MicroJIT: a lightweight, just-in-time compiler to improve startup times.

The startup phase of an application represents a tiny fraction of the total runtime, but it is considered, nevertheless, a critical phase for both client and server environments. In the Java Virtual Machine (JVM), the Just-in-Time compiler (JIT) translates Java bytecode to native machine code to improve the performance of an application. We investigate whether using two different JIT compilers in the same JVM can improve startup time. A lightweight JIT system (i.e., MicroJIT) is integrated into the J9 JVM and performs an initial, low-optimized, but fast compilation while, at a later time, the standard JIT recompiles Java bytecodes with better, but more expensive, optimizations. Experimental results show that enabling MicroJIT, can reduce startup time in some configurations of the JVM by almost 19%, but with a cost in memory usage and, in some cases, a reduction in throughput performance.