Deterministic execution of a multithreaded application guarantees the same output as long as the application runs with the same input parameters. Determinism helps a programmer to test and debug an application and to provide fault-tolerance in the systems based on replicas. Additionally, Transactional Memory (TM) greatly simplifies development of multithreaded applications where applications use transactions (instead of locks) as a concurrency control mechanism to synchronize accesses to shared memory. However, deterministic systems proposed so far are not TM-aware. They violate the main properties of TM (atomicity, consistency and isolation of transactions), and execute TM applications incorrectly. In this paper, we present DeTrans, a runtime system for deterministic execution of multithreaded TM applications. DeTrans executes TM applications deterministically, it executes nontransactional code serially in round-robin order, and transactional code in parallel. Also, we show how DeTrans works with both eager and lazy software TM. We compare DeTrans with Dthreads, a state-of-the-art deterministic execution system. Unlike Dthreads, DeTrans does not use memory protection hardware nor facilities of the underlying operating system (OS) to execute multithreaded applications deterministically. DeTrans uses properties of software TM to ensure deterministic execution. We evaluate DeTrans using the STAMP benchmark suite and we compare DeTrans and Dthreads performance costs. DeTrans incurs less overhead because threads execute in the same address space without any OS system calls overhead. According to our results, DeTrans is 3.99x, 3.39x, 2.44x faster on average than Dthreads for 2, 4 and 8 threads, respectively.

The simplicity of concurrent programming with Transactional Memory (TM) and its recent implementation in mainstream processors greatly motivates researchers and industry to investigate this field and propose new implementations and optimizations. However, there is still no standard C system library which a wide range of TM developers can adopt. TM application developers have been forced to avoid library calls inside of transactions or to execute them irrevocably (i.e. in serial order). In this paper, we present the first TM-aware system library, a complex software implementation integrated with TM principles and suited for software (STM), hardware (HTM) and hybrid TM (HyTM). The library we propose is derived from a modified lock-based implementation and can be used with the existing standard C API. In our work, we describe design challenges and code optimizations that would be specific to any TM-based system library or application. We argue about system call execution within transactions, highlighting the possibility of unexpected results from threads. For this reason we propose: (1) a mechanism for detecting conflicts over kernel data in user space, and (2) a new barrier to allow hybrid TM to be used effectively with system libraries. Our evaluation includes different TM implementations and the focus is on memory management and file operations since they are widely used in applications and require additional mechanisms for concurrent execution. We show the benefit we gain with our libc modifications providing parallel execution as much as possible. The library we propose shows high scalability when linked with STM and HTM. For file operations it shows on average a 1.1, 2.6 and 3.7x performance speedup for 8 cores using HyTM, STM and HTM, respectively (over a lock-based single-threaded execution). For a red-black tree it shows on average 3.14x performance speedup for 8 cores using STM (over a multi-read single-threaded execution).

Previous studies in software transactional memory mostly focused on reducing the overhead of transactional read and write operations. In this article, we introduce transaction coalescing , a profile-guided compiler optimization technique that attempts to reduce the overheads of starting and committing a transaction by merging two or more small transactions into one large transaction. We develop a profiling tool and a transaction coalescing heuristic to identify candidate transactions suitable for coalescing. We implement a compiler extension to automatically merge the candidate transactions at the compile time. We evaluate the effectiveness of our technique using the hash table micro-benchmark and the STAMP benchmark suite. Transaction coalescing improves the performance of the hash table significantly and the performance of Vacation and SSCA2 benchmarks by 19.4% and 36.4%, respectively, when running with 12 threads.