How to compile OpenJDK 8 for embedded devices

Thomas Lorblanch├Ęs

16/10/2015

big coffee
little coffee

Setup (Debian and derivatives)

$ sudo apt-get install mercurial (1)
$ hg clone http://hg.openjdk.java.net/jdk8u/jdk8u60 (2)
$ cd jdk8u60
$ bash ./get_source.sh (3)
$ sudo apt-get install openjdk8-jdk (4)
$ sudo apt-get build-dep openjdk-8 (5)
  1. OpenJDK uses Mercurial for its source repositories

  2. Download root repository

  3. Download sub-repositories ("forest")

  4. Java 7 or 8 is required to compile OpenJDK 8

  5. Installation of build dependencies

Compilation of OpenJDK 8

  • Configuration:

$ bash ./configure --with-jvm-variants=server \
                   --with-debug-level=release \
                   --disable-debug-symbols --disable-zip-debug-info \
                   --enable-unlimited-crypto
  • Help on configuration options:

$ bash ./configure --help
  • Compilation:

$ make all

Initial assessment: 98.3Mo

jdk8 server

VM flavours

Server

default VM, best performances, biggest size.

Client

32bit VM optimised for client applications (startup time).

Minimal

JEP-148, 32bit VM, minimum footprint, perf. degradation < 5%.

Kernel

ancestor of Minimal (Java 6), Windows only, not maintained.

Zero

VM without assembly code, interpreter only.

Zero-Shark

Zero VM with JIT compiler based on LLVM.

Default sizes and performances of x86 VM

VMConfiguration option
--with-jvm-variants=
Size of libjvm.soSPECjvm2008 score

Server 64bits

server

15.5Mo

70.9 ops/m

Server 32bits

server

13.5Mo

62.9 ops/m

Client 32bits

client

8.4Mo

43.9 ops/m

Minimal 32bits

minimal1

5.8Mo

40.4 ops/m

Zero 32 bits

zero

7.0Mo

1.4 ops/m

Shark 32 bits

zeroshark

19.8Mo

16.8 ops/m

libjvm.so optimisation

  • Dynamic linking with C++ lib of the OS:

    • --with-stdc++lib=dynamic

    • Saving: -102ko

  • Compilation optimised for size:

    • --with-extra-cflags=-Os

    • Saving: -1.8Mo on libjvm.so (VM Minimal)

  • Stripping of binaries:

    • find . -exec strip --strip-unneeded {} \;

    • Saving: -1Mo on libjvm.so (VM Minimal)

Summary of the size optimisations of libjvm.so

VMDefault size of libjvm.so"Optimised" size

Server 64bits

15.5Mo

8.8Mo

Server 32bits

13.5Mo

7.1Mo

Client 32bits

8.4Mo

4.1Mo

Minimal 32bits

5.8Mo

2.8Mo

Zero 32 bits

7.0Mo

3.4Mo

Shark 32 bits

19.8Mo

14.5Mo

Security

debian

Use of Debian hardening flags
(cf. https://wiki.debian.org/Hardening).

--with-extra-cflags='-Os -D_FORTIFY_SOURCE=2
-fstack-protector-strong'

--with-extra-ldflags='-Xlinker -z -Xlinker relro'

Cost: +139ko (VM Minimal) without any measurable performance impact

duke wall

What about ARM?

Optimisation of rt.jar: compact profiles

New in Java 8.

Compact profiles are sort of a poor man’s module system.
— Mark Reinhold
compact profiles

Generation of compact profiles

$ make profiles
Type of JRESize of rt.jar

Full JRE

61.7Mo

Compact 3

30.5Mo

Compact 2

24.8Mo

Compact 1

13.9Mo

Compression of rt.jar

  • By default, rt.jar is not compressed!

$ zipinfo images/j2re-compact1-image/lib/rt.jar
[...]
4689 files, 13858994 bytes uncompressed, 13858994 bytes compressed: 0.0%
  • Compile with COMPRESS_JARS=true :

$ make all COMPRESS_JARS=true
$ make profiles COMPRESS_JARS=true
$ zipinfo images/j2re-compact1-image/lib/rt.jar
[...]
4689 files, 13858994 bytes uncompressed, 6574887 bytes compressed: 52.6%

Compressed rt.jar sizes

Type of JREWithout compressionWith compression

Full JRE

61.7Mo

30.1Mo

Compact 3

30.5Mo

14.9Mo

Compact 2

24.8Mo

12.0Mo

Compact 1

13.9Mo

7.1Mo

Drawback: class loading time / application startup time

Where are we now?

Minimal VM with compact 1 profile and compression.

jre8 compact1 jarcompress

The final bits

  • lib/ext (1.6Mo): optional packages

    • localedata.jar (1.1Mo): local data for java.text and java.util (43 languages!)

    • sunec.jar (35ko) + lib/{arch}/libsunec.so (180ko): elliptic curve crypto

    • sunjce_provider.jar (253ko) + lib/jce.jar (91ko) : (a)symmetric crypto (Impl+API)

    • sunpkcs11.jar (230ko) + lib/{arch}/libj2pkcs11.so (63ko): PKCS#11 (crypto tokens)

  • lib/jsse.jar (303ko): Java Secure Socket Extension (TLS/SSL)

What about Java 9?

55 modules!

modules
Figure 1. Modules Java 9 (source : Mark Reinhold, JFokus 2015)

New tools

  • jdeps (since Java 8): dependency analysis.

$ jdeps -profile helloworld.jar
helloworld.jar -> java.base (compact1)
   test (helloworld.jar)
      -> java.io                                            compact1
      -> java.lang                                          compact1
  • jlink (Java 9): creaton of custom JVM images.

Questions ?

duke