使用并行的parLapply:无法访问并行代码中的变量

问题描述

我最近有一台具有多个内核的计算机，并且正在学习使用并行计算.我对lapply相当精通，并被告知parLapply的工作原理非常相似.我没有正确操作它.看来我必须明确地将所有内容放入parLapply中才能使其正常工作(即要使用的函数，变量等).使用lapply时，它将从父环境中读取，而parLapply似乎没有执行此操作.因此，在下面的示例中，我可以通过将所有信息放在parLapply中来使所有信息正常工作，但是如果在用户定义的函数中使用此信息，则无法明确地将text.var放在parLapply中.

I recently got a computer with several cores and am learning to use parallel computing. I'm fairly proficient with lapply and was told parLapply works very similarly. I'm not operating it correctly though. It seems I have to explicitly put everything inside the parLapply to make it work (that is functions to be use, variables etc.). With lapply it reads from the parent environment and parLapply does not seem to do this. So in my example below I could make everything work by placing all info inside parLapply but if I use this inside a user defined function I can't explicitly put text.var inside of parLapply.

library(parallel)
text.var <- rep("I like cake and ice cream so much!", 20)
ntv <- length(text.var)
gc.rate <- 10

pos <-  function(i) {
    paste(sapply(strsplit(tolower(i), " "), nchar), collapse=" | ")
}

lapply(seq_len(ntv), function(i) {
        x <- pos(text.var[i])
        if (i%%gc.rate==0) gc()
        return(x)
    }

)

#doesn't work
cl <- makeCluster(mc <- getOption("cl.cores", 4))
parLapply(cl, seq_len(ntv), function(i) {
        x <- pos(text.var[i])
        if (i%%gc.rate==0) gc()
        return(x)
    }

)

#does work but have to specify all the stuff inside parLapply
cl <- makeCluster(mc <- getOption("cl.cores", 4))
parLapply(cl, seq_len(ntv), function(i) {
        ######stuff I have to put inside parLapply##########
        text.var <- rep("I like cake and ice cream so much!", 20)
        ntv <- length(text.var)
        gc.rate <- 10
        pos <-  function(i) {
            paste(sapply(strsplit(tolower(i), " "), nchar), collapse=" | ")
        }
        ######stuff I have to put inside parLapply##########
        x <- pos(text.var[i])
        if (i%%gc.rate==0) gc()
        return(x)
    }
)

如何将text.var，ntv，gc.rate和pos传递给parLapply而不显式地将它们放入内部? (我想您会以某种方式将它们作为列表传递)

How can I pass text.var, ntv, gc.rate, and pos to parLapply without explicitly putting them inside? (I'm guessing you pass them as a list somehow)

PS Windows 7机器，所以我需要使用parLapply我认为

PS windows 7 machine so I need to use parLapply I think

推荐答案

您需要将这些变量导出到集群中的其他R进程:

You need to export those variables to the other R processes in the cluster:

cl <- makeCluster(mc <- getOption("cl.cores", 4))
clusterExport(cl=cl, varlist=c("text.var", "ntv", "gc.rate", "pos"))

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