; definitions of breeds used breed [ writers writer ] breed [ readers reader ] breed [ queue_readers queue_reader ] breed [ queue_writers queue_writer ] ; Definition of global variables used globals [ storage-x storage-y ; posistion of storage door-x door-y ; entrance location number-of-readers ; actual number of readers number-of-writers ; actual number of writers total_number-of-readers ; total number of readers total_number-of-writers ; total number of writers mutex ; mutex for storage locking ] ; setup values of global variables and turtle shapes to setup ca set storage-x 13 set storage-y 0 set door-x -10 set door-y 0 set mutex 0 set number-of-readers 0 set number-of-writers 0 set total_number-of-readers 0 set total_number-of-writers 0 ; placing of the storage ask patch storage-x storage-y [ sprout 1 [ set color grey set shape "book" set size 8 stamp die ] ] ; placing of the door ask patch door-x door-y [ sprout 1 [ set color brown set shape "door" set size 7 stamp die ] ] ; placing of the readers queue create-queue_readers 1 [ set color blue set shape "circle 2" set size 3 setxy -10 -10 ht ] ; placing of the writers queue create-queue_writers 1 [ set color yellow set shape "circle 2" set size 3 setxy -10 10 ht ] do-plots ; graphs plotting end to go ; setting of random numbers for probabilities let reader-comes random 101 let reader-leaves random 101 let writer-comes random 101 let writer-leaves random 101 if ( probability-writer-comes > writer-comes ) ; if true, the writer is created [ ; creating of writer create-writers 1 [ set color red set size 2 set shape "pencil" setxy door-x door-y set number-of-writers number-of-writers + 1 ; used for counting set total_number-of-writers total_number-of-writers + 1 ; used for counting ] ifelse ( mutex = 0 ) ; true if storage is empty [ ask writers with-min [ who ] ; choose the first writer in the queue ; move writer to the storage [ facexy storage-x storage-y while [ xcor <= storage-x ] [ fd 1 wait 0.05 ] setxy storage-x storage-y set size 4 set mutex 1 ] ] [ ask writers with-max [ who ] ; if the storage is non-empty, the writer is sent to the queue [ facexy -10 10 while [ ycor != 10 ] [ fd 1 wait 0.05 ] ] ] ] ; true if writer writes into the storage and if the writer should leave if ( probability-writer-leaves > writer-leaves and count writers with [ size = 4 ] > 0 ) [ ; writing writer finishes writing ask writers with-min [ who ] [ die ] set number-of-writers number-of-writers - 1 ; used for counting set mutex 0 ; alowing the storage for reading or writing ifelse ( count readers > 0 ) ; true if there are readers in the queue [ ; move all readers to the storage ask readers [ facexy storage-x storage-y while [ xcor <= storage-x ] [ fd 1 wait 0.05 ] setxy storage-x storage-y set size 4 set mutex 1 ; disalowing the storage for reading or writing ] ] [ ask writers with-min [ who ] ; choose the first writer in the queue ; move writer to the storage [ facexy storage-x storage-y while [ xcor <= storage-x ] [ fd 1 wait 0.05 ] setxy storage-x storage-y set size 4 set mutex 1 ; disalowing the storage for reading or writing ] ] ] if ( probability-reader-comes > reader-comes ) ; if true, the reader is created [ ; creating of reader create-readers 1 [ set color yellow set size 2 set shape "glasses" setxy door-x door-y set number-of-readers number-of-readers + 1 ; used for counting set total_number-of-readers total_number-of-readers + 1 ; used for counting ] ifelse ( count writers with [ size = 4 ] < 1) ; true if no writer writes into the storage [ ; move all readers to the storage ask readers [ facexy storage-x storage-y while [ xcor <= storage-x ] [ fd 1 wait 0.05 ] setxy storage-x storage-y set size 4 set mutex 1 ; disalowing the storage for reading or writing ] ] [ ask readers with-max [ who ] ; if the writer writes into the storage, the reader is sent to the queue [ facexy -10 -10 while [ ycor != -10 ] [ fd 1 wait 0.05 ] ] ] ] ; true if readers read from the storage and if the writer should leave if ( probability-reader-leaves > reader-leaves and count readers with [ size = 4 ] > 0) [ ; one of reader leave the storage ask one-of readers [ die ] set number-of-readers number-of-readers - 1 ; used for counting if ( count readers = 0 ) ; true if no readers are in the queue or reading [ set mutex 0 ; alowing the storage for reading or writing if ( count writers > 0) ; true if any writer is in the queue [ ; move writer into the storage ask writers with-min [ who ] [ facexy storage-x storage-y while [ xcor <= storage-x ] [ fd 1 wait 0.05 ] setxy storage-x storage-y set size 4 set mutex 1 ; disalowing the storage for reading or writing ] ] ] ] do-plots ; graphs plotting end to do-plots set-current-plot "Actual number of readers-writers" set-current-plot-pen "Readers" plot number-of-readers set-current-plot-pen "Writers" plot number-of-writers set-current-plot "Number of readers-writers" set-current-plot-pen "Readers" plot-pen-reset plotxy 0 number-of-readers set-current-plot-pen "Writers" plot-pen-reset plotxy 1 number-of-writers set-current-plot-pen "Total readers" plot-pen-reset plotxy 2 total_number-of-readers set-current-plot-pen "Total writers" plot-pen-reset plotxy 3 total_number-of-writers end @#$#@#$#@ GRAPHICS-WINDOW 322 26 761 486 16 16 13.0 1 10 1 1 1 0 0 0 1 -16 16 -16 16 0 0 1 ticks CC-WINDOW 5 606 770 701 Command Center 0 BUTTON 246 26 315 59 NIL setup NIL 1 T OBSERVER NIL NIL NIL NIL BUTTON 246 65 315 98 go once go NIL 1 T OBSERVER NIL NIL NIL NIL SLIDER 10 26 239 59 probability-reader-comes probability-reader-comes 0 100 50 1 1 NIL HORIZONTAL SLIDER 10 65 239 98 probability-reader-leaves probability-reader-leaves 0 100 50 1 1 NIL HORIZONTAL BUTTON 246 104 315 137 NIL go T 1 T OBSERVER NIL NIL NIL NIL MONITOR 10 182 121 227 Readers number-of-readers 17 1 11 SLIDER 10 104 239 137 probability-writer-comes probability-writer-comes 0 100 50 1 1 NIL HORIZONTAL SLIDER 10 143 239 176 probability-writer-leaves probability-writer-leaves 0 100 50 1 1 NIL HORIZONTAL MONITOR 128 182 239 227 Writers number-of-writers 17 1 11 MONITOR 10 233 121 278 Total readers total_number-of-readers\n 17 1 11 MONITOR 128 233 239 278 Total writers total_number-of-writers 17 1 11 PLOT 10 285 314 435 Actual number of readers-writers time count 0.0 10.0 0.0 10.0 true true PENS "Readers" 1.0 1 -1184463 true "Writers" 1.0 1 -2674135 true PLOT 10 442 314 592 Number of readers-writers readers - writers count 0.0 4.0 0.0 10.0 true true PENS "Readers" 1.0 1 -1184463 true "Writers" 1.0 1 -2674135 true "Total readers" 1.0 1 -13345367 true "Total writers" 1.0 1 -10899396 true @#$#@#$#@ WHAT IS IT? ----------- The problem of Readers and Writers describes a computing problem with concurrency. This problem was described for the first time by P. J. Courtois et al. in their work from 1971. Suppose we have a shared document. There are two types of agents in this model - readers, who access the document only for reading and writers, who access the document for modification. The problem is with the agents accessing this document at the same time. HOW IT WORKS ------------ FIRST TYPE OF R/W PROBLEM - readers-preference: There are several rules for this model: - only one type of agent can access the document at the same time - only one writer can modify the document - if the document is locked for reading, an unlimited number of readers can access the document The main rule of this model is that no reader shall be kept waiting if the document is currently opened for reading. HOW TO USE IT ------------- Initial settings: - probability-reader-comes: Interval (0-100) - what is the probability that the next agent, who wants to access the document, will be a reader? - probability-reader-leaves: Interval (0-100) - probability, that the reader will finish his action ("how long he reads") - probability-writer-comes: Interval (0-100) - what is the probability that the next agent who wants to access the document will be a writer? - probability-writer-leaves: Interval (0-100) - probability that the writer will finish his action ("how long he writes") Plots: - Readers - how many readers are actually in the system? - Writers - how many readers are actually in the system? - Total readers - total number of processed readers plus number of readers currently in the system. - Total writers - total number of processed writers plus number of writers currently in the system. - Actual number of readers-writers: chart showing number of agents in the system.. - Number of readers-writers: chart showing number of agents in the system and total number of processed agents THINGS TO NOTICE ---------------- From the number of writers in the system (unprocessed writers) you can see that in the first readers-writers problem the writers are starving. THINGS TO TRY ------------- Try moving the sliders. For example, if you set the "probability-reader-comes" to a very low value and "probability-reader-leaves" to a very high value, the writers won't starve. @#$#@#$#@ default true 0 Polygon -7500403 true true 150 5 40 250 150 205 260 250 airplane true 0 Polygon -7500403 true true 150 0 135 15 120 60 120 105 15 165 15 195 120 180 135 240 105 270 120 285 150 270 180 285 210 270 165 240 180 180 285 195 285 165 180 105 180 60 165 15 arrow true 0 Polygon -7500403 true true 150 0 0 150 105 150 105 293 195 293 195 150 300 150 book false 0 Polygon -7500403 true true 30 195 150 255 270 135 150 75 Polygon -7500403 true true 30 135 150 195 270 75 150 15 Polygon -7500403 true true 30 135 30 195 90 150 Polygon -1 true false 39 139 39 184 151 239 156 199 Polygon -1 true false 151 239 254 135 254 90 151 197 Line -7500403 true 150 196 150 247 Line -7500403 true 43 159 138 207 Line -7500403 true 43 174 138 222 Line -7500403 true 153 206 248 113 Line -7500403 true 153 221 248 128 Polygon -1 true false 159 52 144 67 204 97 219 82 box false 0 Polygon -7500403 true true 150 285 285 225 285 75 150 135 Polygon -7500403 true true 150 135 15 75 150 15 285 75 Polygon -7500403 true true 15 75 15 225 150 285 150 135 Line -16777216 false 150 285 150 135 Line -16777216 false 150 135 15 75 Line -16777216 false 150 135 285 75 bug true 0 Circle -7500403 true true 96 182 108 Circle -7500403 true true 110 127 80 Circle -7500403 true true 110 75 80 Line -7500403 true 150 100 80 30 Line -7500403 true 150 100 220 30 butterfly true 0 Polygon -7500403 true true 150 165 209 199 225 225 225 255 195 270 165 255 150 240 Polygon -7500403 true true 150 165 89 198 75 225 75 255 105 270 135 255 150 240 Polygon -7500403 true true 139 148 100 105 55 90 25 90 10 105 10 135 25 180 40 195 85 194 139 163 Polygon -7500403 true true 162 150 200 105 245 90 275 90 290 105 290 135 275 180 260 195 215 195 162 165 Polygon -16777216 true false 150 255 135 225 120 150 135 120 150 105 165 120 180 150 165 225 Circle -16777216 true false 135 90 30 Line -16777216 false 150 105 195 60 Line -16777216 false 150 105 105 60 car false 0 Polygon -7500403 true true 300 180 279 164 261 144 240 135 226 132 213 106 203 84 185 63 159 50 135 50 75 60 0 150 0 165 0 225 300 225 300 180 Circle -16777216 true false 180 180 90 Circle -16777216 true false 30 180 90 Polygon -16777216 true false 162 80 132 78 134 135 209 135 194 105 189 96 180 89 Circle -7500403 true true 47 195 58 Circle -7500403 true true 195 195 58 circle false 0 Circle -7500403 true true 0 0 300 circle 2 false 0 Circle -7500403 true true 0 0 300 Circle -16777216 true false 30 30 240 cow false 0 Polygon -7500403 true true 200 193 197 249 179 249 177 196 166 187 140 189 93 191 78 179 72 211 49 209 48 181 37 149 25 120 25 89 45 72 103 84 179 75 198 76 252 64 272 81 293 103 285 121 255 121 242 118 224 167 Polygon -7500403 true true 73 210 86 251 62 249 48 208 Polygon -7500403 true true 25 114 16 195 9 204 23 213 25 200 39 123 cylinder false 0 Circle -7500403 true true 0 0 300 door false 0 Rectangle -7500403 true true 75 90 225 285 Circle -7500403 true true 75 15 150 Circle -16777216 true false 105 45 90 Line -16777216 false 105 150 105 255 Line -16777216 false 135 150 135 255 Line -16777216 false 165 150 165 255 Line -16777216 false 195 150 195 255 Polygon -16777216 true false 180 165 210 165 210 180 195 180 180 180 180 165 210 165 195 165 180 180 195 165 dot false 0 Circle -7500403 true true 90 90 120 face happy false 0 Circle -7500403 true true 8 8 285 Circle -16777216 true false 60 75 60 Circle -16777216 true false 180 75 60 Polygon -16777216 true false 150 255 90 239 62 213 47 191 67 179 90 203 109 218 150 225 192 218 210 203 227 181 251 194 236 217 212 240 face neutral false 0 Circle -7500403 true true 8 7 285 Circle -16777216 true false 60 75 60 Circle -16777216 true false 180 75 60 Rectangle -16777216 true false 60 195 240 225 face sad false 0 Circle -7500403 true true 8 8 285 Circle -16777216 true false 60 75 60 Circle -16777216 true false 180 75 60 Polygon -16777216 true false 150 168 90 184 62 210 47 232 67 244 90 220 109 205 150 198 192 205 210 220 227 242 251 229 236 206 212 183 fish false 0 Polygon -1 true false 44 131 21 87 15 86 0 120 15 150 0 180 13 214 20 212 45 166 Polygon -1 true false 135 195 119 235 95 218 76 210 46 204 60 165 Polygon -1 true false 75 45 83 77 71 103 86 114 166 78 135 60 Polygon -7500403 true true 30 136 151 77 226 81 280 119 292 146 292 160 287 170 270 195 195 210 151 212 30 166 Circle -16777216 true false 215 106 30 flag false 0 Rectangle -7500403 true true 60 15 75 300 Polygon -7500403 true true 90 150 270 90 90 30 Line -7500403 true 75 135 90 135 Line -7500403 true 75 45 90 45 flower false 0 Polygon -10899396 true false 135 120 165 165 180 210 180 240 150 300 165 300 195 240 195 195 165 135 Circle -7500403 true true 85 132 38 Circle -7500403 true true 130 147 38 Circle -7500403 true true 192 85 38 Circle -7500403 true true 85 40 38 Circle -7500403 true true 177 40 38 Circle -7500403 true true 177 132 38 Circle -7500403 true true 70 85 38 Circle -7500403 true true 130 25 38 Circle -7500403 true true 96 51 108 Circle -16777216 true false 113 68 74 Polygon -10899396 true false 189 233 219 188 249 173 279 188 234 218 Polygon -10899396 true false 180 255 150 210 105 210 75 240 135 240 glasses false 0 Circle -7500403 true true 45 45 90 Circle -7500403 true true 165 45 90 Rectangle -7500403 true true 120 60 180 75 Circle -16777216 true false 60 60 60 Circle -16777216 true false 180 60 60 Circle -1 true false 89 91 24 Circle -1 true false 208 91 24 Polygon -7500403 true true 45 90 60 90 60 240 45 240 house false 0 Rectangle -7500403 true true 45 120 255 285 Rectangle -16777216 true false 120 210 180 285 Polygon -7500403 true true 15 120 150 15 285 120 Line -16777216 false 30 120 270 120 leaf false 0 Polygon -7500403 true true 150 210 135 195 120 210 60 210 30 195 60 180 60 165 15 135 30 120 15 105 40 104 45 90 60 90 90 105 105 120 120 120 105 60 120 60 135 30 150 15 165 30 180 60 195 60 180 120 195 120 210 105 240 90 255 90 263 104 285 105 270 120 285 135 240 165 240 180 270 195 240 210 180 210 165 195 Polygon -7500403 true true 135 195 135 240 120 255 105 255 105 285 135 285 165 240 165 195 line true 0 Line -7500403 true 150 0 150 300 line half true 0 Line -7500403 true 150 0 150 150 paintbrush false 0 Polygon -1 true false 87 191 103 218 238 53 223 38 Polygon -13345367 true false 104 204 104 218 239 53 235 47 Polygon -7500403 true true 99 173 83 175 71 186 64 207 52 235 45 251 77 238 108 227 124 205 118 185 pencil false 0 Polygon -7500403 true true 255 60 255 90 105 240 90 225 Polygon -7500403 true true 60 195 75 210 240 45 210 45 Polygon -7500403 true true 90 195 105 210 255 60 240 45 Polygon -6459832 true false 90 195 60 195 45 255 105 240 105 210 Polygon -7500403 true true 45 255 74 248 75 240 60 225 51 225 Line -16777216 false 60 195 210 45 Line -16777216 false 210 45 240 45 Line -16777216 false 240 45 255 60 Line -16777216 false 255 60 255 90 Line -16777216 false 255 90 105 240 Line -16777216 false 105 240 105 210 Line -16777216 false 105 210 90 195 Line -16777216 false 90 195 60 195 Line -16777216 false 240 45 90 195 Line -16777216 false 255 60 105 210 pentagon false 0 Polygon -7500403 true true 150 15 15 120 60 285 240 285 285 120 person false 0 Circle -7500403 true true 110 5 80 Polygon -7500403 true true 105 90 120 195 90 285 105 300 135 300 150 225 165 300 195 300 210 285 180 195 195 90 Rectangle -7500403 true true 127 79 172 94 Polygon -7500403 true true 195 90 240 150 225 180 165 105 Polygon -7500403 true true 105 90 60 150 75 180 135 105 person soldier false 0 Rectangle -7500403 true true 127 79 172 94 Polygon -10899396 true false 105 90 60 195 90 210 135 105 Polygon -10899396 true false 195 90 240 195 210 210 165 105 Circle -7500403 true true 110 5 80 Polygon -10899396 true false 105 90 120 195 90 285 105 300 135 300 150 225 165 300 195 300 210 285 180 195 195 90 Polygon -6459832 true false 120 90 105 90 180 195 180 165 Line -6459832 false 109 105 139 105 Line -6459832 false 122 125 151 117 Line -6459832 false 137 143 159 134 Line -6459832 false 158 179 181 158 Line -6459832 false 146 160 169 146 Rectangle -6459832 true false 120 193 180 201 Polygon -6459832 true false 122 4 107 16 102 39 105 53 148 34 192 27 189 17 172 2 145 0 Polygon -16777216 true false 183 90 240 15 247 22 193 90 Rectangle -6459832 true false 114 187 128 208 Rectangle -6459832 true false 177 187 191 208 plant false 0 Rectangle -7500403 true true 135 90 165 300 Polygon -7500403 true true 135 255 90 210 45 195 75 255 135 285 Polygon -7500403 true true 165 255 210 210 255 195 225 255 165 285 Polygon -7500403 true true 135 180 90 135 45 120 75 180 135 210 Polygon -7500403 true true 165 180 165 210 225 180 255 120 210 135 Polygon -7500403 true true 135 105 90 60 45 45 75 105 135 135 Polygon -7500403 true true 165 105 165 135 225 105 255 45 210 60 Polygon -7500403 true true 135 90 120 45 150 15 180 45 165 90 square false 0 Rectangle -7500403 true true 30 30 270 270 square 2 false 0 Rectangle -7500403 true true 30 30 270 270 Rectangle -16777216 true false 60 60 240 240 star false 0 Polygon -7500403 true true 151 1 185 108 298 108 207 175 242 282 151 216 59 282 94 175 3 108 116 108 target false 0 Circle -7500403 true true 0 0 300 Circle -16777216 true false 30 30 240 Circle -7500403 true true 60 60 180 Circle -16777216 true false 90 90 120 Circle -7500403 true true 120 120 60 tree false 0 Circle -7500403 true true 118 3 94 Rectangle -6459832 true false 120 195 180 300 Circle -7500403 true true 65 21 108 Circle -7500403 true true 116 41 127 Circle -7500403 true true 45 90 120 Circle -7500403 true true 104 74 152 triangle false 0 Polygon -7500403 true true 150 30 15 255 285 255 triangle 2 false 0 Polygon -7500403 true true 150 30 15 255 285 255 Polygon -16777216 true false 151 99 225 223 75 224 truck false 0 Rectangle -7500403 true true 4 45 195 187 Polygon -7500403 true true 296 193 296 150 259 134 244 104 208 104 207 194 Rectangle -1 true false 195 60 195 105 Polygon -16777216 true false 238 112 252 141 219 141 218 112 Circle -16777216 true false 234 174 42 Rectangle -7500403 true true 181 185 214 194 Circle -16777216 true false 144 174 42 Circle -16777216 true false 24 174 42 Circle -7500403 false true 24 174 42 Circle -7500403 false true 144 174 42 Circle -7500403 false true 234 174 42 turtle true 0 Polygon -10899396 true false 215 204 240 233 246 254 228 266 215 252 193 210 Polygon -10899396 true false 195 90 225 75 245 75 260 89 269 108 261 124 240 105 225 105 210 105 Polygon -10899396 true false 105 90 75 75 55 75 40 89 31 108 39 124 60 105 75 105 90 105 Polygon -10899396 true false 132 85 134 64 107 51 108 17 150 2 192 18 192 52 169 65 172 87 Polygon -10899396 true false 85 204 60 233 54 254 72 266 85 252 107 210 Polygon -7500403 true true 119 75 179 75 209 101 224 135 220 225 175 261 128 261 81 224 74 135 88 99 wheel false 0 Circle -7500403 true true 3 3 294 Circle -16777216 true false 30 30 240 Line -7500403 true 150 285 150 15 Line -7500403 true 15 150 285 150 Circle -7500403 true true 120 120 60 Line -7500403 true 216 40 79 269 Line -7500403 true 40 84 269 221 Line -7500403 true 40 216 269 79 Line -7500403 true 84 40 221 269 x false 0 Polygon -7500403 true true 270 75 225 30 30 225 75 270 Polygon -7500403 true true 30 75 75 30 270 225 225 270 @#$#@#$#@ NetLogo 4.0.2 @#$#@#$#@ @#$#@#$#@ @#$#@#$#@ setup go total_number-of-readers - number-of-readers total_number-of-writers - number-of-writers setup go total_number-of-readers - number-of-readers total_number-of-writers - number-of-writers setup go total_number-of-readers - number-of-readers total_number-of-writers - number-of-writers @#$#@#$#@ @#$#@#$#@ default 0.0 -0.2 0 0.0 1.0 0.0 1 1.0 0.0 0.2 0 0.0 1.0 link direction true 0 Line -7500403 true 150 150 90 180 Line -7500403 true 150 150 210 180 @#$#@#$#@