Browsing by Subject "Cache coherence"

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Open Access
Boosting performance of directory-based cache coherence protocols with coherence bypass at subpage granularity and a novel on-chip page table
(ACM, 2016- 05) Soltaniyeh, M.; Kadayıf, I.; Öztürk, Özcan
Chip multiprocessors (CMPs) require effective cache coher-ence protocols as well as fast virtual-To-physical address trans-lation 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 struc-ture. 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 consid-erably more private data blocks. Consequently, it reduces the percentage of blocks required to be tracked in the di-rectory 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 struc-ture 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) in-volvement in updating the maintenance bits belonging to subpages stored in page table entries, nullifying some por-tion of performance benefits of subpage level data classification. To overcome this, we propose a distributed on-chip page table as a our second contribution. © 2016 Copyright held by the owner/author(s).
Open Access
Classifying data blocks at subpage granularity with an on-chip page table to improve coherence in tiled CMPs
(Institute of Electrical and Electronics Engineers, 2018) Soltaniyeh, M.; Kadayif, I.; Öztürk, Özcan
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.