Energy-Efficient Compilation of Irregular Task-Parallel Loops.

Energy-efficient compilation is an important problem for multi-core systems. In this context, irregular programs with task-parallel loops  present interesting challenges: the threads with lesser work-loads (non-critical-threads) wait at the join-points for the thread with maximum work-load (critical-thread); this leads to significant energy wastage. This problem becomes more interesting in the context of multi-socket-multi-core (MSMC) systems, where different sockets may run at different frequencies, but all the cores connected to a socket run at a single frequency. In such a configuration, even though the load-imbalance among the cores may be significant, an MSMC-oblivious technique may miss the opportunities to reduce energy consumption, if the load-imbalance across the sockets is minimal. This problem becomes further challenging in the presence of mutual-exclusion, where scaling the frequencies of a socket executing the non-critical-threads can impact the execution time of the critical-threads. In this article, we propose a scheme (X10Ergy) to obtain energy gains with minimal impact on the execution time, for task-parallel languages, such as X10, HJ, and so on. X10Ergy takes as input a loop-chunked program (parallel-loop iterations divided into chunks and each chunk is executed by a unique thread). X10Ergy follows a mixed compile-time + runtime approach that (i) uses static analysis to efficiently compute the work-load of each chunk at runtime, (ii) computes the “remaining” work-load of the chunks running on the cores of each socket at regular intervals and tunes the frequency of the sockets accordingly, (iii) groups the threads into different sockets (based on the remaining work-load of their respective chunks), and (iv) in the presence of atomic-blocks, models the effect of frequency-scaling on the critical-thread. We implemented X10Ergy for X10 and have obtained encouraging results for the IMSuite kernels.