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    Boosting Performance of Directory-based Cache Coherence Protocols with Coherence Bypass at Subpage Granularity and A Novel On-chip Page Table
    (Assoc Computing Machinery, 2016) Soltaniyeh, Mohammadreza; Kadayif, Ismail; Ozturk, Ozcan
    Chip multiprocessors (CMPs) require effective cache coherence protocols as well as fast virtual-to-physical address translation mechanisms for high performance. Directory-based cache coherence protocols are the state-of-the-art approaches in many-core CMPs to keep the data blocks coherent at the last level private caches. However, the area overhead and high associativity requirement of the directory structures may not scale well with increasingly higher number of cores. As shown in some prior studies, a significant percentage of data blocks are accessed by only one core, therefore, it is not necessary to keep track of these in the directory structure. In this study, we have two major contributions. First, we show that compared to the classification of cache blocks at page granularity as done in some previous studies, data block classification at subpage level helps to detect considerably more private data blocks. Consequently, it reduces the percentage of blocks required to be tracked in the directory significantly compared to similar page level classification approaches. This, in turn, enables smaller directory caches with lower associativity to be used in CMPs without hurting performance, thereby helping the directory structure to scale gracefully with the increasing number of cores. Memory block classification at subpage level, however, may increase the frequency of the Operating System's (OS) involvement in updating the maintenance bits belonging to subpages stored in page table entries, nullifying some portion of performance benefits of subpage level data classification. To overcome this, we propose a distributed on-chip page table as a our second contribution.
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    Classifying Data Blocks at Subpage Granularity With an On-Chip Page Table to Improve Coherence in Tiled CMPs
    (IEEE-Inst Electrical Electronics Engineers Inc, 2018) Soltaniyeh, Mohammadreza; Kadayif, Ismail; Ozturk, Ozcan
    As shown in some prior studies, a significant percentage of data blocks accessed in parallel codes are private, and not keeping track of those blocks can improve the effectiveness of directory structures in Chip multiprocessors (CMPs). In this paper, we have two major contributions. First, we showed that compared to the classification of cache blocks at page granularity, data block classification (DBC) at subpage level helps to detect considerably more private data blocks. Based on this idea, we propose two different approaches for enhancing the effectiveness of directory caches in tiled CMPs. In the first approach, which is called quasi-dynamic subpage level DBC (QDBC), a data block is assumed to be private from the beginning of the program execution and stays private as long as the corresponding subpage is accessed by only one core. Our second approach, which is called dynamic subpage level DBC, turns a data block into private again after all blocks within the corresponding subpage are evicted from private cache hierarchy. Memory block classification at subpage level, however, may increase the frequency of the operating system involvement in updating the maintenance bits in page table entries. To overcome this, we propose, as a second contribution, a distributed table called as on-chip page table (o-CPT), which stores recently accessed page translations in the system. Our simulation results show that, compared to page level data classification, QDBC and DBC approaches relying on the o-CPT can detect significantly more private data blocks and considerably improve system performance.
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    Coherency Traffic Reduction in Manycore Systems
    (IEEE, 2022) Derebasoglu, Erdem; Kadayif, Ismail; Ozturk, Ozcan
    With the increasing number of cores in manycore accelerators and chip multiprocessors (CMPs), it gets more challenging to provide cache coherency efficiently. Although the snooping-based protocols are appropriate solutions to small-scale systems, they are inefficient for large systems because of the limited bandwidth. Therefore, large-scale manycores require directory-based solutions where a hardware structure called directory holds the information. This directory keeps track of all memory blocks and which cache stores a copy of these blocks. The directory sends messages only to caches that store relevant blocks and also coordinate simultaneous accesses to a cache block. As directory-based protocols scale to many cores, performance, network-on-chip (NoC) traffic, and bandwidth become major problems. In this paper, we present software mechanisms to improve the effectiveness of directory-based cache coherency in manycore and multicore systems with shared memory. In multithreaded applications, some of the data accesses do not disrupt cache coherency, but they still produce coherency messages among cores such as read-only (private) data. However, if data is accessed by at least two cores and at least one of them is a write operation, it is called shared data and requires cache coherency. In our proposed system, private data and shared data are determined at compile time, and cache coherency protocol only applies to shared data. We implement our approach in two stages. First, we use Andersen's static pointer analysis to analyze the program and mark its private instructions, i.e., instructions that load or store private data. Then, we use these analyses to decide if cache coherency protocol will be applied or not at runtime. Our simulation results on parallel benchmarks show that our approach reduces cycle count, dynamic random access memory (DRAM) accesses, and coherency traffic up to 13%.
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    Energy reduction in 3D NoCs through communication optimization
    (Springer Wien, 2015) Ozturk, Ozcan; Akturk, Ismail; Kadayif, Ismail; Tosun, Suleyman
    Network-on-Chip (NoC) architectures and three-dimensional (3D) integrated circuits have been introduced as attractive options for overcoming the barriers in interconnect scaling while increasing the number of cores. Combining these two approaches is expected to yield better performance and higher scalability. This paper explores the possibility of combining these two techniques in a heterogeneity aware fashion. Specifically, on a heterogeneous 3D NoC architecture, we explore how different types of processors can be optimally placed to minimize data access costs. Moreover, we select the optimal set of links with optimal voltage levels. The experimental results indicate significant savings in energy consumption across a wide range of values of our major simulation parameters.
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    Hardware/Software Approaches for Reducing the Process Variation Impact on Instruction Fetches
    (Assoc Computing Machinery, 2013) Kadayif, Ismail; Turkcan, Mahir; Kiziltepe, Seher; Ozturk, Ozcan
    As technology moves towards finer process geometries, it is becoming extremely difficult to control critical physical parameters such as channel length, gate oxide thickness, and dopant ion concentration. Variations in these parameters lead to dramatic variations in access latencies in Static Random Access Memory (SRAM) devices. This means that different lines of the same cache may have different access latencies. A simple solution to this problem is to adopt the worst-case latency paradigm. While this egalitarian cache management is simple, it may introduce significant performance overhead during instruction fetches when both address translation (instruction Translation Lookaside Buffer (TLB) access) and instruction cache access take place, making this solution infeasible for future high-performance processors. In this study, we first propose some hardware and software enhancements and then, based on those, investigate several techniques to mitigate the effect of process variation on the instruction fetch pipeline stage in modern processors. For address translation, we study an approach that performs the virtual-to-physical page translation once, then stores it in a special register, reusing it as long as the execution remains on the same instruction page. To handle varying access latencies across different instruction cache lines, we annotate the cache access latency of instructions within themselves to give the circuitry a hint about how long to wait for the next instruction to become available.