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| Authored by: indyandy on Sunday, May 20 2012 @ 07:48 AM EDT |
For the eight-month period from 1-Jan-2007 to 31-Aug-2007
...<br>
<br>
I spent most of my time working on the Dalvik
VM (the Java-ish virtual machine used on the Android
platform).<br>
The VM was first made generally available within
Android in January, replaced our previous VM (JamVM) as the default in March,
and was sufficiently fast and trouble-free that we dropped support for JamVM in
July.<br>
<br>
Highlights:<br>
<br>
-
Reworked various pieces to maximize data-sharing between processes. For
example, the Dalvik classfiles are now uncompressed to cache files after being
byte-swapped and aligned, allowing direct access from C to shared read-only
memory-mapped pages. The VM' s "write once" data, such as
method names and signatures, are stored in a separate allocation area to
minimize the ''write'' in ''copy-on-write'' after a
fork().<br>
<br>
- Improved VM performance significantly. I
developed a "DEX optimizer" that rewrites bytecodes as the
classfiles are uncompressed, and created a low-overhead
"inline" native call mechanism. I removed debugger overhead
from the interpreter while retaining support for the debugger. I learned ARM
assembly and rewrote the JNI call bridge in it. I wrote several Java
performance tests to exercise the VM, and ran them frequently to measure
results.<br>
<br>
- Implemented bytecode verification. This
involves verifying that classes are constructed properly, and
"executing" every bit of code in every class to ensure that
"bad stuff" can't happen, e.g. passing an integer when an
obJect reference is expected.<br>
<br>
- Continued
development of core features. Most of the major items were in place, but we
still needed a lot of little things (uncaught exception handling, Java language
asserts, etc.) that are expected in a commercial
VM.<br>
<br>
- Improved support for Java debuggers.
IntelliJ is now fully supported, and all common JDWP command-line options are
handled.<br>
<br>
For the first half of the year we
supported both Dalvik VM and JamVM simultaneously (switchable at
runtime).<br>
I maintained JamVM until we dropped
it.<br>
<br>
<br>
Another major item was the
development of the Dalvik Debug Monitor system. Debugging a Java app on the
device required a complicated series of steps. The Dalvik Debug Monitor Server
(DDMS) monitors all Java processes running on a device, allows you to examine
thread and heap information, and provides pass-through ports for easy debugger
connections. I borrowed a book on the SWT toolkit to write the GUI interface,
and piggybacked the DDM messages on top of JDWP (the Java Debug Wire Protocol) .
(The stand-alone DDMS app has since been transformed by xav into a much nifter
Eclipse plug-in.)<br>
<br>
I used what I learned while
working on the Dalvik VM to significantly rewrite and expand our
"Efficient Java" document, which describes best practices for
Java development on small embedded systems.<br>
<br>
I
frequently worked with others to measure and resolve performance problems,
providing analysis or adding instrumentation to the
VM.<br>
<br>
<br>
<br>
- I write good
code. It does what it's supposed to, and is well commented. (The Dalvik VM
sources have extensive descriptions of ''why'' as well as ''what''.) I fix bugs
as they come up, even if they're not related to the feature I'm currently
working on.<br>
<br>
- I find and fill needs before their
absence becomes critical. Debugging Java apps on the device was difficult, so I
made it easy with DDMS. The tools were developed and stable before the team
shifted from the simulator to the device as the primary development
platform.<br>
<br>
- I pick up new technologies quickly. I
learned how to write GUI apps with the SWT toolkit so I could develop DDMS, and
I learned how to program in ARM assembly language to make a piece of the VM run
correctly (and
quickly!).<br>
<br>
<br>
<br>
Last
review, horowitz wrote:<br>
<br>
"I would like to
see Andy reach out more and offer his advice and guidance on system level
architecture, even in areas he is not responsible
for.''<br>
<br>
I've stayed pretty focused on the
VM.<br>
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