Transactifying Apache's cache module

Apache is a large-scale industrial multi-process and multithreaded application, which uses lock-based synchronization. We report on our experience in modifying Apache's cache module to employ transactional memory instead of locks, a process we refer to as transactification; we are not aware of any previous efforts to transactify legacy software of such a large scale. Along the way, we learned some valuable lessons about which tools one should use, which parts of the code one should transactify and which are better left untouched, as well as on the intricacy of commit handlers. We also stumbled across weaknesses of existing software transactional memory (STM) toolkits, leading us to identify desirable features they are currently lacking. Finally, we present performance results from running Apache on a 32-core machine, showing that, there are scenarios where the performance of the STM-based version is close to that of the lock-based version. These results suggest that there are applications for which the overhead of using a software-only implementation of transactional memory is insignificant.