statement each case arm was reduced in size by one instruction. Similar
optimizations were also done in other parts of the compiler.
-
\section{Interfacing with Java Code}
-Java source code can create a copy of the translated binary by
-instantiating the corresponding class, which extends {\tt Runtime}.
-Invoking the {\tt main()} method on this class is equivalent to
-calling the {\tt main()} function within the binary; the {\tt String}
-arguments to this function are copied into the binary's memory space
-and made available as {\tt **argv} and {\tt argc}.
+NestedVM has two primary ways of executing code, the interpreter, and the binary translators. Both the interpreter and the output from the binary translators sit on top of a Runtime class. This class provides the public interface to both the interpreter and the translated binaries.
-The translated binary communicates with the rest of the VM by
-executing MIPS {\tt SYSCALL} instructions, which are translated into
-invocations of the {\tt syscall()} method. This calls back to the
-native Java world, which can manipulate the binary's environment by
-reading and writing to its memory space, checking its exit status,
-pausing the VM, and restarting the VM.
+\subsection{The Runtime Class}
+The Runtime class does the work that the operating system usually does. Conceptually the Runtime class can be though of as the operating system and itÕs subclasses (translated binaries and the interpreter) the CPU. The Runtime fulfills 5 primary goals:
-\subsection{Virtualization}
+\begin{itemize}
-The {\tt Runtime} class implements the majority of the standard {\tt
-libc} syscalls, providing a complete interface to the filesystem,
-network socket library, time of day, (Brian: what else goes here?).
+\item Provides a consistent external interface - Because only Runtime exposes a public interface the method of actually executing the code (currently only translated binaries and the interpreter) can be changed without any code changes.
-\begin{itemize}
+\item Provide an easy to use interface - The interpreter and the output from the binary translators only know how to execute code. The Runtime class provides an easy to use interface to the code. It contains methods to pass arguments to the main() function, read and write from memory, and call individual functions in the binary.
-\item ability to provide the same interface to CNI code and
- NestedVMified code
-
-\item security advantages (chroot the {\tt fork()}ed process)
+\item Manage the processÕs memory - The Runtime class contains large int[] arrays that represent the processÕs entire memory space. Subclasses read and write to these arrays as required by the instructions they are executing. Subclasses can expend their memory space using the sbrk syscall.
+
+\item Provide access to the file system and streams - Subclasses access the file system through standard UNIX syscalls (read, write, open, etc). The Runtime manages the file descriptor table that maps UNIX file descriptors to Java RandomAccessFiles, InputStreams, OutputStreams, and sockets.
+
+\item Miscellaneous other syscalls - In additions to those mentioned above the Runtime class implements a variety of other syscalls (sleep, gettimeofday, getpagesize, sysconf, fcntl, etc).
\end{itemize}
+\subsection{Interacting with the Binary}
+
+Java source code can create a copy of the translated binary by instantiating the class generated by the binary translator or instantiating the interpreter. It can then interact with the process through the many facilities provided by the Runtime interface. Invoking the run() method of the Runtime interface will load the given arguments into the processÕs memory as invoke the binaries entry point (typically \_start() in crt0.o). This will pass control on to the main() function which will have the arguments passed to run() loaded into argv and argc.
+
+As the binary executes it often passes control back to the Runtime class through the MIPS {\tt SYSCALL} instruction. The interpreter and translated binaries invoke the {\tt syscall()} method of the Runtime class when the {\tt SYSCALL} instruction is executed. The Runtime class then can manipulate the processÕs environment (read and write to memory, modify the file descriptor table, etc) and interact with the rest of the JVM on behalf of the process (read and write to a file or stream, etc). There is even a syscall to pause the VM and temporarily return control to the caller.
+
+In addition to the interfaces provided by NestedVM, users can create their own interfaces between the MIPS and Java world. The Runtime provides a method called call() that will call a function by name in the MIPS binary. The call() method looks up the function name in the binaryÕs ELF symbol table and manipulating the stack and registers accordingly to execute the given function. This allows Java code to seamlessly invoke functions in the binary.
+
+{\footnotesize\begin{verbatim}
+// Java
+private Runtime rt = new MyBinary();
+public void foo(int n) {
+ for(int i=0;i<10;i++) {
+ int result = rt.call("do_work",i);
+ System.err.println("do_work(i) = " + result);
+ }
+}
+// C
+void do_work(int n) {
+ int i;
+ int ret=0;
+ for(i=0;i<n;i++) ret += i;
+ return n;
+}
+\end{verbatim}}
+
+The MIPS binaries can also invoke a special method of Runtime called callJava().When the MIPS binary invokes the {\tt CALL\_JAVA} syscall (usually done through the {\tt \_call\_java()} function provided by the NestedVM support library) the callJava() method in Runtime is invoked with the arguments passes to the syscall.
+
+{\footnotesize\begin{verbatim}
+// Java
+private Runtime rt = new MyBinary() {
+ pubilc int callJava(int a, int b, int c, int d) { System.err.println("Got " + a + " " + b);
+};
+public void foo() { rt.run(); }
+// C
+void main(int argc, char **argv) {
+ \_call\_java(1,2);
+}
+\end{verbatim}}
+
+These two methods can even be combined. MIPS can call Java through the CALL\_JAVA syscall, which can in turn invoke a MIPS function in the binary with the call() method.\r\r
+Users preferring a simpler communication mechanism can also use Java StreamÕs and file descriptors. Runtime provides a simple interface for mapping a Java Input or OutputStream to a File Descriptor.
+
+%Java source code can create a copy of the translated binary by
+%instantiating the corresponding class, which extends {\tt Runtime}.
+%Invoking the {\tt main()} method on this class is equivalent to
+%calling the {\tt main()} function within the binary; the {\tt String}
+%arguments to this function are copied into the binary's memory space
+%and made available as {\tt **argv} and {\tt argc}.
+
+%The translated binary communicates with the rest of the VM by
+%executing MIPS {\tt SYSCALL} instructions, which are translated into
+%invocations of the {\tt syscall()} method. This calls back to the
+%native Java world, which can manipulate the binary's environment by
+%reading and writing to its memory space, checking its exit status,
+%pausing the VM, and restarting the VM.
+
+
+%\subsection{Virtualization}
+
+%The {\tt Runtime} class implements the majority of the standard {\tt
+%libc} syscalls, providing a complete interface to the filesystem,
+%network socket library, time of day, (Brian: what else goes here?).
+
+%\begin{itemize}
+
+%\item ability to provide the same interface to CNI code and
+% NestedVMified code
+
+%\item security advantages (chroot the {\tt fork()}ed process)
+%
+%\end{itemize}
+
\section{Quantitative Performance}
\subsection{Optimizations}
-Brian, can you write something to go here? Just mention which
-optimizations helped and which ones hurt.
+Although NestedVM perfectly emulates a MIPS R2000 CPU its performance characteristics aren't anything like an actual MIPS R2000 CPU. GCC makes several optimizations that increase performance on an actually MIPS CPU but actually decrease performance when run through the NestedVM binary translator. Fortunately, GCC provides many options to customize its code generations and eliminate these optimizations. GCC also has optimization options that aren't helpful on a real MIPS CPU but are very helpful under NestedVM
+
+Adam, we should cite "Using the GNU Compiler Collection" somewhere in here.
\begin{itemize}
-\item {\tt trampoline}
-\item {\tt optimal method size}
-\item {\tt -msingle-float}
-\item {\tt -mmemcpy}
-\item {\tt fastmem}
-\item {\tt local vars for registers (useless)}
+
+\item {\tt -falign-functions}
+Normally a function's location in memory has no effect on its execution speed. However, in the NestedVM binary translator the .text segment is split up on a power of two boundary. If a function is unlucky enough to start near the end of one of these boundaries a performance critical part of the function could end up spanning two methods. There is a significant amount of overhead in switching between two methods so this must be avoided at all costs. By telling GCC to align all functions to the boundary that the .text segment is split on the chances of a critical part of a function spanning two methods is significantly reduced.
+
\item {\tt -fno-rename-registers}
-\item {\tt -ffast-math}
-\item {\tt -fno-trapping-math}
-\item {\tt -fsingle-precision-constant}
-\item {\tt -mfused-madd}
-\item {\tt -freg-struct-return}
-\item {\tt -freduce-all-givs}
-\item {\tt -fno-peephole}
-\item {\tt -fno-peephole2}
-\item {\tt -fmove-all-movables}
-\item {\tt -fno-sched-spec-load}
-\item {\tt -fno-sched-spec}
-\item {\tt -fno-schedule-insns}
-\item {\tt -fno-schedule-insns2}
+Some processors can better schedule code when registers aren't reused for two different purposes. By default GCC will try to use as many registers as possibly when it can. This excess use of registers just confuses JIT's trying to compile the output from the binary translator. All the JIT compilers we tested do much better with a few frequently used registers.
+
\item {\tt -fno-delayed-branch}
-\item {\tt -fno-function-cse}
-\item {\tt -ffunction-sections}
-\item {\tt -fdata-sections}
-\item {\tt array bounds checking}
-\item {\tt -falign-functions=n}
-\item {\tt -falign-labels=n}
-\item {\tt -falign-loops=n}
-\item {\tt -falign-jumps=n}
-\item {\tt -fno-function-cse}
+The MIPS CPU has a delay slot (see above). Earlier versions of NestedVM didn't efficiently emulate delay slots. This option causes GCC to avoid using delay slots for anything (a NOP is simply placed in the delay slot). This had a small performance benefit. However, recent versions of NestedVM emulate delay slots with no performance overhead so this options has little effect. Nonetheless, these delay slots provide no benefit under NestedVM either so they are avoided with this option.
+
+\item {\tt -fno-schedule-insns}
+Load operations in the MIPS ISA also have a delay slot. The results of a load operation are not available for use until one instruction later. Several other instructions also have similar delay slots. GCC tries to do useful work wile waiting for the results of one of these operations by default. However, this, like register renaming, tends to confuse JIT compilers. This option prevents GCC from going out of its way to take advantage of these delay slots and makes the code generated by NestedVM easier for JIT compilers to handle.
+
+\item {\tt -mmemcpy}
+GCC sometimes has to copy somewhat large areas of memory. The most common example of this is assigning one struct to another. Memory copying can be done far more efficiently in Java than under NestedVM. Calls to the memcpy libc function are treated specially by the binary translator. They are turned into calls to a memcpy method in Runtime. The {\tt -mmemcpy} option causes GCC to invoke libc's memcpy() function when it needs to copy a region of memory rather than generating its own memcpy code. This call in then turned into a call to this Java memcpy function which is significantly faster than the MIPS implementation.
+
+\item {\tt -ffunction-sections -fdata-sections}
+These two options are used in conjunction with the {\tt --gc-section} linker option. These three options cause the linker to aggressively discard unused functions and data sections. In some cases this leads to significantly smaller binaries.
+
+%\item {\tt trampoline}
+%\item {\tt optimal method size}
+%\item {\tt -msingle-float}
+%\item {\tt -mmemcpy}
+%\item {\tt fastmem}
+%\item {\tt local vars for registers (useless)}
+%\item {\tt -fno-rename-registers}
+%\item {\tt -ffast-math}
+%\item {\tt -fno-trapping-math}
+%\item {\tt -fsingle-precision-constant}
+%\item {\tt -mfused-madd}
+%\item {\tt -freg-struct-return}
+%\item {\tt -freduce-all-givs}
+%\item {\tt -fno-peephole}
+%\item {\tt -fno-peephole2}
+%\item {\tt -fmove-all-movables}
+%\item {\tt -fno-sched-spec-load}
+%\item {\tt -fno-sched-spec}
+%\item {\tt -fno-schedule-insns}
+%\item {\tt -fno-schedule-insns2}
+%\item {\tt -fno-delayed-branch}
+%\item {\tt -fno-function-cse}
+%\item {\tt -ffunction-sections}
+%\item {\tt -fdata-sections}
+%\item {\tt array bounds checking}
+%\item {\tt -falign-functions=n}
+%\item {\tt -falign-labels=n}
+%\item {\tt -falign-loops=n}
+%\item {\tt -falign-jumps=n}
+%\item {\tt -fno-function-cse}
\end{itemize}
\section{Future Directions}
projects / nestedvm.git / commitdiff