Skip to main content

Java Ergonomics and JVM Flags

Java Virtual Machine can tune itself depending on the environment and this smart tuning is referred to as Ergonomics.

When tuning Java, it's important to know which values were used as default for Garbage collector, Heap Sizes, Runtime Compiler by Java Ergonomics

There are many JVM command line flags. So, how do we find out which JVM flags are used? It turns out that JVM has flags to print flags. :)

I tested following commands with Java 8.

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 15.04
Release: 15.04
Codename: vivid
$ uname -a
Linux isurup-ThinkPad-T530 3.19.0-26-generic #28-Ubuntu SMP Tue Aug 11 14:16:32 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux



Printing Command Line Flags


We can use "-XX:+PrintCommandLineFlags" to print the command line flags used by the JVM. This is a useful flag to see the values selected by Java Ergonomics.

$ java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=259964992 -XX:MaxHeapSize=4159439872 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC 
java version "1.8.0_60"
Java(TM) SE Runtime Environment (build 1.8.0_60-b27)
Java HotSpot(TM) 64-Bit Server VM (build 25.60-b23, mixed mode)


My laptop is detected as a server-class machine as I'm running Ubuntu 64-bit version. I have 16GB memory.

$ free 
             total       used       free     shared    buffers     cached
Mem:      16247812   13500596    2747216    1020860     172252    7987388
-/+ buffers/cache:    5340956   10906856
Swap:     18622460      42072   18580388


So, as explained in Java Ergonomics, we can see that the initial heap size is 1/64 of physical memory and maximum heap size is 1/4 of physical memory.

$ echo $(((16247812 * 1024) / 259964992))
64
$ echo $(((16247812 * 1024) / 4159439872))
4


You can also notice that the parallel garbage collector (-XX:+UseParallelGC) is selected by the JVM.

Printing Initial JVM Flags


$ java -XX:+PrintFlagsInitial -version


When printing the JVM flags, we can see there are several columns. Those columns are type, name, assignment operator, value and the flag type.

Use this command to see the default values.

Printing Final JVM Flags


See blog post on -XX:+PrintFlagsFinal to learn more details on each column.

$ java -XX:+PrintFlagsFinal -version


When printing final flags, we can see there are some flags with assignment operator ":=", which indicates that the flag values were modified (by manually or by Java Ergonomics)


$ java -XX:+PrintFlagsFinal -version | grep ':='



JVM Flag Types


All JVM Flags are categorized in to different types, which can be seen inside curly brackets in the -XX:+PrintFlagsInitial/-XX:+PrintFlagsFinal output.



$ java -XX:+UnlockDiagnosticVMOptions -XX:+UnlockExperimentalVMOptions -XX:+UnlockCommercialFeatures -XX:+PrintFlagsFinal -version  |  awk -F ' {2,}' '{print $4}' | sort -u 
java version "1.8.0_60"
Java(TM) SE Runtime Environment (build 1.8.0_60-b27)
Java HotSpot(TM) 64-Bit Server VM (build 25.60-b23, mixed mode)

{ARCH diagnostic}
{ARCH experimental}
{ARCH product}
{C1 diagnostic}
{C1 pd product}
{C1 product}
{C2 diagnostic}
{C2 experimental}
{C2 pd product}
{C2 product}
{commercial}
{diagnostic}
{experimental}
{lp64_product}
{manageable}
{pd product}
{product}
{product rw}


As mentioned in the blog post on -XX:+PrintFlagsFinal , we can find some details on these flag types from the JDK source file:
http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/share/vm/runtime/globals.hpp

Following are the meanings of above flag types

  • product - General Flags for JVM, which are officially supported.
  • product rw - Writable internal product flags
  • manageable - Writable external product flags.
  • diagnostic - These flags are not meant for JVM tuning or for product modes. Can be used for JVM debugging. Need to use -XX:+UnlockDiagnosticVMOptions
  • experimental - These flags are in support of features, which are not part of officially supported product. Need to use -XX:+UnlockExperimentalVMOptions
  • commercial - These flags are related to commercial features of the JVM. Need a license to use in production. Need to use -XX:+UnlockCommercialFeatures
  • C1 - Client JIT Compiler specific flags
  • C2 - Server JIT Compiler specific flags
  • pd - Platform dependent flags
  • lp64 - Flags for 64bit JVM
  • ARCH - Architecture (CPU: x86, sparc etc) dependent flags

JVM Flag Data Types



$ java -XX:+PrintFlagsFinal -version | awk '{if (NR!=1) {print $1}}' | sort -u
java version "1.8.0_60"
Java(TM) SE Runtime Environment (build 1.8.0_60-b27)
Java HotSpot(TM) 64-Bit Server VM (build 25.60-b23, mixed mode)
bool
ccstr
ccstrlist
double
intx
uint64_t
uintx


The data types for JVM Flags are intuitive.

  • bool - Boolean (true/false)
  • ccstr - String
  • ccstrlist - Represents String arguments, which can accumulate
  • double - Double
  • intx - Integer
  • uint64_t - Unsigned long
  • uintx - Unsigned integer


Summary


If you ever wanted to find out all possible JVM flags, the -XX:+PrintFlagsFinal flag is the solution for you. Java Ergonomics select the best values depending on the environment and it's important to be aware of the Java Ergonomics when tuning your application.


Labels:

Comments

Post a Comment

Popular posts from this blog

Finding how many processors

I wanted to find out the processor details in my laptop and I found out that there are several ways to check. For example, see The RedHat community discussion on  Figuring out CPUs and Sockets . In this blog post, I'm listing few commands to find out details about CPUs. I'm using Ubuntu in my Lenovo ThinkPad T530 laptop and following commands should be working any Linux system. Display information about CPU architecture $ lscpu Architecture: x86_64 CPU op-mode(s): 32-bit, 64-bit Byte Order: Little Endian CPU(s): 4 On-line CPU(s) list: 0-3 Thread(s) per core: 2 Core(s) per socket: 2 Socket(s): 1 NUMA node(s): 1 Vendor ID: GenuineIntel CPU family: 6 Model: 58 Model name: Intel(R) Core(TM) i7-3520M CPU @ 2.90GHz Stepping: 9 CPU MHz: 1199.988 CPU max MHz: 3600.0000 CPU min MHz: 1200.0000 BogoMIPS: 5787.1...
Post a Comment
Read more

Java Mission Control & Java Flight Recorder

Last year, I got two opportunities to talk about Java Mission Control & Java Flight Recorder. I first talked about " Using Java Mission Control & Java Flight Recorder " as an internal tech talk at WSO2 . I must thank Srinath for giving me that opportunity. After that, Prabath also invited me to do a talk at Java Colombo Meetup . Prabath, Thank you for inviting me and giving me the opportunity to talk at the Java Colombo Meetup! I'm also very excited to see that Marcus Hirt , the Team Lead for Java Mission Control has mentioned about the Java Colombo Meetup in his blog post: " My Favourite JMC Quotes ". It's so nice to see "Sri Lanka" was mentioned in his blog post! :) From Marcus' Blog Here are the slides used at the meetup. Java Colombo Meetup: Java Mission Control & Java Flight Recorder from Isuru Perera Marcus Hirt's blog posts really helped me to understand JMC & JFR concepts and his tutorials were very helpful...
Post a Comment
Read more

Flame Graphs with Java Flight Recordings

Flame Graphs Brendon D. Gregg , who is a computer performance analyst , has created  Flame Graphs to visualize stack traces in an interactive way. You must watch his talk at USENIX/LISA13 , titled Blazing Performance with Flame Graphs , which explains Flame Graphs in detail. There can be different types of flame graphs and I'm focusing on  CPU Flame Graphs  with Java in this blog post. Please look at the Flame Graphs Description  to understand the Flame Graph visualization. CPU Flame Graphs and Java Stack Traces As  Brendon  mentioned in his talk, understanding why CPUs are busy is very important when analyzing performance.  CPU Flame Graphs  is a good way to identify hot methods from sampled stack traces. In order to generate CPU Flame Graphs for Java Stack Traces , we need a way to get sample stack traces. Brendon has given examples to use jstack  and Google's lightweight-java-profiler . Please refer to his perl program on g...
Post a Comment
Read more