A Survey on Compiler Autotuning using Machine Learning

Since the mid-1990s, researchers have been trying to use machine-learning based approaches to solve a number of different compiler optimization problems. These techniques primarily enhance the quality of the obtained results and, more importantly, make it feasible to tackle two main compiler optimization problems: optimization selection (choosing which optimizations to apply) and phase-ordering (choosing the order of applying optimizations). The compiler optimization space continues to grow due to the advancement of applications, increasing number of compiler optimizations, and new target architectures. Generic optimization passes in compilers cannot fully leverage newly introduced optimizations and, therefore, cannot keep up with the pace of increasing options. This survey summarizes and classifies the recent advances in using machine learning for the compiler optimization field, particularly on the two major problems of (1) selecting the best optimizations and (2) the phase-ordering of optimizations. The survey highlights the approaches taken so far, the obtained results, the fine-grain classification among different approaches and finally, the influential papers of the field.Comment: version 5.0 (updated on September 2018)- Preprint Version For our Accepted Journal @ ACM CSUR 2018 (42 pages) - This survey will be updated quarterly here (Send me your new published papers to be added in the subsequent version) History: Received November 2016; Revised August 2017; Revised February 2018; Accepted March 2018

On the asymptotic flux of ultrapermeable seawater reverse osmosis membranes due to concentration polarisation

Just as thermodynamic considerations impose a finite limit on the energy requirements of reverse osmosis, concentration polarisation imposes a finite limit on flux, or equivalently, on system size. In the limit of infinite permeability, we show the limiting flux to be linearly dependent on the mass transfer coefficient and show this to be true for low recovery systems just as well as moderate and high recovery single stage and batch reverse osmosis system designs. At low recovery, the limiting flux depends on the logarithm of the ratio of hydraulic to bulk osmotic pressure and at moderate or higher recovery, the relationship with this pressure ratio is a little more complex but nonetheless can be expressed as an explicit analytical formula. For a single stage seawater reverse osmosis system operating at a hydraulic pressure, recovery ratio, and value of mass transfer coefficient that are typical today, the flux asymptote is roughly 60 L m[superscript −2] h[superscript −1] – roughly four times where average fluxes in seawater reverse osmosis systems currently stand

On the potential of forward osmosis to energetically outperform reverse osmosis desalination

We provide a comparison of the theoretical and actual energy requirements of forward osmosis and reverse osmosis seawater desalination. We argue that reverse osmosis is significantly more energy efficient and that forward osmosis research efforts would best be fully oriented towards alternate applications. The underlying reason for the inefficiency of forward osmosis is the draw-dilution step, which increases the theoretical and actual energy requirements for draw regeneration. As a consequence, for a forward osmosis technology to compete with reverse osmosis, the regeneration process must be significantly more efficient than reverse osmosis. However, even considering the optimisation of the draw solution and the benefits of reduced fouling during regeneration, the efficiency of an optimal draw regeneration process and of reverse osmosis are unlikely to differ significantly, meaning the energy efficiency of direct desalination with reverse osmosis is likely to be superior

THE EFFECT OF VERY HIGH HYDRAULIC PRESSURE ON THE PERMEABILITY AND SALT REJECTION OF REVERSE OSMOSIS MEMBRANES

We employ a stirred-cell reverse osmosis setup to demonstrate that a seawater reverse osmosis membrane can maintain excellent salt rejection at pressures as high as 172 bar. However, we also demonstrate a very significant drop in membrane permeability at high pressures–likely due to membrane compaction. At 172 bar, permeability is more than 50% lower than at a pressure of 34.5 bar. In addition, our results illustrate how flux fluctuates significantly in time when the pressure is removed and then reapplied, even for very short periods, in high pressure reverse osmosis processes–an effect that requires careful consideration from the perspective of process control and operation. From the perspective of membrane performance, RO is feasible at high pressures but distinct challenges are presented by reduced permeability and increased variability in flux

Raising forward osmosis brine concentration efficiency through flow rate optimization

An exergetic efficiency is defined in order to compare brine concentration processes including forward osmosis (FO) across a wide range of salinities. We find that existing FO pilot plants have lower efficiency than reverse osmosis plants in the brackish and seawater salinity ranges. High salinity FO, in its current form, is still less efficient than mechanical vapor compression. We show that efficiency is the product of FO exchanger and draw regenerator efficiencies, and therefore FO system energy efficiency benefits from improvements to both. The mass flow rate ratio (between draw and feed flow rates) is identified as a crucial parameter in the design of efficient FO systems because of its effect on exchanger efficiency. We demonstrate a method of thermodynamically balancing an FO system by choosing flow rates that lead to equal osmotic pressure differences at both ends of the exchanger, and show the method's potential to increase the efficiency of current systems by 3–21%.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R4-CW-08)National Science Foundation (U.S.). Graduate Research Fellowship (Grant 1122374)Hugh Hampton Young Memorial Fellowshi

The benefits of hybridising electrodialysis with reverse osmosis

A cost analysis reveals that hybridisation of electrodialysis with reverse osmosis is only justified if the cost of water from the reverse osmosis unit is less than 40% of that from a stand-alone electrodialysis system. In such cases the additional reverse osmosis costs justify the electrodialysis cost savings brought about by shifting salt removal to higher salinity, where current densities are higher and equipment costs lower. Furthermore, the analysis suggests that a simple hybrid configuration is more cost effective than a recirculated hybrid, a simple hybrid being one where the reverse osmosis concentrate is fed to the electrodialysis stack and the products from both units are blended, and a recirculated being one hybrid involving recirculation of the electrodialysis product back to the reverse osmosis unit. The underlying rationale is that simple hybridisation shifts salt removal away from the lowest salinity zone of operation, where salt removal is most expensive. Further shifts in the salinity at which salt is removed, brought about by recirculation, do not justify the associated increased costs of reverse osmosis.Hugh Hampton Young Memorial FellowshipCenter for Clean Water and Clean Energy at MIT and KFUPM (Project R15-CW-11

Hybrid electrodialysis reverse osmosis system design and its optimization for treatment of highly saline brines

The demand is rising for desalination technologies to treat highly saline brines arising from hydraulic fracturing processes and inland desalination. Interest is growing in the use of electrical desalination technologies for this application. The hybridization of electrodialysis (ED) with reverse osmosis (RO) allows high salinities (beyond the range of RO alone) to be reached while avoiding the operation of ED with a low conductivity diluate stream. Such hybrid systems have been experimentally investigated for concentrates from brackish and seawater desalination. However, progress is required in the modelling and optimization of hybrid systems at higher concentrations. A novel hybrid arrangement of counterflow ED systems with reverse osmosis is presented to concentrate a saline feed at 120 ppt. The system is considered from the perspective of efficiency, membrane productivity and the levelised cost of water, with emphasis on the optimisation of current density. In contrast to brackish ED systems, membrane resistances are found to dominate diluate and concentrate resistances at high salinity. The current density found to minimise LCW (levelised cost of water) is significantly greater than the current density found to maximise efficiency, indicating the high current capital cost of ED per unit membrane area and poor membrane transport properties relative to RO. Finally, performance at high recoveries is found to be limited by high stream-to-stream concentration differences, increasing water transport via osmosis, decreasing efficiency and increasing the LCW.Fulbright ProgramMartin Family Society of Fellows for SustainabilityInternational Desalination Association (Channabasappa Memorial Scholarship

Increasing the power density and reducing the levelized cost of electricity of a reverse electrodialysis stack through blending

We increase the power density of a reverse electrodialysis (RED) stack by blending the low salinity feed with a higher salinity stream before the stack entrance. This lowers the capital cost of the system and the resulting levelized cost of electricity, enhancing the viability of RED renewable energy generation. Blending increases the power density by decreasing the dominating electrical resistance in the diluate channel as well as the effective resistance caused by concentration polarization, but not without sacrificing some driving potential. To quantify this trade-off and to evaluate the power density improvement blending can provide, a one-dimensional RED stack model is employed and validated with experimental results from the literature. For a typical stack configured with a feed velocity of 1 cm/s, power density improvements of over 20% and levelized cost of energy reductions of over 40% are achievable, provided the salinity of the available river water is below 200 ppm. Additional cost reductions are realized through back-end blending, whereby the diluate exit stream is used as the higher salinity blend stream. Also, improvements from blending increase for higher feed velocities, shorter stack lengths, and larger channel heights.King Fahd University of Petroleum and Minerals (Center for Clean Water and Clean Energy at MIT and KFUPM, project number R15-CW-11)Massachusetts Life Sciences Center (Hugh Hampton Memorial Fellowship

The cost effectiveness of electrodialysis for diverse salinity applications

We provide a thermoeconomic assessment of electrodialysis indicating that the technology is most productive and efficient for the partial desalination of feed streams at the higher end of the brackish range of salinities. After optimising the current density to minimise the sum of energy and equipment costs, we demonstrate that at low feed salinities the productivity, and hence equipment costs, of electrodialysis are hampered by the limiting current density. By contrast, at higher feed salinities both productivity and efficiency are hampered by the reduced chemical potential difference of salt in the diluate (low salinity) and concentrate (high salinity) streams. This analysis indicates the promise of further developing electrodialysis for the treatment of waters from oil, gas and coal-bed methane as well as flue-gas de-sulphurisation, where the partial desalination of streams at the high-end of the brackish range can be beneficial.Center for Clean Water and Clean Energy at MIT and KFUPM (Project R15-CW-11)United States. Dept. of State (International Fulbright Science & Technology Award)International Desalination Association (Channabasappa Memorial Scholarship)MIT Martin Family Society of Fellows for SustainabilityHugh Hampton Young Memorial Fellowshi

A new reverse electrodialysis design strategy which significantly reduces the levelized cost of electricity

We develop a framework for choosing the optimal load resistance, feed velocity and residence time for a reverse electrodialysis stack based on minimizing the levelized cost of electricity. The optimal load resistance maximizes the gross stack power density and results from a trade-off between stack voltage and stack current. The primary trade-off governing the optimal feed velocity is between stack pumping power losses, which reduce the net power density and concentration polarization losses, which reduce the gross stack power density. Lastly, the primary trade-off governing the optimal residence time is between the capital costs of the stack and pretreatment system. Implementing our strategy, we show that a smaller load resistance, a smaller feed velocity and a larger residence time than are currently proposed in the literature reduces costs by over 40%. Despite these reductions, reverse electrodialysis remains more expensive than other renewable technologies.King Fahd University of Petroleum and Minerals (Center for Clean Water and Clean Energy at MIT and KFUPM, project number R15-CW-11)Massachusetts Institute of Technology (Hugh Hampton Memorial Fellowship