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A Split Cache Hierarchy for Enabling Data-oriented Optimizations
Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Computer Architecture and Computer Communication. (UART)
Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Computer Systems. Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Computer Architecture and Computer Communication. (UART)
Uppsala University, Disciplinary Domain of Science and Technology, Mathematics and Computer Science, Department of Information Technology, Computer Architecture and Computer Communication. (UART)
2017 (English)Conference paper (Refereed)
Abstract [en]

Today’s caches tightly couple data with metadata at the cache line granularity. One result of this is that they require a multitude of different solutions for finding data (associative way searches and level-by-level searches), evictions (coherent writebacks buffers and associative level-by-level searches) and coherence (directory indirections and associative levelby-level searches) due to the co-location of the data and its identifying metadata. This results in complex implementations with many corner cases, increased latency and energy, and limited flexibility for data optimizations.

We propose splitting the metadata and data into two separate cache hierarchies. The metadata hierarchy tracks the location of the data and makes the hierarchy more efficient and flexible as we need only keep the metadata coherent. This allows us to easily apply many different cache optimizations on the data hierarchy, including smart data placement, dynamic coherence, and direct accesses.

The new split cache hierarchy, Direct-to-Master (D2M), provides a unified mechanism for cache searching, eviction, and coherence, all without the need for indirections through a central directory, level-by level data movement and searches, and with no associative cache address tags for locating data. Optimizations such as near-side LLC slices and private/shared data classification can easily be built on top off D2M to further improve its efficiency. This approach delivers a 54% improvement in cache hierarchy EDP vs. a mobile processor and 40% vs. a server processor, reduces network traffic by an average of 65%, and is especially effective for workloads with large instruction footprints.

Place, publisher, year, edition, pages
2017.
National Category
Computer Systems Computer Science
Research subject
Computer Science
Identifiers
URN: urn:nbn:se:uu:diva-306368OAI: oai:DiVA.org:uu-306368DiVA: diva2:1040373
Conference
International Symposium on High Performance Computer Architecture (HPCA)
Projects
UART
Available from: 2016-10-27 Created: 2016-10-27 Last updated: 2016-10-27
In thesis
1. Hiding and Reducing Memory Latency: Energy-Efficient Pipeline and Memory System Techniques
Open this publication in new window or tab >>Hiding and Reducing Memory Latency: Energy-Efficient Pipeline and Memory System Techniques
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Memory accesses in modern processors are both far slower and vastly more energy-expensive than the actual computations. To improve performance, processors spend a significant amount of energy and resources trying to hide and reduce the memory latency. To hide the latency, processors use out-order-order execution to overlap memory accesses with independent work and aggressive speculative instruction scheduling to execute dependent instructions back-to-back. To reduce the latency, processors use several levels of caching that keep frequently used data closer to the processor. However, these optimizations are not for free. Out-of-order execution requires expensive processor resources, and speculative scheduling must re-execute instructions on incorrect speculations, and multi-level caching requires extra energy and latency to search the cache hierarchy. This thesis investigates several energy-efficient techniques for: 1) hiding the latency in the processor pipeline, and 2) reducing the latency in the memory hierarchy.

Much of the inefficiencies of hiding latency in the processor come from two sources. First, processors need several large and expensive structures to do out-of-order execution (instructions queue, register file, etc.). These resources are typically allocated in program order, effectively giving all instructions equal priority. To reduce the size of these expensive resources without hurting performance, we propose Long Term Parking (LTP). LTP parks non-critical instructions before they allocate resources, thereby making room for critical memory accessing instructions to continue and expose more memory-level parallelism. This enables us to save energy by shrinking the resources sizes without hurting performance. Second, when a load's data returns, the load's dependent instructions need to be scheduled and executed. To execute the dependent instructions back-to-back, the processor will speculatively schedule instructions before the processor knows if the input data will be available at execution time. To save energy, we investigate different scheduling techniques that reduce the number of re-executions due to misspeculation.

The inefficiencies of traditional memory hierarchies come from the need to do level-by-level searches to locate data. The search starts at the L1 cache, then proceeds level by level until the data is found, or determined not to be in any cache, at which point the processor has to fetch the data from main memory. This wastes time and energy for every level that is searched. To reduce the latency, we propose tracking the location of the data directly in a separate metadata hierarchy. This allows us to directly access the data without needing to search. The processor simply queries the metadata hierarchy for the location information about where the data is stored. Separating metadata into its own hierarchy brings a wide range of additional benefits, including flexibility in how we place data storages in the hierarchy, the ability to intelligently store data in the hierarchy, direct access to remote cores, and many other data-oriented optimizations that can leverage our precise knowledge of where data are located.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 70 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1450
National Category
Computer Systems Computer Science
Research subject
Computer Science
Identifiers
urn:nbn:se:uu:diva-306369 (URN)978-91-554-9744-6 (ISBN)
Public defence
2016-12-15, Gustavianum, Akademigatan 3, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2016-11-23 Created: 2016-10-27 Last updated: 2016-11-28

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