HH.md

hh command

Extended version of jmap -histo command.

Warning: Heap histogram requires stop the world pause and some options require full GC for target process.

Usage

> java -jar sjk.jar --help hh
Usage: hh [options]
  Options:
       Default: false
        --dead
       Dead objects histogram
       Default: false
        --dead-young
       Histogram for sample of dead young objects
       Default: false
        --help

       Default: false
        --live
       Live objects histogram
       Default: false
    -p, --pid
       Process ID
       Default: 0
    -d, --sample-depth
       Used with --dead-young option. Specific time duration to collect young
       population.
       Default: 10000
    -n, --top-number
       Show only N top buckets
       Default: 2147483647
    -X, --verbose
       Enable detailed diagnostics
       Default: false

--dead option make two subsequent histograms (first for all objects then live only) and calculates difference.

--dead-young options forces full GC then wait for specified period of time (default 10 seconds) then produce --dead histogram.

Idea is following: after full GC there are no garbage in heap. So --dead histogram is showing only objects recenctly allocated (young garbage). There are two problems with that aprouch though. First, finalizer may prevent some objects to be collected in first full GC. Second, if young GC is performed between full GC and --dead histogram sampling, garbage population can be skewed. Command tracks young GC counter and warns if young GC was detected.