The computing world today is in the middle of a revolution: mobile clients and cloud computing have emerged as the dominant paradigms driving programming and hardware innovation today. The Fifth Edition of Computer Architecture focuses on this dramatic shift, exploring the ways in which software and technology in the "cloud" are accessed by cell phones, tablets, laptops, and other mobile computing devices. Each chapter includes two real-world examples, one mobile and one datacenter, to illustrate this revolutionary change. Updated to cover the mobile computing revolutionEmphasizes the two most important topics in architecture today: memory hierarchy and parallelism in all its forms.Develops common themes throughout each chapter: power, performance, cost, dependability, protection, programming models, and emerging trends ("What's Next")Includes three review appendices in the printed text. Additional reference appendices are available online.Includes updated Case Studies and completely new exercises.

Computer Architecture, Fifth Edition: A Quantitative Approach

The performance tradeoff between hardware complexity and clock speed is studied. First, a generic superscalar pipeline is defined. Then the specific areas of register renaming, instruction window wakeup and selection logic, and operand bypassing are analyzed. Each is modeled and Spice simulated for feature sizes of 0.8µm, 0.35µm, and 0.18µm. Performance results and trends are expressed in terms of issue width and window size. Our analysis indicates that window wakeup and selection logic as well as operand bypass logic are likely to be the most critical in the future.A microarchitecture that simplifies wakeup and selection logic is proposed and discussed. This implementation puts chains of dependent instructions into queues, and issues instructions from multiple queues in parallel. Simulation shows little slowdown as compared with a completely flexible issue window when performance is measured in clock cycles. Furthermore, because only instructions at queue heads need to be awakened and selected, issue logic is simplified and the clock cycle is faster --- consequently overall performance is improved. By grouping dependent instructions together, the proposed microarchitecture will help minimize performance degradation due to slow bypasses in future wide-issue machines.

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Complexity-effective superscalar processors

Smaller feature sizes, reduced voltage levels, higher transistor counts, and reduced noise margins make future generations of microprocessors increasingly prone to transient hardware faults. Most commercial fault-tolerant computers use fully replicated hardware components to detect microprocessor faults. The components are lockstepped (cycle-by-cycle synchronized) to ensure that, in each cycle, they perform the same operation on the same inputs, producing the same outputs in the absence of faults. Unfortunately, for a given hardware budget, full replication reduces performance by statically partitioning resources among redundant operations.We demonstrate that a Simultaneous and Redundantly Threaded (SRT) processor—derived from a Simultaneous Multithreaded (SMT) processor—provides transient fault coverage with significantly higher performance. An SRT processor provides transient fault coverage by running identical copies of the same program simultaneously as independent threads. An SRT processor provides higher performance because it dynamically schedules its hardware resources among the redundant copies. However, dynamic scheduling makes it difficult to implement lockstepping, because corresponding instructions from redundant threads may not execute in the same cycle or in the same order.This paper makes four contributions to the design of SRT processors. First, we introduce the concept of the sphere of replication, which abstracts both the physical redundancy of a lockstepped system and the logical redundancy of an SRT processor. This framework aids in identifying the scope of fault coverage and the input and output values requiring special handling. Second, we identify two viable spheres of replication in an SRT processor, and show that one of them provides fault detection while checking only committed stores and uncached loads. Third, we identify the need for consistent replication of load values, and propose and evaluate two new mechanisms for satisfying this requirement. Finally, we propose and evaluate two mechanisms—slack fetch and branch outcome queue—that enhance the performance of an SRT processor by allowing one thread to prefetch cache misses and branch results for the other thread. Our results with 11 SPEC95 benchmarks show that an SRT processor can outperform an equivalently sized, on-chip, hardware-replicated solution by 16% on average, with a maximum benefit of up to 29%.

Transient fault detection via simultaneous multithreading

Is your memory hierarchy stopping your microprocessor from performing at the high level it should be? Memory Systems: Cache, DRAM, Disk shows you how to resolve this problem. The book tells you everything you need to know about the logical design and operation, physical design and operation, performance characteristics and resulting design trade-offs, and the energy consumption of modern memory hierarchies. You learn how to to tackle the challenging optimization problems that result from the side-effects that can appear at any point in the entire hierarchy.As a result you will be able to design and emulate the entire memory hierarchy. . Understand all levels of the system hierarchy -Xcache, DRAM, and disk. . Evaluate the system-level effects of all design choices. . Model performance and energy consumption for each component in the memory hierarchy.

Memory Systems: Cache, DRAM, Disk

In a chip-multiprocessor (CMP) system, the DRAM system isshared among cores. In a shared DRAM system, requests from athread can not only delay requests from other threads by causingbank/bus/row-buffer conflicts but they can also destroy other threads’DRAM-bank-level parallelism. Requests whose latencies would otherwisehave been overlapped could effectively become serialized. As aresult both fairness and system throughput degrade, and some threadscan starve for long time periods.This paper proposes a fundamentally new approach to designinga shared DRAM controller that provides quality of service to threads,while also improving system throughput. Our parallelism-aware batchscheduler (PAR-BS) design is based on two key ideas. First, PARBSprocesses DRAM requests in batches to provide fairness and toavoid starvation of requests. Second, to optimize system throughput,PAR-BS employs a parallelism-aware DRAM scheduling policythat aims to process requests from a thread in parallel in the DRAMbanks, thereby reducing the memory-related stall-time experienced bythe thread. PAR-BS seamlessly incorporates support for system-levelthread priorities and can provide different service levels, includingpurely opportunistic service, to threads with different priorities.We evaluate the design trade-offs involved in PAR-BS and compareit to four previously proposed DRAM scheduler designs on 4-, 8-, and16-core systems. Our evaluations show that, averaged over 100 4-coreworkloads, PAR-BS improves fairness by 1.11X and system throughputby 8.3% compared to the best previous scheduling technique, Stall-Time Fair Memory (STFM) scheduling. Based on simple request prioritizationrules, PAR-BS is also simpler to implement than STFM.

http://users.ece.cmu.edu/~omutlu/pub/parbs_isca08.pdf

Parallelism-Aware Batch Scheduling: Enhancing both Performance and Fairness of Shared DRAM Systems

Five solutions to the precise interrupt problem in pipelined processors are described and evaluated. An interrupt is precise if the saved process state corresponds to a sequential model of program execution in which one instruction completes before the next begins. In a pipelined processor, precise interrupts are difficult to implement because an instruction may be initiated before its predecessors have completed. The first solution forces instructions to complete and modify the process state in architectural order. The other four solutions allow instructions to complete in any order, but additional hardware is used, so that a precise state can be restored when an interrupt occurs. All the methods are discussed in the context of a parallel pipeline structure. Simulation results for the Cray-1S scalar architecture are used to show that the first solution results in a performance degradation of at least 16%. The remaining four solutions offer better performance, and three of them result in as little as a 3% performance loss. Several extensions, including vector architectures, virtual memory, and linear pipeline structures, are briefly discussed. >

Implementing precise interrupts in pipelined processors

An interrupt is precise if the saved process state corresponds with the sequential model of program execution where one instruction completes before the next begins. In a pipelined processor, precise interrupts are difficult to achieve because an instruction may be initiated before its predecessors have been completed. This paper describes and evaluates solutions to the precise interrupt problem in pipelined processors. The precise interrupt problem is first described. Then five solutions are discussed in detail. The first forces instructions to complete and modify the process state in architectural order. The other four allow instructions to complete in any order, but additional hardware is used so that a precise state can be restored when an interrupt occurs. All the methods are discussed in the context of a parallel pipeline struck sure. Simulation results based on the CRAY-1S scalar architecture are used to show that, at best, the first solution results in a performance degradation of about 16%. The remaining four solutions offer similar performance, and three of them result in as little as a 3% performance loss. Several extensions, including virtual memory and linear pipeline structures, are briefly discussed.

Implementation of precise interrupts in pipelined processors

Absm~-PIPE (Parallel Instructions and Pipelined Execution) is a research vehicle for stud)~ng high performance VLSI architectures and organizations. Its principal features are: 1) it is pipelined, 2) it makes extensive use of architectural queues, 3) it is capable of a decoupled mode of operation where two processors cooperate in executing the same task and communicate via hardware queues. 4) it has an instruction cache, and 5) it has a memory interface that allows overlapped memory transactions. This paper describes the architecture and its implementation. Results of extensive simulation studies are reported.

PIPE: a VLSI decoupled architecture

In this paper, we look at the interaction of pipelining and multiple functional units in single processor machines. When implementing a high performance machine, a number of hardware techniques maybe used to improve the performance of the final system. Our goal is to gain an understanding of how each of these techniques contribute to performance improvement. As a basis for our studies we use a CRAY-like processor model and the issue rate (instructions per clock cycle) as the performance measure. We then systematically augment this base, non-pipelined, machine with more and more hardware features and evaluate the performance impact of each feature. We find, for example, that in non-vector machines, pipelining multiple function units does not provide significant performance improvements. Dataflow limits are then derived for our benchmark programs to determine the performance potential of each benchmark. In addition, other limits are computed which apply more realistic constraints on a computation. Based on these more realistic limits, we determine it is worthwhile to investigate the performance improvements that can be achieved from issuing multiple instructions each clock cycle. Several hardware approaches are evaluated for issuing multiple instructions each clock cycle.

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The performance potential of multiple functional unit processors

In this paper, the WISQ architecture is described. This architecture is designed to achieve high performance by exploiting new compiler technology and using a highly segmented pipeline. By having a highly segmented pipeline, a very-high-speed clock can be used. Since a highly segmented pipeline will require relatively long pipelines, a way must be provided to minimize the effects of pipeline bubbles that are formed due to data and control dependencies. It is also important to provide a way of supporting precise interrupts. These goals are met, in part, by providing a reorder buffer to help restore the machine to a precise state. The architecture then makes the pipelining visible to the programmer/compiler by making the reorder buffer accessible and by explicitly providing that issued instructions cannot be affected by immediately preceding ones. Compiler techniques have been identified that can take advantage of the reorder buffer and permit a sustained execution rate approaching or exceeding one per clock. These techniques include using trace scheduling and providing a relatively easy way to “undo” instructions if the predicted branch path is not taken. We have also studied ways to further reduce the effects of branches by not having them executed in the execution unit. In particular, branches are detected and resolved in the instruction fetch unit. Using this approach, the execution unit is sent a stream of instructions (without branches) that are guaranteed to execute.

Andrew R. Pleszkun

Papers

Implementing precise interrupts in pipelined processors

Implementation of precise interrupts in pipelined processors

PIPE: a VLSI decoupled architecture

The performance potential of multiple functional unit processors

WISQ: a restartable architecture using queues