问题
When mclapply(X, FUN) encounters errors for some of the values of X, the errors propagate to some (but not all) of the other values of X:
require(parallel)
test <- function(x) if(x == 3) stop() else x
mclapply(1:3, test, mc.cores = 2)
#[[1]]
#[1] "Error in FUN(c(1L, 3L)[[2L]], ...[cut]
#
#[[2]]
#[1] 2
#
#[[3]]
#[1] "Error in FUN(c(1L, 3L)[[2L]], ... [cut]
#Warning message:
#In mclapply(1:3, test, mc.cores = 2) :
# scheduled core 1 encountered error in user code, all values of the job will be affected
How can I stop this happening?
回答1:
The trick is to set mc.preschedule = FALSE
mclapply(1:3, test, mc.cores = 2, mc.preschedule = FALSE)
#[[1]]
#[1] 1
#[[2]]
#[1] 2
#[[3]]
#[1] "Error in FUN(X[[nexti]], ...[cut]
#Warning message:
#In mclapply(1:3, test, mc.cores = 2, mc.preschedule = FALSE) :
# 1 function calls resulted in an error
This works because by default mclapply seems to divide X into mc.cores groups and applies a vectorized version of FUN to each group. As a result if any member of the group yields an error, all values in that group will yield the same error (but values in other groups are unaffected).
Setting mc.preschedule = FALSE has adverse effects and may make it impossible to reproduce a sequence of pseudo-random numbers where the same job always receives the same number in the sequence, see ?mcparallel under the heading Random numbers.
来源：https://stackoverflow.com/questions/18330274/an-error-in-one-job-contaminates-others-with-mclapply