COUNTDOWN: a Run-time Library for Performance-Neutral Energy Saving in MPI Applications

TL;DR

COUNTDOWN is a runtime library that reduces energy consumption during communication idle times in MPI applications without significantly impacting execution time, tested on real HPC systems.

Contribution

It introduces a methodology and tool for transparent, performance-neutral energy savings in MPI applications during communication idle periods.

Findings

Saves 6-50% energy on NAS benchmarks with <5% time increase.

Achieves 22.36% energy savings on Quantum ESPRESSO with <3% performance penalty.

Energy savings increase to 37% with higher performance penalty when communication tuning is disabled.

Abstract

Power and energy consumption is becoming key challenges to deploy the first exascale supercomputer successfully. Large-scale HPC applications waste a significant amount of power in communication and synchronization-related idle times. However, due to the time scale at which communication happens, transitioning in low power states during communication's idle times may introduce unacceptable overhead in applications' execution time. In this paper, we present COUNTDOWN, a runtime library, supported by a methodology and analysis tool for identifying and automatically reducing the power consumption of the computing elements during communication and synchronization. COUNTDOWN saves energy without imposing significant time-to-completion increase by lowering CPUs power consumption only during idle times for which power state transition overhead are negligible. This is done transparently to the user, without requiring labor-intensive and error-prone application code modifications, nor requiring recompilation of the application. We test our methodology in a production Tier-0 system. For the NAS benchmarks, COUNTDOWN saves between 6% and 50% energy, with a time-to-solution penalty lower than 5%. In a complete production --- Quantum ESPRESSO --- for a 3.5K cores run, COUNTDOWN saves 22.36% energy, with a performance penalty below 3%. Energy saving increases to 37% with a performance penalty of 6.38%, if the application is executed without communication tuning.