Parallelize a Vector Map Function using Forking
pvec parellelizes the execution of a function on vector elements
by splitting the vector and submitting each part to one core. The
function must be a vectorized map, i.e.\sspaceit takes a vector input and
creates a vector output of exactly the same length as the input which
doesn't depend on the partition of the vector.
It relies on forking and hence is not available on Windows unless
mc.cores = 1.
vector to operate on
function to call on each part of the vector
any further arguments passed to
if set to
The number of cores to use, i.e.\sspaceat most how many child processes will be run simultaneously. Must be at least one, and at least two for parallel operation. The option is initialized from environment variable MC_CORES if set.
See the description of this argument in
FUN(x, ...) where
FUN is a
function that returns a vector of the same length as
FUN must also be pure (i.e., without side-effects)
since side-effects are not collected from the parallel processes. The
vector is split into nearly identically sized subvectors on which
FUN is run. Although it is in principle possible to use
functions that are not necessarily maps, the interpretation would be
case-specific as the splitting is in theory arbitrary (a warning is
given in such cases).
The major difference between
mclapply will run
FUN on each element separately
pvec assumes that
c(FUN(x), FUN(x)) is
FUN(x[1:2]) and thus will split into as many
FUN as there are cores (or elements, if fewer), each
handling a subset vector. This makes it more efficient than
mclapply but requires the above assumption on
mc.cores == 1 this evaluates
FUN(v, ...) in the
The result of the computation – in a successful case it should be of
the same length as
v. If an error occurred or the function was
not a map the result may be shorter or longer, and a warning is given.
Due to the nature of the parallelization, error handling does not
follow the usual rules since errors will be returned as strings and
results from killed child processes will show up simply as
non-existent data. Therefore it is the responsibility of the user to
check the length of the result to make sure it is of the correct size.
pvec raises a warning if that is the case since it does not
know whether such an outcome is intentional or not.
mcfork for the inadvisability of using this with
Simon Urbanek and R Core.
Derived from the \CRANpkgmulticore package.
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x <- pvec(1:1000, sqrt) stopifnot(all(x == sqrt(1:1000))) # One use is to convert date strings to unix time in large datasets # as that is a relatively slow operation. # So let's get some random dates first # (A small test only with 2 cores: set options("mc.cores") # and increase N for a larger-scale test.) N <- 1e5 dates <- sprintf('%04d-%02d-%02d', as.integer(2000+rnorm(N)), as.integer(runif(N, 1, 12)), as.integer(runif(N, 1, 28))) system.time(a <- as.POSIXct(dates)) # But specifying the format is faster system.time(a <- as.POSIXct(dates, format = "%Y-%m-%d")) # pvec ought to be faster, but system overhead can be high system.time(b <- pvec(dates, as.POSIXct, format = "%Y-%m-%d")) stopifnot(all(a == b)) # using mclapply for this would much slower because each value # will require a separate call to as.POSIXct() # as lapply(dates, as.POSIXct) does system.time(c <- unlist(mclapply(dates, as.POSIXct, format = "%Y-%m-%d"))) stopifnot(all(a == c))