ECE PhD Prospectus Defense- Yenai Ma

3:30 pm on Thursday, May 28, 2020
5:30 pm on Thursday, May 28, 2020

Title: Cross-Layer Design of Thermally-Aware 2.5D Systems

Advisor: Ajay Joshi, ECE

Chair: TBD

Committee: Wenchao Li, ECE; Ayse Coskun, ECE; Milos Popovic, ECE

Abstract: Over the past decade, CMOS technology scaling has slowed down. To sustain the historic performance improvement predicted by Moore's Law, in the mid-2000s the computing industry moved to using manycore systems and exploiting parallelism. The on-chip power densities of manycore systems, however, continued to increase after the breakdown of Dennard's Scaling. This leads to the `dark silicon' problem, whereby not all cores can operate at the highest frequency or can be turned on simultaneously due to thermal constraints. As a result, we have not been able to take full advantage of the parallelism in manycore systems. `More than Moore' approach that is being explored to address this problem is integration of diverse functional components onto a substrate using 2.5D integration technology. 2.5D integration provides opportunities to exploit chiplet placement flexibility to address the dark silicon problem and mitigate the thermal stress of today's high-performance systems. These opportunities can be leveraged to improve the overall performance of the manycore heterogeneous computing systems.

This thesis aims at designing thermally-aware 2.5D systems. To address the dark silicon problem of manycore systems, we first propose a single-layer thermally-aware chiplet organization methodology for homogeneous 2.5D systems. The key idea is to strategically insert spacing between the chiplets of a 2.5D manycore system to lower the operating temperature, and thus reclaim dark silicon by allowing more active cores and/or higher operating frequency under a temperature threshold. We investigate manufacturing cost and thermal behavior of 2.5D systems, then formulate and solve an optimization problem that jointly maximizes performance and minimizes manufacturing cost. We then enhance our methodology by incorporating a cross-layer co-optimization approach. We jointly maximize performance and minimize manufacturing cost and operating temperature across logical, physical, and circuit layers. We propose a novel gas-station link design that enables pipelining in passive interposers. We then extend our thermally-aware optimization methodology for network routing and chiplet placement of heterogeneous 2.5D systems, which consist of CPU chiplets, GPU chiplets, accelerator chiplets, and/or memory stacks. We jointly minimize the total wirelength and the system temperature. Our enhanced methodology increases the thermal design power budget and thereby improves thermal-constraint performance of the system.