File-Based Sharing For Dynamically Compiled Code On Dalvik Virtual Machine

Abstract

Memory footprint is considered as an important design issue for embedded systems. Sharing dynamically compiled code among virtual machines can reduce memory footprint and recompilation overhead. On the other hand, sharing writable native code may cause security problems, due to support of native function call such as Java Native Interface (JNI). We propose a native-code sharing mechanism that ensures the security for Dalvik virtual machine (VM) on the Android platform. Dynamically generated code is saved in a file and is shared with memory mapping when other VMs need the same code. Protection is granted by controlling of file writing permissions. To improve the security, we implement a daemon process, named Query Agent, to control all accesses to the native code and maintain all the information of traces, which are the units of the compilation in the Dalvik VM. We implement our code sharing mechanism on Android version 2.1 system, and experiment on an arm-based system. We get 45% code-cache size reduction and 9% performance improvement from eliminating recompilation overhead.