Power/Performance Bits: May 6

Compressing objects
Computer scientists at MIT propose a way to improve data compression in memory by focusing on objects rather than cache lines.

“The motivation was trying to come up with a new memory hierarchy that could do object-based compression, instead of cache-line compression, because that’s how most modern programming languages manage data,” said Po-An Tsai, a graduate student at MIT’s CSAIL.

In experiments using a modified Java virtual machine, the technique compressed twice as much data and reduced memory usage by half over traditional cache-based methods. It was also effective when tested on two object-heavy C/C++ workloads.

The latest effort is based on the team’s previous work developing a system called Hotpads. Hotpads stores entire objects, tightly packed into hierarchical levels, or “pads,” which reside entirely on efficient, on-chip, directly addressed memories.

Programs then directly reference the location of all objects across the hierarchy of pads. Newly allocated and recently referenced objects, and the objects they point to, stay in the faster level. When the faster level fills, it runs an “eviction” process that keeps recently referenced objects but kicks down older objects to slower levels and recycles objects that are no longer useful, to free up space. Pointers are then updated in each object to point to the new locations of all moved objects. The researchers say this allows programs to access objects much more cheaply than searching through cache levels.

To address compression, the team created Zippads, a technique that uses Hotpads architecture to compress objects. When objects first start at the faster level, they’re uncompressed. But when they’re evicted to slower levels, they’re all compressed. Pointers in all objects across levels then point to those compressed objects, which makes them easy to recall back to the faster levels and able to be stored more compactly than prior techniques.

The compression algorithm leverages redundancy across objects, which the team says creates more opportunities for compression than looking for redundancies in small, fixed blocks.

“All computer systems would benefit from this,” said Daniel Sanchez, a professor of computer science and electrical engineering at MIT and a researcher at CSAIL. “Programs become faster because they stop being bottlenecked by memory bandwidth.”

Caffeinated solar
Scientists at UCLA found a way to improve the thermal stability of perovskite solar cells by adding caffeine.

While promising due to their low cost and high efficiency, perovskite solar cells degrade rapidly under real-world conditions, including when subjected to heat generated by the sun.

“Solar cells need high thermal stability since they are constantly exposed to sunlight, which warms up the devices,” said Yang Yang, a professor of materials science and engineering at UCLA. “While perovskites are an attractive option for solar cells, the materials degrade and become less stable over time. We need them to last 20 to 30 years like traditional solar cells.”

The idea to test whether caffeine would interact positively with the components of perovskite cells arose, naturally, whilst the team was drinking coffee. “The boiling point of caffeine is 300 degrees Celsius, which is higher than the operational temperature of solar cells, so it seemed like a possible candidate,” said Rui Wang, the UCLA graduate student who came up with the idea.

Adding caffeine to perovskite solar cells could help make them commercially viable because the chemical improves their ability to withstand sustained heat from sunlight. (Source: Marc Roseboro/CNSI)

The team made a custom perovskite film by mixing dimethylformamide, methylammonium iodide and lead iodide to create a liquid solution, added caffeine, then poured the solution onto indium tin oxide glass to form a black layer of perovskite.

They incorporated the new film into a solar cell and tested its ability to withstand high temperatures by placing it on a plate heated to 85 degrees Celsius (about 185 degrees Fahrenheit). Measuring its energy output every four days for two months, the researchers found that the device retained its thermal stability for more than 1,300 hours, or about 55 days, and retained 86% power conversion efficiency. In comparison, a test cell made without caffeine retained only 60% of its power conversion efficiency after 175 hours, or about seven days.

Using a transmittance electron microscope, the team analyzed changes to the new film’s crystalline structure and found a strong interaction, or molecular lock, between the caffeine and lead ions.

“Parts of caffeine’s chemical structure were forming very strong binding with the lead ions and stabilizing the crystals,” said Jingjing Xue, a UCLA graduate student. “The molecular lock between caffeine and lead also slowed down the growth of perovskite crystals, allowing them to align into an orientation that is beneficial for electric charge transfer.”

The team next plans to further investigate the chemical structure of the caffeine-incorporated perovskite material and to identify whether other chemicals could provide similar protective effects.