Summary

The current "jmap -histo" command accepts a parallel=[N] option to indicate the number of parallel threads, which exposes too much details to user and makes spec hard to be clear and not to be ambiguous.

Propose to enable parallel heap inspection by default and remove "parallel=[N]" option. Instead introduce a new option "noparallel" to allow user to do heap inspection non-parallel.

Specification

The change of jmap help info looks like:

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 (takes precedence if both "live" and "all" are specified)
  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>
  gz=<number>  If specified, the heap dump is written in gzipped format using the given compression level.
               1 (recommended) is the fastest, 9 the strongest compression.

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

histo-options:
  live         count only live objects (takes precedence if both "live" and "all" are specified)
  all          count all objects in the heap (default if one of "live" or "all" is not specified)
  file=<file>  dump data to <file>
- parallel=<number>  parallel threads number for heap iteration: 
-                           parallel=0 default behavior, use predefined number of threads
-                           parallel=1 disable parallel heap iteration
-                           parallel=<N> use N threads for parallel heap iteration
+ noparallel   If specified, the heap is inspected serially.

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