OPTIMIZING THE JAVA VIRTUAL MACHINE INSTRUCTION SET BY ...

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provided by only two vendors. IBM’s Jikes RVM executed on the Pentium III and
Pentium 4 represent ‘compile only’ strategies that do not make use of an interpreter
at any time during execution. At the opposite end of the scale, Sun Java build
1.5.0 05-b05, interpreted mode executes the application using only an interpreter.
This is accomplished using the standard Sun virtual machine. However, the -X:int
command line argument is employed which disables the JIT compiler. The remaining
Sun virtual machines were executed in mixed mode, using both an interpreter and a
JIT compiler.
5.2.1 Performance Results
Performance results were generated by measuring the runtime of 20 distinct benchmarks
on the platforms outlined in Table 5.2. A total of 5 distinct test conditions
were considered. The baseline condition executed the original unmodified benchmarks
using the original, unmodified Java class library in order to establish the expected
runtime. All other conditions were reported as a percentage of this runtime.
The remaining four test conditions despecialized an increasingly large number of
the bytecodes listed in Table 5.1. These included the despecialization of 16, 32, 48 and
62 specialized bytecodes respectively. Because the bytecodes are listed in increasing
order of frequency, it was expected that the first despecializations would have little if
any impact on application runtime while later despecializations might show a decrease
in performance.
Each benchmark was executed at least six times in order to minimize the the
possibility that factors beyond the author’s control would skew the results. Such
factors include the behaviour of background processes, the process scheduler, memory
manager and disk controller on the test machines. After the tests were performed,
the highest and lowest result were discarded and the average of the four remaining
results was presented.
Figure 5.1 shows the average performance impact despecialization had on each individual
benchmark across all computing platforms. The y-axis indicates the relative
difference between execution of the unmodified benchmark and the performance of
each of the despecialization conditions. The x-axis provides a separate histogram for
each benchmark, showing the original runtime and the performance achieved after
despecializing 16, 32, 48 and 62 bytecodes, in the order shown in Table 5.1. Finally,
the right most histogram shows the average performance observed across all of
the benchmarks and configurations tested. This overall result shows that specialized