{"id":2304,"date":"2015-04-21T00:21:49","date_gmt":"2015-04-20T23:21:49","guid":{"rendered":"http:\/\/roboblog.fatal-fury.de\/?p=2304"},"modified":"2015-04-21T12:05:27","modified_gmt":"2015-04-21T11:05:27","slug":"calling-multiple-random-generator-in-parallel-c11","status":"publish","type":"post","link":"http:\/\/roboblog.fatal-fury.de\/?p=2304","title":{"rendered":"Calling multiple random generator in parallel c++11"},"content":{"rendered":"

Standard random functions are not thread safe. Calling them in parallel may result in undefined behavior or the threads blocking each other. This is a little test to create the same random sequences in serial as in parallel too.<\/p>\n

Setting the same seed on a random generator as a start point should produce the same random sequence. First we create one generator and print the first three numbers. Restart it with different seed numbers.
\nThen we create three random generators with the same seeds and call them parallel. They should create the same sequences even if they are called interleaved.<\/p>\n

Program output:<\/p>\n

seriell
\nseed 1
\n48271
\n182605794
\n1291394886<\/p>\n

seed 2
\n96542
\n365211588
\n435306125<\/p>\n

seed 3
\n144813
\n547817382
\n1726701011<\/p>\n

parallel
\nThread 0 seed 1: 48271
\nThread 0 seed 1: 182605794
\nThread 0 seed 1: 1291394886
\nThread 1 seed 2: 96542
\nThread 1 seed 2: 365211588
\nThread 1 seed 2: 435306125
\nThread 2 seed 3: 144813
\nThread 2 seed 3: 547817382
\nThread 2 seed 3: 1726701011\n<\/p><\/blockquote>\n

Yep, work as expected :D
\nThe program looks not as simple as it might can be. But I think it is okay.<\/p>\n

void waitlong() {\r\n  long a = 0;\r\n  for(int i=0; i < 1000000; i++) {\r\n      a += i;\r\n  }\r\n}\r\n\r\nint main() {\r\n  \/\/ create some random number and store them in expectedResult\r\n  int expectedResult[3][3] = { 0 };\r\n  std::minstd_rand generator;\r\n\r\n  std::cout << \"seriell\\n\";\r\n\r\n  for(int id=0; id < 3; id++) {\r\n    generator.seed(id+1);\r\n    std::cout << \"seed \" << id+1 << \"\\n\";\r\n    for(int i=0; i < 3; i++) {\r\n      expectedResult[id][i] = generator();\r\n      std::cout << expectedResult[id][i] << \"\\n\";\r\n    }\r\n  }\r\n\r\n  \/\/ create three generators with the same seed and call them parallel\r\n  std::cout << \"\\nparallel\\n\";\r\n\r\n  std::minstd_rand generators[3];\r\n  generators[0].seed(1);\r\n  generators[1].seed(2);\r\n  generators[2].seed(3);\r\n\r\n  int result[3][3] = { 0 };\r\n  int count = 0;\r\n\r\n  \/\/ create 3 threads and interleave them with schedule(static, 1) num_threads(3)\r\n  \/\/ We can not call cout in a threaded enviroment. So we must store the random numbers\r\n  \/\/ in a array and print them laster. count and result is for this.\r\n#pragma omp parallel for firstprivate(count) shared(result) schedule(static, 1) num_threads(3)\r\n  for(int i=0; i < 3*3; i++) {\r\n    int id = omp_get_thread_num();\r\n    int rand = generators[id]();\r\n\r\n    \/\/ not error free even with omp critical\r\n\/\/     std::cout << \"Thread \" << id << \" seed \" << id+1 << \": \" << rand << \"\\n\";\r\n\r\n    result[id][count++] = rand;\r\n\r\n    \/\/ wait a litte bit to give the next thread a chance to start\r\n    waitlong();\r\n  }\r\n\r\n  \/\/ now print and compre the results\r\n  for(int id=0; id < 3; id++) {\r\n    for(int i=0; i < 3; i++) {\r\n      std::cout << \"Thread \" << id << \" seed \" << id+1 << \": \" << result[id][i] << \"\\n\";\r\n      if(result[id][i] != expectedResult[id][i]) {\r\n        std::cout << \"but expected \" << expectedResult[id][i] << \"\\n\";\r\n      }\r\n    }\r\n  }\r\n\r\n  std::cout << \"\\nValues per seed must be equal\\n\";\r\n  return 0;\r\n}\r\n<\/code><\/pre>\n","protected":false},"excerpt":{"rendered":"
Standard random functions are not thread safe. Calling them in parallel may result in undefined behavior or the threads blocking each other. This is a little test to create the same random sequences in serial as in parallel too. Setting the same seed on a random generator as a start point should produce the same […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[4],"tags":[17],"class_list":["post-2304","post","type-post","status-publish","format-standard","hentry","category-allgemein","tag-cpp"],"_links":{"self":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/2304","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=2304"}],"version-history":[{"count":7,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/2304\/revisions"}],"predecessor-version":[{"id":2314,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=\/wp\/v2\/posts\/2304\/revisions\/2314"}],"wp:attachment":[{"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2304"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2304"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/roboblog.fatal-fury.de\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2304"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}