seqParallel {SeqArray} | R Documentation

## Apply Functions in Parallel

### Description

Applies a user-defined function in parallel.

### Usage

    seqParallel(cl=seqGetParallel(), gdsfile, FUN,
        split=c("by.variant", "by.sample", "none"), .combine="unlist",
        .selection.flag=FALSE, ...)
    seqParApply(cl=seqGetParallel(), x, FUN, load.balancing=TRUE, ...)

### Arguments

`cl` | `NULL` or `FALSE`: serial processing; `TRUE`: multicore processing (the maximum number of cores minor one); a numeric value: the number of cores to be used; a `cluster` object for parallel processing, created by the functions in the package `parallel`, like `makeCluster`; a `BiocParallelParam` object from the BiocParallel package. See details
`gdsfile` | a `SeqVarGDSClass` object, or `NULL`
`FUN` | the function to be applied, should be like `FUN(gdsfile, ...)` or `FUN(...)`
`split` | split the dataset by variant or sample according to multiple processes, or "none" for no split
`.combine` | define a fucntion for combining results from different processes; by default, `"unlist"` is used, to produce a vector which contains all the atomic components, via `unlist(..., recursive=FALSE)`; `"list"`, return a list of results created by child processes; `"none"`, no return; or a function, like "+".
`.selection.flag` | `TRUE` – passes a logical vector of selection to the second argument of `FUN(gdsfile, selection, ...)`
`x` | a vector (atomic or list), passed to `FUN`
`load.balancing` | if `TRUE`, call `clusterApplyLB` instead of `clusterApply`
`...` | optional arguments to `FUN`

### Details

When `cl` is `TRUE` or a numeric value, forking techniques are used to create a new child process as a copy of the current R process, see `?parallel::mcfork`. However, forking is not available on Windows, and `makeCluster` is called to make a cluster which will be deallocated after calling `FUN`.

It is strongly suggested to use `seqParallel` together with `seqParallelSetup`. `seqParallelSetup` could work around the problem of forking on Windows, without allocating clusters frequently.

The user-defined function could use two predefined variables `SeqArray:::process_count` and `SeqArray:::process_index` to tell the total number of cluster nodes and which cluster node being used.

`seqParallel(, gdsfile=NULL, FUN=..., split="none")` could be used to setup multiple streams of pseudo-random numbers, and see `nextRNGStream` or `nextRNGSubStream` in the package `parallel`.

### Value

A vector or list of values.

### Author(s)

Xiuwen Zheng

### See Also

`seqSetFilter`, `seqGetData`, `seqApply`, `seqParallelSetup`, `seqGetParallel`

### Examples

    library(parallel)

    # choose an appropriate cluster size or number of cores
    seqParallelSetup(2)

    # the GDS file
    (gds.fn <- seqExampleFileName("gds"))

    # display
    (gdsfile <- seqOpen(gds.fn))

    # the uniprocessor version
    afreq1 <- seqParallel(, gdsfile, FUN = function(f) {
            seqApply(f, "genotype", as.is="double",
                FUN=function(x) mean(x==0, na.rm=TRUE))
        }, split = "by.variant")

    length(afreq1)
    summary(afreq1)

    # run in parallel
    afreq2 <- seqParallel(, gdsfile, FUN = function(f) {
            seqApply(f, "genotype", as.is="double",
                FUN=function(x) mean(x==0, na.rm=TRUE))
        }, split = "by.variant")

    length(afreq2)
    summary(afreq2)

    # check
    length(afreq1)  # 1348
    all(afreq1 == afreq2)

    ################################################################
    # check -- variant splits

    seqParallel(, gdsfile, FUN = function(f) {
            v <- seqGetFilter(f)
            sum(v$variant.sel)
        }, split = "by.variant")
    # [1] 674 674

    ################################################################

    seqParallel(, NULL, FUN = function() {
            paste(SeqArray:::process_index, SeqArray:::process_count, sep=" / ")
        }, split = "none")

    ################################################################

    # close the GDS file
    seqClose(gdsfile)

    seqParallelSetup(FALSE)

[Package SeqArray version 1.22.6 Index]