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Reading and Using JStat to Read Java Heap Usage and Java GC

Date: (March 30, 2016)

Most Hadoop Components are made out of Java. Sometimes they won't have a straightforward way of telling you how much heap they're using, or if they don't have GC logs, they might already be experiencing Full GC's. Knowing how to read JStat values is useful if, for example, a high impacting component such as Kafka is behaving weirdly.

First off, to understand how GC's work, read up on understanding how java garbage collection works.

Next is to actually get GC data. You can do this by running jstat -gc <pid>. To get the java pid, you can run jps on the command line to list all the pid's of the java processes.

A caveat though, you must use the user that was used to launch the java process. For example, you can see which user through the ps command, ex. ps aux | grep kafka.

Example: I want to read the jstat values of the Kafka Broker running on my machine

user  $ ps aux | grep kafka
kafka   1002   0.0  0.2  2568016  17232   ??  S     7:58AM   0:00.33 java /kafka/path

# I see that the user "kafka" was used to launch this java process
user  $ sudo su kafka

kafka $ jps
1002 kafka
1001 jps

kafka $ jstat -gc 1002
 S0C    S1C    S0U    S1U      EC       EU        OC         OU       PC     PU    YGC     YGCT    FGC    FGCT     GCT
500.0 500.0 500.0  0.0   5000.0 5000.0  100000.0  100000.0  50000.0 2500.0  5000 500.0 500   100.500 100.500

The meaning of these values are taken from this article.

  • S0C Current survivor space 0 capacity (KB)
  • S1C Current survivor space 1 capacity (KB)
  • S0U Survivor space 0 utilization (KB)
  • S1U Survivor space 1 utilization (KB)
  • EC Current eden space capacity (KB)
  • EU Eden space utilization (KB)
  • OC Current old space capacity (KB)
  • OU Old space utilization (KB)
  • PC Current permanent space capacity (KB)
  • PU Permanent space utilization (KB)
  • YGC Number of young generation GC Events
  • YGCT Young generation garbage collection time
  • FGC Number of full GC events
  • FGCT Full garbage collection time
  • GCT Total garbage collection time

Reading Java Usage

Just add the following values:

OU + PU + EU + S0U + S1U = Total Utilization

Reading Java GC

I'm leaving the rest to the "Understanding Java Garbage Collection", but the only thing to watch out for are the following: FGC, FGCT and GCT. If they grow too quickly, somethings wrong with the Garbage Collection.