How to compile OpenJDK 8 for embedded devices

Thomas Lorblanchès

16/10/2015

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)

OpenJDK uses Mercurial for its source repositories
Download root repository
Download sub-repositories ("forest")
Java 7 or 8 is required to compile OpenJDK 8
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

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

VM Configuration option
--with-jvm-variants= Size of libjvm.so SPECjvm2008 score

VM	Configuration option `--with-jvm-variants=`	Size of libjvm.so	SPECjvm2008 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

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

VM	Default 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

Default 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

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

What about ARM?

ARMv8 (64 bits) :
- AArch64 port since March 2014
- Repository: http://hg.openjdk.java.net/aarch64-port/jdk8
- Merged inside jdk9 repository
ARMv6/7 (32 bits) :
- Zero / Shark
- Oracle Java SE Embedded 8 (proprietary license) : http://www.oracle.com/technetwork/java/embedded/embedded-se/

Optimisation of rt.jar: compact profiles

New in Java 8.

Compact profiles are sort of a poor man’s module system.

— Mark Reinhold

Generation of compact profiles

$ make profiles

Type of JRE	Size of rt.jar
Full JRE	61.7Mo
Compact 3	30.5Mo
Compact 2	24.8Mo
Compact 1	13.9Mo

Type of JRE

Size 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 JRE	Without compression	With 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

Type of JRE

Without compression

With 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.

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!

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 ?