Mitigating bank conflicts in main memory via selective data duplication and migration

Date

2021-05-07

Authors

Lin, Ching-Pei

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Abstract

Main memory is organized as a hierarchy of banks, rows, and columns. Only data from a single row can be accessed from each bank at any given time. Switching between different rows of the same bank requires serializing long latency operations to the bank. Consequently, memory performance suffers on bank conflicts when concurrent requests access different rows of the same bank.

Many prior solutions to the bank conflict problem required modifications to the memory device and/or the memory access protocol. Such modifications create hurdles for adoption due to the commodity nature of the memory business. Instead, I propose two new runtime solutions that work with unmodified memory devices and access protocols. The first, Duplicon Cache, duplicates select data to multiple banks, allowing duplicated data to be sourced from either the original bank or the alternate bank, whichever is more lightly loaded. The second, Continuous Row Compaction, identifies data that are frequently accessed together, then migrates them to non-conflicting rows across different banks.

To limit the data transfer overhead from data duplication and migration, only select data are duplicated/migrated. The key is to identify large working sets of the running applications that remain stable over very long time intervals, and slowly duplicate/migrate them over time, amortizing the cost of duplication/migration. In effect, the set of duplicated/migrated data form a cache within main memory that captures large stable working sets of the application.

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