c - Openmp parallel sections vs parallel for -


i have these 2 sections of code

#pragma omp parallel #pragma omp sections   { #pragma omp section     printf("h"); #pragma omp section     printf("e"); #pragma omp section     printf("l"); #pragma omp section     printf("l"); #pragma omp section     printf("o"); #pragma omp section     printf(" "); #pragma omp section     printf("w"); #pragma omp section     printf("o"); #pragma omp section     printf("r"); #pragma omp section     printf("l"); #pragma omp section     printf("d"); #pragma omp section     printf("!");   }

and

char word[] = "hello world!"; int n;  #pragma omp parallel   for(n=0; n<12; n++)   {   printf("%c", word[n]);   }

while first 1 prints hello world! second 1 prints hello world! , prints helld!lo wor

why first 1 seems deterministic , other not?

first of all, official gcc 4.1.2 does not support openmp. have redhat-derived linux distribution (rhel, fedora, centos, scientific linux, etc.) has openmp support backported gcc 4.1.2 newer version. rh used maintain backport quite time until switched newer gcc version.

writing shared stream results in non-deterministic behaviour in both openmp sections , parallel loops. observe here result of dynamic scheduling nature of sections implementation in gcc. libgomp (gcc openmp runtime) distributes sections on first-come first-served basis among threads in team. happens in case sections short in size , therefore in execution time, first thread exit docking barrier @ beginning of parallel region consumes work items before other threads have caught up, resulting in serial execution of sections.

as parallel for loop, observe result of default loop scheduling in libgomp being static, i.e. 12 iterations evenly split among threads in linear fashion. guess there 6 threads in case (based on text segments scrambled output), thread 0 gets iterations 0 1, thread 1 gets iterations 2 3, , on. again, execution of iterations in each thread defined, there no guarantee in order threads execute.

note behaviour gcc-specific. openmp standard says:

the method of scheduling structured blocks among threads in team implementation defined.

for example, intel's compiler distributes sections in round-robin fashion, i.e. section n given thread n % num_threads, parallel for loop static scheduling , chunk size of 1 do.


Comments

Post a Comment

Popular posts from this blog

authentication - Mongodb revoke acccess to connect test database -

i have ec2 instance mongodb 3.2.0 installed. have user admin access. can connect user , perform operation remotely. 1 can connect database test remote if knows ip mongo 11.11.11.11. although not able make operations, want restrict access user credentials can connect server. first need create users , grant roles users, example: use admin db.createuser( { user: "myuseradmin", pwd: "abc123", roles: [ { role: "useradminanydatabase", db: "admin" } ] } ) then need start server --auth parameter mongod --auth more information here
Read more

r - Update two sets of radiobuttons reactively - shiny -

i have read ( how make choices in radiobuttons reactive in shiny? ) shows me how update radiobutton s in reactive way. however, when try , update 2 sets of buttons same data, 1 set renders. example: server: # create example data wafer <- rep(c(1:3), each=3) length <- c(1,1,2,1,1,1,3,5,1) width <- c(3,1,6,1,1,1,1,1,6) dd <- data.frame(wafer, length, width) shinyserver(function(input, output, session){ # create reactive dataframe store data values <- reactivevalues() values$df <- data.frame() # lengths , widths of wafer user input <- eventreactive(input$do, { subset(dd, wafer %in% input$wafer, select = length:width) }) # update reactive data frame lengths , widths have been selected user input observe({ if(!is.null(a())) { values$df <- rbind(isolate(values$df), a()) } }) output$wl <- renderdatatable({ a() }) # update radio buttons unique length , widths stored in values$df # ever "observe" put first in co...
Read more

ios - Realm over CoreData should I use NSFetchedResultController or a Dictionary? -

i'm working on app using core data , nsfetchedresultcontroller many reasons switch use realm. saw there "realm version" of nsfetchedresultcontroller on github wouldn't compatible current code using core data. the view controller displaying list of people same school, address book. list sublist of people studied in same city. so thinking make 1 unique request database retrieve list of people , filter locally list within 1 dictionary per school [string: anyobject] string section name within tableview , anyobject array of people. first solution, make nsfetchedresultcontroller each school predicate filtering location. delete actions etc.. handled delegates -> not compatible realm create dictionaries , update them each actions... -> works realm it's annoying code. any better solution? edit: i need clarify request: i'd write class inherit uitableviewcontroller. this class has list of people sorted in alphabetical order the tableview has ...
Read more