Section: New Results

Hardware/Software helper thread prefetching

Heterogeneous Many Cores (HMC) architectures that mix many simple/small cores with a few complex/large cores are emerging as a design alternative that can provide both fast sequential performance for single threaded workloads and power-efficient execution for through-put oriented parallel workloads. The availability of many small cores in a HMC presents an opportunity to utilize them as low-power helper cores to accelerate memory-intensive sequential programs mapped to a large core. However, the latency overhead of accessing small cores in a loosely coupled system limits their utility as helper cores. Also, it is not clear if small cores can execute helper threads sufficiently in advance to benefit applications running on a larger, much powerful, core.

In this project, we designed a hardware/software framework called core-tethering to support efficient helper threading on heterogeneous manycores. Core-tethering provides a co-processor like interface to the small cores that (a) enables a large core to directly initiate and control helper execution on the helper core and (b) allows efficient transfer of execution context between the cores, thereby reducing the performance overhead of accessing small cores for helper execution. Our evaluation on a set of memory intensive programs chosen from the standard benchmark suites shows that helper threads using moderately sized small cores can significantly accelerate a larger core compared to using a hardware prefetcher alone. We find that a small core provides a good trade-off against using an equivalent large core to run helper threads in a HMC. Additionally, helper prefetching on small cores when used along with hardware prefetching, can provide an alternate design point to growing instruction window size for achieving higher sequential performance on memory intensive applications.

This work is a collaboration between the ALF team in Rennes and CAMUS in Strasbourg. Our contribution is mainly on the generation of helper thread code (as a followup to our work on program skeletonization). The result of the work has been published in October 2014 in the Proceedings of the SBAC-PAD conference [17] .