Memory ordering

Résumé

Memory ordering describes the order of accesses to computer memory by a CPU. The term can refer either to the memory ordering generated by the compiler during compile time, or to the memory ordering generated by a CPU during runtime. In modern microprocessors, memory ordering characterizes the CPU's ability to reorder memory operations – it is a type of out-of-order execution. Memory reordering can be used to fully utilize the bus-bandwidth of different types of memory such as caches and memory banks. On most modern uniprocessors memory operations are not executed in the order specified by the program code. In single threaded programs all operations appear to have been executed in the order specified, with all out-of-order execution hidden to the programmer – however in multi-threaded environments (or when interfacing with other hardware via memory buses) this can lead to problems. To avoid problems, memory barriers can be used in these cases. Compile-time memory ordering

Source officielle

https://en.wikipedia.org/wiki/Memory_ordering

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When the ARM weakly consistent memory model meets speculation: is it necessary?

Marc-Alexandre Boéchat, Tao-Chun Lee

Aggressive memory-level-parallelism techniques have provided significant performance gain in Distributed Share Memory Designs. In this paper, we reevaluate speculative memory ordering in the context of Chip Multi-Processors (CMPs) and power-limited computation. We evaluate relative performance between Sequential Consistency, Total Store Order and Relaxed Memory Order on a selection of modern workloads to predict the performance of the ARM weakly consistent memory model.

2012

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A simple cellular automaton that solves the density and ordering problems

Mathieu Capcarrère

1998

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Dynamically-scheduled elastic circuits generated by High-Level Synthesis (HLS) tools are inherently out-of-order, following the flow of data rather than the evolution of an instruction pointer. Components of the circuit which access memory need to be connected to a Load-Store Queue (LSQ) that dynamically checks for memory dependencies, performs store ordering and forwarding, and allows unordered access to Random-Access Memory (RAM) whenever possible. While connecting every memory access (load/store) component to an LSQ ensures correctness of program execution, the hardware and power cost makes this solution unattractive. Statically ruling out dependencies allows circuits to access memory via lightweight components that use an arbitrator to handle RAM port sharing. Reducing the number of components using the LSQ allows the compiler to generate smaller queues which results in superlinear savings in hardware and power for the memory subsystem. This work describes additions to the Elastic Compiler (EC) that allow it to analyze algorithms expressed in LLVM-IR, an intermediate code representation, to rule out memory dependencies between load/store instructions and their underlying insights. These analyses leverage pointer analysis as well as array access patterns to narrow down the list of possibly dependent instructions. We also enhance the compiler to leverage our analyses and automatically generate relevant memory-access components for the circuit and to connect them to the relevant arbitrator or LSQ.

2018

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