Paxos is one of the most popular protocols for state machine replication (a technique used for making services highly available). We are the first to propose a Paxos-based state machine replication framework which is aimed at persistent (non-volatile) memory, pmem in short-a new class of memory offering direct byte-addressable access to memory (e.g., Optane ™ DC Persistent Memory). In the paper, we describe two variants of the framework, called mPaxosSM and mPaxos, which support efficient recovery of processes after crash with the use of pmem. In the latter variant, a part of Paxos's state, and in the former also the entire state machine's state that should survive crashes, are stored in the persistent memory. This allows to achieve low failure recovery time. We used a key-value map to compare our frameworks equipped with different memory backends (pmem, DRAM, and emulated pmem), with the classical Paxos that recovers state from snapshots and logs stored in stable storage, and with Paxos equipped with EpochSS-a state-of-the-art protocol ensuring state recovery from peer replicas. Our results show the advantages of pmem and our approach.

In this article, we propose and evaluate three different state recovery algorithms aimed for Paxos-one of the most popular distributed agreement protocols. Paxos is commonly used to maintain consistency among state machine replicas despite of failures of processes. The first algorithm, that we call FullSS, originates from the original Paxos and requires that the system frequently uses stable storage during regular (non-faulty) execution. The other two state recovery algorithms, ViewSS and EpochSS, scarcely require access to stable storage, and the recovering process must do much less work to restore its lost state, and to catch up on the current state of the system. We thoroughly analyze and compare the behavior of the three algorithms during state recovery and also during regular, non-faulty system execution, under various workloads (e.g., causing the network or CPU saturation). The experimental results show that by using ViewSS and EpochSS, we can significantly improve process recovery with respect to the original Paxos, if only it can be assumed that at any time a majority of replicas are up running (excluding those replicas that are just recovering). Moreover, these algorithms do not impact the performance of Paxos during regular (non-faulty) operation. However, FullSS is the only choice out of the three, if the system must tolerate catastrophic failures.

Transactional memory (TM) aims to be a general purpose concurrency mechanism. However, operations which cause side-effects cannot be easily managed by a TM system, in which transactions are executed optimistically. In particular, networking, I/O, and some system calls cannot be executed within a transaction that may abort and restart (e.g., due to conflicts). Thus, many TM systems let transactions become irrevocable, i.e., they are guaranteed to commit. Supporting this in TM is a challenge, but there exist fast and highly parallel TM systems that allow for irrevocable transactions. However, no such system so far provides guarantees that all transactional operations terminate in a finite time. In this paper, we show that support for irrevocable operations does not entail inherent waiting. We present a TM algorithm that guarantees wait-freedom for any transactional operation. The algorithm is based on the weakest synchronization primitive possible (test-and-set), and guarantees opacity and strong progressiveness. To experimentally evaluate the algorithm, we developed a proof-of-concept TM system and tested it using the STMBench7 benchmark.