Add parallel heap iteration for jmap -histo

Summary

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

"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

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

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>