Non-volatile memory is expected to coexist with (or even displace) volatile DRAM for main memory in upcoming architectures. This has led to increasing interest in the problem of designing and specifying durable data structures that can recover from system crashes. Data structures may be designed to satisfy stricter or weaker durability guarantees to provide a balance between the strength of the provided guarantees and performance overhead. This paper proposes three novel implementations of a concurrent lock-free queue. These implementations illustrate algorithmic challenges in building persistent lock-free data structures with different levels of durability guarantees. In presenting these challenges, the proposed algorithmic designs, and the different durability guarantees, we hope to shed light on ways to build a wide variety of durable data structures. We implemented the various designs and compared their performance overhead to a simple queue design for standard (volatile) memory.

The growing gap between the speed of memory access and cache access has made cache misses an influential factor in program efficiency. Much effort has been spent recently on reducing the number of cache misses during program run. This effort includes wise rearranging of program code, cache-conscious data placement, and algorithmic modifications that improve the program cache behavior. In this work we investigate the complexity of finding the optimal placement of objects (or code) in the memory, in the sense that this placement reduces the cache misses to the minimum. We show that this problem is one of the toughest amongst the interesting algorithmic problems in computer science. In particular, suppose one is given a sequence of memory accesses and one has to place the data in the memory so as to minimize the number of cache misses for this sequence. We show that if P ≠ NP, then one cannot efficiently approximate the optimal solution even up to a very liberal approximation ratio. Thus, this problem joins the small family of extremely inapproximable optimization problems. The other two famous members in this family are minimum coloring and maximum clique.

Concurrent garbage collection is highly attractive for real-time systems, because offloading the collection effort from the executing threads allows faster response, allowing for extremely short deadlines at the microseconds level. Concurrent collectors also offer much better scalability over incremental collectors. The main problem with concurrent real-time collectors is their complexity. The first concurrent real-time garbage collector that can support fine synchronization, STOPLESS, has recently been presented by Pizlo et al. In this paper, we propose two additional (and different) algorithms for concurrent real-time garbage collection: CLOVER and CHICKEN. Both collectors obtain reduced complexity over the first collector STOPLESS, but need to trade a benefit for it. We study the algorithmic strengths and weaknesses of CLOVER and CHICKEN and compare them to STOPLESS. Finally, we have implemented all three collectors on the Bartok compiler and runtime for C# and we present measurements to compare their efficiency and responsiveness.

Reference counting is not naturally suitable for running on multiprocessors. The update of pointers and reference counts requires atomic and synchronized operations. We present a novel reference counting algorithm suitable for a multiprocessor that does not require any synchronized operation in its write barrier (not even a compare-and-swap type of synchronization). The algorithm is efficient and may compete with any tracing algorithm.We have implemented our algorithm on SUN's Java Virtual Machine 1.2.2 and ran it on a 4-way IBM Netfinity 8500R server with 550MHz Intel Pentium III Xeon and 2GB of physical memory. It turns out that our algorithm has an extremely low latency and throughput that is comparable to the mark and sweep algorithm used in the original JVM.

The queue data structure is fundamental and ubiquitous. Lockfree versions of the queue are well known. However, an important open question is whether practical wait-free queues exist. Until now, only versions with limited concurrency were proposed. In this paper we provide a design for a practical wait-free queue. Our construction is based on the highly efficient lock-free queue of Michael and Scott. To achieve wait-freedom, we employ a prioritybased helping scheme in which faster threads help the slower peers to complete their pending operations. We have implemented our scheme on multicore machines and present performance measurements comparing our implementation with that of Michael and Scott in several system configurations.