Lock-Based cache coherence protocol for chip multiprocessors

Chip multiprocessor (CMP) is replacing the superscalar processor due to its huge performance gains in terms of processor speed, scalability, power consumption and economical design. Since the CMP consists of multiple processor cores on a single chip usually with share cache resources, process synchronization is an important issue that needs to be dealt with. Synchronization is usually done by the operating system in case of shared memory multiprocessors (SMP). This work studies the effect of performing synchronization by the hardware through its integration with the cache coherence protocol. A novel cache coherence protocol, called Lock-based Cache Coherence Protocol (LCCP) was designed and its performance was compared with MESI cache coherence protocol. Experiments were performed by a functional multiprocessor simulator, MP_Simplesim, that was modified to do this work. A novel interconnection network was also designed and tested in terms of performance against the traditional bus approach by means of simulation

Library Cache Coherence

Directory-based cache coherence is a popular mechanism for chip multiprocessors and multicores. The directory protocol, however, requires multicast for invalidation messages and the collection of acknowledgement messages, which can be expensive in terms of latency and network traffic. Furthermore, the size of the directory increases with the number of cores. We present Library Cache Coherence (LCC), which requires neither broadcast/multicast for invalidations nor waiting for invalidation acknowledgements. A library is a set of timestamps that are used to auto-invalidate shared cache lines, and delay writes on the lines until all shared copies expire. The size of library is independent of the number of cores. By removing the complex invalidation process of directory-based cache coherence protocols, LCC generates fewer network messages. At the same time, LCC also allows reads on a cache block to take place while a write to the block is being delayed, without breaking sequential consistency. As a result, LCC has 1.85X less average memory latency than a MESI directory-based protocol on our set of benchmarks, even with a simple timestamp choosing algorithm; moreover, our experimental results on LCC with an ideal timestamp scheme (though not implementable) show the potential of further improvement for LCC with more sophisticated timestamp schemes

HWP: Hardware Support to Reconcile Cache Energy, Complexity, Performance and WCET Estimates in Multicore Real-Time Systems

High-performance processors have deployed multilevel cache (MLC) systems for decades. In the embedded real-time market, the use of MLC is also on the rise, with processors for future systems in space, railway, avionics and automotive already featuring two or more cache levels. One of the most critical elements for MLC is the write policy that not only affects several key metrics such as performance, WCET estimates, energy/power, and reliability, but also the design of complexity-prone cache coherence protocol and cache reliability solutions. In this paper we make an extensive analysis of existing write policies, namely write-through (WT) and write-back (WB). In the context of the real-time domain, we show that no write policy is superior for all metrics: WT simplifies the design of the coherence and reliability solutions at the cost of performance, WCET, and energy; while WB improves performance and energy results, but complicates cache design. To take the best of each policy, we propose Hybrid Write Policy (HWP) a low-complexity hardware mechanism that reconciles the benefits of WT in terms of simplifying the cache design (e.g. coherence solution) and the benefits of WB in improved average performance and WCET estimates as the pressure on the interconnection network increases. Guaranteed performance results show that HWP scales with core count similar to WB. Likewise, HWP reduces cache energy usage of WT, to levels similar to those of WB. These benefits are obtained while retaining the reduced coherence complexity of WT, in contrast to high coherence costs under WB

Moving the shared memory closer to the processors: DDM

Multiprocessors with shared memory are considered more general and easier to program than message-passing machines. The scalability is, however, in favor of the latter. There are a number of proposals showing how the poor scalability of shared memory multiprocessors can be improved by the introduction of private caches attached to the processors. These caches are kept consistent with each other by cache-coherence protocols. In this paper we introduce a new class of architectures called Cache Only Memory Architectures (COMA). These architectures provide the programming paradigm of the shared-memory architectures, but are believed to be more scal- able. COMAs have no physically shared memory; instead, the caches attached to the processors contain all the memory in the system, and their size is therefore large. A datum is allowed to be in any or many of the caches, and will automatically be moved to where it is needed by a cache-coherence protocol, which also ensures that the last copy of a datum is never lost. The location of a datum in the machine is completely decoupled from its address. We also introduce one example of COMA: the Data Diffusion Machine (DDM). The DDM is based on a hierarchical network structure, with processor/memory pairs at its tips. Remote accesses generally cause only a limited amount of traffic over a limited part of the machine. The architecture is scalable in that there can be any number of levels in the hierarchy, and that the root bus of the hierarchy can be implemented by several buses, increasing the bandwidth

Basis Token Consistency: A Practical Mechanism for Strong Web Cache Consistency

With web caching and cache-related services like CDNs and edge services playing an increasingly significant role in the modern internet, the problem of the weak consistency and coherence provisions in current web protocols is becoming increasingly significant and drawing the attention of the standards community [LCD01]. Toward this end, we present definitions of consistency and coherence for web-like environments, that is, distributed client-server information systems where the semantics of interactions with resource are more general than the read/write operations found in memory hierarchies and distributed file systems. We then present a brief review of proposed mechanisms which strengthen the consistency of caches in the web, focusing upon their conceptual contributions and their weaknesses in real-world practice. These insights motivate a new mechanism, which we call "Basis Token Consistency" or BTC; when implemented at the server, this mechanism allows any client (independent of the presence and conformity of any intermediaries) to maintain a self-consistent view of the server's state. This is accomplished by annotating responses with additional per-resource application information which allows client caches to recognize the obsolescence of currently cached entities and identify responses from other caches which are already stale in light of what has already been seen. The mechanism requires no deviation from the existing client-server communication model, and does not require servers to maintain any additional per-client state. We discuss how our mechanism could be integrated into a fragment-assembling Content Management System (CMS), and present a simulation-driven performance comparison between the BTC algorithm and the use of the Time-To-Live (TTL) heuristic.National Science Foundation (ANI-9986397, ANI-0095988

Locality-aware data replication in the Last-Level Cache

Next generation multicores will process massive data with varying degree of locality. Harnessing on-chip data locality to optimize the utilization of cache and network resources is of fundamental importance. We propose a locality-aware selective data replication protocol for the last-level cache (LLC). Our goal is to lower memory access latency and energy by replicating only high locality cache lines in the LLC slice of the requesting core, while simultaneously keeping the off-chip miss rate low. Our approach relies on low overhead yet highly accurate in-hardware run-time classification of data locality at the cache line granularity, and only allows replication for cache lines with high reuse. Furthermore, our classifier captures the LLC pressure at the existing replica locations and adapts its replication decision accordingly. The locality tracking mechanism is decoupled from the sharer tracking structures that cause scalability concerns in traditional coherence protocols. Moreover, the complexity of our protocol is low since no additional coherence states are created. On a set of parallel benchmarks, our protocol reduces the overall energy by 16%, 14%, 13% and 21% and the completion time by 4%, 9%, 6% and 13% when compared to the previously proposed Victim Replication, Adaptive Selective Replication, Reactive-NUCA and Static-NUCA LLC management schemes