Summary
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Add a new sub-command option "parallel" to the existing "histo" jmap command line option, which specifies how many threads to use to generate the heap histogram.

Problem
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"jmap -histo" inspects the heap serially, which takes a long time when the heap is large. Heap iteration time can be significantly reduced by parallel iteration over the object graph.

Solution
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Add a new sub-command option "parallel" to the existing "histo" jmap command line option, which specifies how many threads to use to generate the heap histogram.

Specification
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Updated specification.

    Usage:
    jmap -clstats <pid>
        to connect to running process and print class loader statistics
    jmap -finalizerinfo <pid>
        to connect to running process and print information on objects awaiting finalization
    jmap -histo[:[<histo-options>]] <pid>
        to connect to running process and print histogram of java object heap
    jmap -dump:<dump-options> <pid>
        to connect to running process and dump java heap
    jmap -? -h --help
        to print this help message

    dump-options:
      live         dump only live objects
      all          dump all objects in the heap (default if one of "live" or "all" is not specified
      format=b     binary format
      file=<file>  dump heap to <file>

    Example: jmap -dump:live,format=b,file=heap.bin <pid>

    histo-options:
      live             count only live objects
      all              count all objects in the heap (default if one of "live" or "all" is not specified)
      file=<file>      dump data to <file>
      parallel=<count> generate histogram using this many parallel threads, default 0
                         0 use system determined number of threads
                         1 use one thread, i.e., disable parallelism
                         n use n threads, n must be positive

    Example: jmap -histo:live,file=/tmp/histo.data,parallel=5 <pid>