As Peter pointed out earlier, this is better addressed by disabling the Tcl/Tk event loop in forked processes.
Cheers, Simon On Feb 2, 2013, at 5:02 PM, Paul Johnson wrote: > Dear R-devel friends: > > I'm back to bother you again about the conflict between mclapply and > tcltk. I've been > monitoring several packages that want to use mclapply to parallelize > computations and > need to figure out what should be done. > > It appears tcltk cannot be safely unloaded, so the best we can do is > check for the presence of tcltk and stop if it is found before > mclapply() is used. > > I wish you would please review my suggestion below. Maybe checkForTcltk() > could be used in the parallel package. Otherwise, we are letting > people run with scissors. > > There's a warning about this in ?mclapply > > It is _strongly discouraged_ to use these functions in GUI or > embedded environments, because it leads to several processes > sharing the same GUI which will likely cause chaos (and possibly > crashes). Child processes should never use on-screen graphics > devices.(Warning in ?mclapply) > > Bug report: (https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=15040 ) > > By the way, what is "embedded environments" in ?mclapply > > ## I don't want to crash your system, but if you want to see a freeze-up: > ## change 0 to 1 and run this: > if (0){ > library(parallel) > library(tcltk) > example(mclapply) > } > > ## What are packagers supposed to do if they want to call mclapply? > ## It appears to me the best a package can do is scan for tcltk and > ## stop. Here's a function that does so. > > checkForTcltk <- function(){ > if ("tcltk" %in% loadedNamespaces()){ > stop("This function cannot be used because the R tcltk > package is loaded. It is necessary to Close R, and re-run > the program making sure that tcltk is never loaded before > this function is called.") > } > } > > ## put that to use. > MCLApply <- function(){ > if (!require(parallel)) stop ("parallel wouldn't load") > checkForTcltk() > example(mclapply) > } > > ## test that: > > checkForTcltk() > MCLApply() > > library(tcltk) > checkForTcltk() > > > ## Why can't tcltk just be unloaded? I don't understand, but it is a deep > ## problem. > > ## Consider the ominous warnings in R detach's help: > ## > ## "Unloading some namespaces has undesirable side effects: e.g. > ## unloading ‘grid’ closes all graphics devices, and on most systems > ## ‘tcltk’ cannot be reloaded once it has been unloaded and may crash > ## R if this is attempted." (Note: section of ?detach). > ## > ## To be fair, that does not say unloading tcltk is fatal, but > ## reloading it is fatal. And I've seen plenty of times when > ## unloading it is fatal. > > ## Example 1. Crash on library.dynam.unload() > detach("package:tcltk", unload = TRUE) > library.dynam.unload("tcltk", system.file(package = "tcltk")) > > ## Output > ## > library.dynam.unload("tcltk", system.file(package = "tcltk")) > ## > > ## *** caught segfault *** > ## address 0x7f69c9d99580, cause 'memory not mapped' > > ## Possible actions: > ## 1: abort (with core dump, if enabled) > ## 2: normal R exit > ## 3: exit R without saving workspace > ## 4: exit R saving workspace > ## Selection: > ## Process R segmentation fault at Sat Feb 2 13:55:08 2013 > > > ## Example 2. > library(tcltk) > detach("package:tcltk", unload = TRUE) > library.dynam.unload("tcltk", system.file(package = "tcltk")) > example(mclapply) > > ## Output: > > ## > example(mclapply) > > ## *** caught segfault *** > ## address 0x7f25ccbfe000, cause 'memory not mapped' > > ## Possible actions: > ## 1: abort (with core dump, if enabled) > ## 2: normal R exit > ## 3: exit R without saving workspace > ## 4: exit R saving workspace > ## Selection: > > > pj > > -- > Paul E. Johnson > Professor, Political Science Assoc. Director > 1541 Lilac Lane, Room 504 Center for Research Methods > University of Kansas University of Kansas > http://pj.freefaculty.org http://quant.ku.edu > > ______________________________________________ > [email protected] mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > > ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
