Effective method for Java Lock Reservation for Java Virtual Machines that Have Cooperative Multithreading

Java programs often execute code (especially in the class library) that is synchronized but the synchronization is actually not required for most common programs that are not heavily multithreaded. Synchronized code usually requires special platform instructions to ensure memory coherency which are usually expensive and can significantly slow down the running program. Lock reservation is a known compiler optimization that aims to reduce the synchronization overhead in programs with low contention. When lock reservation is used, a thread reserves the object being synchronized on for itself unless the object is also wanted by another thread. Once an object is reserved, the reserving thread can enter and exit the synchronized region of code without executing the expensive memory coherency instructions. However, lock reservation can cause increased contention or higher locking overhead if an object is truly contended and the reservation has to be cancelled.

In this talk, we describe an effective technique for lock reservation in a Virtual Machine (VM) environment that implements a cooperative threading model (i.e. threads voluntarily give up control at well defined yield points in the code). We also describe static and run-time heuristics for evaluating lock reservation effectiveness, as well as ways to further optimize locked code regions that do not contain any yield points.

Greg Steffan

Last modified: Fri Aug 31 10:24:16 EDT 2007