Real Java applications in software transactional memory

Abstract

Transactional Memory (TM) shows promise as a new concurrency control mechanism to replace lock-based synchronization. However, there have been few studies of TM systems with real applications, and the real-world benefits and barriers of TM remain unknown. In this paper, we present a detailed analysis of the behavior of real applications on a software transactional memory system. Based on this analysis, we aim to clarify what programming work is required to achieve reasonable performance in TM-based applications. We selected three existing Java applications: (1) HSQLDB, (2) the Geronimo application server, and (3) the GlassFish application server, because each application has a scalability problem caused by lock contentions. We identified the critical sections where lock contentions frequently occur, and modified the source code so that the critical sections are executed transactionally. However, this simple modification proved insufficient to achieve reasonable performance because of excessive data conflicts. We found that most of the data conflicts were caused by application-level optimizations such as reusing objects to reduce the memory usage. After modifying the source code to disable those optimizations, the TM-based applications showed higher or competitive performance compared to lock-based applications. Another finding is that the number of variables that actually cause data conflicts is much smaller than the number of variables that can be accessed in critical sections. This implies that the performance tuning of TM-based applications may be easier than that of lock-based applications where we need to take care of all of the variables that can be accessed in the critical sections.