Way Combination for an Adaptive and Scalable Coherence Directory

Abstract

© 2019 IEEE. Personal use of this material is permitted. Permissíon from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertisíng or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.[EN] This manuscript opens the way to a new class of coherence directory structures that are based on the brand-new concept of way combining. A Way-Combining Directory (WC-dir) builds on a typical sparse directory but allows to take advantage of several ways in the same set to codify the sharing information of each memory block. The result is a sparse directory with variable effective associativity per set and variable length entries, thus being able to dynamically adapt the directory structure to the particular requirements of each application. In particular, our proposal uses just enough bits per entry to store a single pointer, which is optimal for the common case of having just one sharer. For those addresses that have more than one sharer, we have observed that in the majority of cases extra bits could be taken from other empty ways in the same set. All in all, our proposal minimizes the storage overheads without losing the flexibility to adapt to several sharing degrees and without the complexities of other previously proposed techniques. Detailed simulations of a 128-core multicore architecture running benchmarks from PARSEC-3.0 and SPLASH-3 demonstrate that WC-dir can closely approach the performance of a non-scalable bit vector sparse directory, beating the state-of-the-art Scalable Coherence Directory (SCD) and Pool directory proposals.This work has been supported by the Spanish MCIU and AEI, as well as European Commission FEDER funds, under grant "RTI2018-098156-B-C53".Titos-Gil, R.; Flores, A.; Fernández-Pascual, R.; Ros, A.; Petit Martí, SV.; Sahuquillo Borrás, J.; Acacio, ME. (2019). Way Combination for an Adaptive and Scalable Coherence Directory. IEEE Transactions on Parallel and Distributed Systems. 30(11):2608-2623. https://doi.org/10.1109/TPDS.2019.2917185S26082623301