end
     10.times {
     job = Proc.new { puts "hello" }
     my_queue.push(job)
     }
     my_queue.push(POISON)
     my_thread.join
     

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206. Exercise: Build a
     thread pool that
     has multiple
     threads
     

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207. To poison multiple
     workers, add multiple
     poison objects into
     pool
     

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208. Threads are not…
     

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209. Threads are not
     evil
     

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210. GVL is not magic
     

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211. There are other
     thread constructs
     

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212. Mutex
     

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213. Condition
     Variable
     

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214. You can ignore most
     of them if you use a
     queue and boss/
     worker pattern.
     

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215. How do I
     _actually_ use
     threads?
     

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216. concurrent_ruby
     

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217. require 'concurrent'
     10.times {
     Concurrent::Promise.execute(
     executor: :fast) do
     puts "hello"
     end
     }
     

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218. require 'concurrent'
     10.times {
     Concurrent::Promise.execute(
     executor: :fast) do
     puts "hello"
     end
     }
     

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219. require 'concurrent'
     10.times {
     Concurrent::Promise.execute(
     executor: :fast) do
     puts "hello"
     end
     }
     

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220. Protip!
     

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221. Always call wait!
     

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222. Sprockets
     Example
     

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223. After we process
     an asset we have
     to write it to disk
     

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224. We also want a
     gzip file etc.
     

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225. File writes release
     the GVL
     

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226. Use Concurrent
     Ruby!
     

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227. promise = Concurrent::Promise.execute(
     executor: executor) do
     exporter.call
     end
     concurrent_exporters << promise
     

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228. promise = Concurrent::Promise.execute(
     executor: executor) do
     exporter.call
     end
     concurrent_exporters << promise
     

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229. concurrent_exporters.each(&:wait!)
     

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230. Here’s how to use
     threads
     

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231. Find where the
     GVL is released
     

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232. GVL is released
     where there is IO
     

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233. Where there is IO,
     threads are an
     amazing
     performance fit
     

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234. Create a job
     

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235. Make sure the job
     finishes
     

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236. Handle shared
     variable access
     (via atomic
     structures)
     

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237. Handle shared
     variable access
     (via atomic
     structures)
     

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238. What about
     guilds?
     

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239. Guilds aren’t
     released
     

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240. Can still use
     threads from
     within a guild
     

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241. Remember our
     explorers
     

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242. View Slide

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244. View Slide

245. View Slide

246. ☠☠☠
     

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247. Webrick is slow
     

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248. Webrick is slow
     

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249. Talk about what
     you DON’T
     know
     

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250. Talk about what
     you DON’T
     know
     

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251. Talk about what
     you DON’T
     know
     

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252. Share
     assumptions
     

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253. Share
     assumptions
     

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254. Threads are a tool
     

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255. Threads can be
     useful
     

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256. Threads can be
     fast
     

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257. View Slide

258. View Slide

259. View Slide

260. View Slide

261. View Slide

262. View Slide

263. Threads
     

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264. Are
     

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265. Not
     

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266. Evil
     

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267. Any Questions?
     

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268. View Slide