An Analysis of I/O and Syscalls in Critical Sections and Their Implications for Transactional Memory

Presented at The Second ACM SIGPLAN Workshop on Transactional Computing (TRANSACT 07), Portland, Oregon, August 16, 2007.
Transactional memory (TM) is a scalable and concurrent way to build atomic sections. One aspect of TM that remains unclear is how side-effecting operations – that is, those which cannot be transparently undone by a TM system – should be handled. This uncertainty poses a significant barrier to the general applicability and acceptance of TM. Further, the absence of transactional workloads makes it difficult to study this aspect In this paper, we characterize the usage of I/O, and in particular system calls, within critical sections in two large applications, exploring both the actions performed and the characteristics of the critical sections in which they are performed. Shared memory programs employing critical sections are the closest approximation available to transactional workloads, so using this characterization, we attempt to reason about how the behavior we observed relates to the previous proposals for handling side-effecting operations within transactions. We find that the large majority of syscalls performed within critical sections can be handled with a range of existing techniques in a way transparent to the application developer. We also find that while sideeffecting critical sections are rare, they tend to be quite longlasting, and that many of these critical sections perform their first syscall (and thus become side-effecting) relatively early in their execution. Finally, we show that while these longlived, side-effecting critical sections tend to execute concurrently with many critical sections on other threads, we observe little concurrency between side-effecting critical sections.