Browsing by Author "Xu, Chao"
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Item Automated OS-level Device Runtime Power Management(2014-12-01) Xu, Chao; Zhong, Lin; Cavallaro, Joseph R; Vardi, Moshe Y; Wallach, Dan SHardware devices on a modern System-on-Chip (SoC), ranging from accelerators to IO controllers, usually account for the largest portion of the chip area. It is therefore vital for Operating Systems (OS) to disable and enable these devices at run time, so that idle devices can enter low-power state timely while meeting user’s performance expectation. This is called device runtime Power Management (PM), for which individual device drivers in commodity OSes are held responsible. Based on the observations of existing drivers and their evolution, we consider counting on drivers for device runtime PM harmful. We identify three pieces of information essential to device runtime PM, and show that all of them can be obtained without involving drivers, either by using a software inference approach atop existing ARM-based SoC, or more efficiently, by adding one register bit to each device. We thus argue for a structural change to the current Linux runtime PM framework, replacing PM code in each individual driver with one kernel module called central PM agent. We experimentally show that central PM agent is just as effective as hand-tuned driver PM code. We also present a software tool called PowerAdvisor, as a remedy to simplify driver PM efforts without overhauling the current Linux runtime PM framework. PowerAdvisor analyzes trace generated from historic executions and suggests PM calls to be inserted at certain driver source locations. Although a best-effort tool, PowerAdvisor not only reproduces hand-tuned PM code that already exists in stock drivers, but also correctly suggests PM code never known before . Overall, our experiences show that it is promising to ultimately free driver developers from manual PM.Item Giant photothermoelectric effect in silicon nanoribbon photodetectors(Springer Nature, 2020) Dai, Wei; Liu, Weikang; Yang, Jian; Xu, Chao; Alabastri, Alessandro; Liu, Chang; Nordlander, Peter; Guan, Zhiqiang; Xu, Hongxing; Laboratory for NanophotonicsThe photothermoelectric (PTE) effect enables efficient harvesting of the energy of photogenerated hot carriers and is a promising choice for high-efficiency photoelectric energy conversion and photodetection. Recently, the PTE effect was reported in low-dimensional nanomaterials, suggesting the possibility of optimizing their energy conversion efficiency. Unfortunately, the PTE effect becomes extremely inefficient in low-dimensional nanomaterials, owing to intrinsic disadvantages, such as low optical absorption and immature fabrication methods. In this study, a giant PTE effect was observed in lightly doped p-type silicon nanoribbons caused by photogenerated hot carriers. The open-circuit photovoltage responsivity of the device was 3-4 orders of magnitude higher than those of previously reported PTE devices. The measured photovoltage responses fit very well with the proposed photothermoelectric multiphysics models. This research proposes an application of the PTE effect and a possible method for utilizing hot carriers in semiconductors to significantly improve their photoelectric conversion efficiency.