Transcript Dalvik Virtual Machine Vs Java Virtual Machine

Arpit Jain
Mtech1
Outline
 Introduction
 Dalvik VM
 Java VM
 Examples
 Comparisons
 Experimental Evaluation
Virtual Machine
 Virtual machines (VMs) are commonly used to
distribute programs in an architecture-neutral format,
which can easily be interpreted or compiled.
Android OS
Android OS is a software stack consisting of
java app running on a java app framework on
top of java core library running on dalvik VM.
Android uses dalvik virtual machine. Compile
java code to dalvik executables and that is run
on dalvik VM.
Android Framework
Dalvik VM
 Dalvik is the virtual machine (VM) in Google's
Android operating system. It is the software that runs
the apps on Android devices.
 Programs are commonly written in Java and compiled
to byte code. They are then converted from Java
Virtual Machine-compatible .class files to Dalvikcompatible .dex (Dalvik Executable) files before
installation on a device
Dalvik VM
 Provides application portability.
 Runs optimised file format(.dex) and dalvik byte code.
 Minimal memory footprint.
DEX format
 A tool called dx is used to convert some (but not all)
Java .class files into the .dex format. Multiple classes
are included in a single .dex file. Duplicate strings and
other constants used in multiple class files are
included only once in the .dex output to conserve
space.
 An uncompressed .dex file is typically a few percent
smaller in size than a compressed .jar (Java Archive)
derived from the same .class files.
Dex file
 On the Android platform, Java source code is still
compiled into .class files. But after .class files are
generated, the “dx” tool is used to convert the .class
files into a .dex, or Dalvik Executable, file.
 Whereas a .class file contains only one class, a .dex file
contains multiple classes.
 It is the .dex file that is executed on the Dalvik VM.
The .dex file has been optimized for memory usage.
Zygote
 Since every application runs in its own instance of the
VM, VM instances must be able to start quickly when a
new application is launched and the memory footprint
of the VM must be minimal.
 Android uses a concept called the Zygote to enable
both sharing of code across VM instances and to
provide fast start up time of new VM instances.
Java VM
 A Java virtual machine (JVM) is a virtual machine that
can execute Java bytecode. It is the code execution
component of the Java software platform.
 A JVM is distributed along with a set of standard class
libraries that implement the Java application
programming interface (API). Appropriate APIs
bundled together with JVM form the Java Runtime
Environment.
 Java has always been marketed as “write once, run
anywhere.”
Java VM
 In standard Java environments, Java source code is
compiled into Java bytecode, which is stored within
.class files. The .class files are read by the JVM at
runtime.
 Each class in your Java code will result in one .class file.
This means that if you have, say, one .java source file
that contains one public class, one static inner class,
and three anonymous classes, the compilation process
(javac) will output 5 .class files.
Comparisons
Dalvik VM
Java VM
Register Based
Stack Based
Executes .dex file
Executes .class file
It uses shared, type-specific constant
pools as it’s primary mechanism for
conserving memory. Rather than store
these values throughout the class, they
are always referred to by their index in
the constant pool.
In the case of the .class file, each class
has its own private, heterogeneous
constant
pool.
Disadvantages of Dex
 By allowing for classes to share constants pools,
repetition of constant values is kept to a minimum.
The consequence of the minimal repetition is that
there are significantly more logical pointers or
references within a .dex file compared to a .class file.
 Simplicity of JVM implementation, ease of writing a
compiler back-end
STACK VERSUS REGISTERS
The cost of executing a VM instruction in an interpreter
consists of three components:
 Dispatching the instruction
 Accessing the operands
 Performing the computation
Dispatching instruction
 It involves fetching next VM instruction and jump to
the corresponding segment of interpreter code.
 Example: A=B+C can be
 ILOAD c, ILOAD b, IADD, ISTORE a(stack JVM)
 IADD a, b, c (virtual register Machine)
Operands Accessing
 Location of operands appears explicit in register code,
while in stack based operands are found relative to
stack pointer.
 So, average register instruction is longer than the
corresponding stack instruction.
 This is the reason why stack architecture is popular.
Translating Stack to Register
 In stack based JVM, local variable is accessed using an
index, and the operands stack is accessed using stack
pointer. So all variables are short lived.
 While Register based JVM considers both local
variables and operand stack as virtual register.
Contd...
Translating Stack to Register
 So, most of the stack-based JVM instructions are
translated to corresponding register based virtual
machine instruction with implicit operands translated
to explicit operand registers.
 Example:
Copy propagation
 In stack based JVM, operands are pushed from local
variables to operand stack before they can be used
and result again back to local variable.
 This causes redundancy in our register based JVM as
instructions can directly use local variables without
going through the stack. So we use forward and
backward copy propagation to eliminate redundancy.
Example
Instruction after translation for register based
JVM
After forward and backward copy propagation
Experimental Evaluation
 Studies shows that a register-based architecture
requires and average of 47% less executed VM
instructions than the stack based.
 On the other hand the register code is 25% larger than
the corresponding stack code but this increased cost of
fetching more VM instructions due to larger code size
involves only 1.07% extra real machine loads per VM
instruction which is negligible.
Contd...
References
 [1] Virtual Machine Showdown: Stack Versus Registers
by Yunhe Shi, David Gregg, Andrew Beatty
Department of Computer Science University of
Dublin, Trinity College Dublin 2, Ireland
 [2] J. Park and S. mook Moon. Optimistic register
coalescing, Mar. 30 1999.
 [3] The dalvik virtual machine architecture by David
Ehringer.