Algorithms that use hardware transactional memory (HTM) must provide a software-only fallback path to guarantee progress. The design of the fallback path can have a profound impact on performance. If the fallback path is allowed to run concurrently with hardware transactions, then hardware transactions must be instrumented, adding significant overhead. Otherwise, hardware transactions must wait for any processes on the fallback path, causing concurrency bottlenecks, or move to the fallback path. We introduce an approach that combines the best of both worlds. The key idea is to use three execution paths: an HTM fast path, an HTM middle path, and a software fallback path, such that the middle path can run concurrently with each of the other two. The fast path and fallback path do not run concurrently, so the fast path incurs no instrumentation overhead. Furthermore, fast path transactions can move to the middle path instead of waiting or moving to the software path. We demonstrate our approach by producing an accelerated version of the tree update template of , which can be used to implement fast lock-free data structures based on down-trees. We used the accelerated template to implement two lock-free trees: a binary search tree (BST), and an (a,b)-tree (a generalization of a B-tree). Experiments show that, with 72 concurrent processes, our accelerated (a,b)-tree performs between 4.0x and 4.2x as many operations per second as an implementation obtained using the original tree update template.
@cite_14: Transactional lock elision (TLE) is a well-known technique that exploits hardware transactional memory (HTM) to introduce concurrency into lock-based software. It achieves that by attempting to execute a critical section protected by a lock in an atomic hardware transaction, reverting to the lock if these attempts fail. One significant drawback of TLE is that it disables hardware speculation once there is a thread running under lock. In this paper we present two algorithms that rely on existing compiler support for transactional programs and allow threads to speculate concurrently on HTM along with a thread holding the lock. We demonstrate the benefit of our algorithms over TLE and other related approaches with an in-depth analysis of a number of benchmarks and a wide range of workloads, including an AVL tree-based micro-benchmark and ccTSA, a real sequence assembler application.
@cite_20: Hardware lock-elision HLE introduces concurrency into legacy lock-based code by optimistically executing critical sections in a fast-path as hardware transactions. Its main limitation is that in case of repeated aborts, it reverts to a fallback-path that acquires a serial lock. This fallback-path lacks hardware-software concurrency, because all fast-path hardware transactions abort and wait for the completion of the fallback. Software lock elision has no such limitation, but the overheads incurred are simply too high. We propose amalgamated lock-elision ALE, a novel lock-elision algorithm that provides hardware-software concurrency and efficiency: the fallback-path executes concurrently with fast-path hardware transactions, while the common-path fast-path reads incur no overheads and proceed without any instrumentation. The key idea in ALE is to use a sequence of fine-grained locks in the fallback-path to detect conflicts with the fast-path, and at the same time reduce the costs of these locks by executing the fallback-path as a series segments, where each segment is a dynamic length short hardware transaction. We implemented ALE into GCC and tested the new system on Intel Haswell 16-way chip that provides hardware transactions. We benchmarked linked-lists, hash-tables and red-black trees, as well as converting KyotoCacheDB to use ALE in GCC, and all show that ALE significantly outperforms HLE.
Different options for concurrency have recently begun to be explored in the context of TLE. Refined TLE @cite_14 and Amalgamated TLE @cite_20 both improve the concurrency of TLE when a process is on the fallback path by allowing HTM transactions to run concurrently with a on the fallback path. Both of these approaches still serialize processes on the fallback path. They also use locks, so they cannot be used to produce lock-free data structures.