OS Support for Virtualizing Hardware Transactional Memory

Transactional memory promises to simplify multithreaded programming. Hardware TM (HTM) implementations promise better performance by augmenting processors with transactional state. However, HTMs interact poorly with the operating system or virtual machine monitor. For example, they often do not tolerate OS actions that virtualize processors and memory, such as context switching and paging. Without support for these actions, an HTM may not execute programs correctly or guarantee forward progress. We investigate virtualizing transactional memory in the context of LogTM-SE. First, we describe an implementation of a kernel module in OpenSolaris that implements transactional virtualization and requires only 1120 lines of code. Second, we find that LogTMSE interacts poorly with virtual machine monitors due to a reliance on physical addresses. We propose an extension to LogTM-SE, called LogTM-VSE, that addresses these problems and improves context-switching performance. Third, through application tracing on real hardware and full system simulation, we show virtualizing transactions can be necessary for system stability and to support code that voluntarily context switches. However, we find that aborting a transaction is generally faster than virtualizing it, and hence preferable in some cases.