Optical Interconnections for 3D Multiprocessor Architectures
To address the issue of interconnection bottleneck in multiprocessor on a single chip, we study how an optical Network-on-Chip (ONoC) can leverage 3D technology by stacking a specific photonics die. The objectives of this study target:
- the definition of a generic architecture including both electrical and optical components,
- the interface between electrical and optical domains,
- the definition of strategies (communication protocol) to manage this communication medium,
- new techniques to manage and reduce the power consumption of optical communications.
To address each of the previous points, we worked on the following topics:
- The first point is required to ensure that electrical and optical components can be used together to define a global architecture. Indeed, optical components are generally larger than electrical components, so a trade-off must be found between the size of optical and electrical parts.
- For the second point, we study how the interface can be designed to take applications needs into account. From the different possible interface designs, we extract a high-level performance model of optical communications from losses induced by all optical components to efficiently manage Laser parameters.
- Then, the third point concerns the definition of high-level mechanisms which can handle the allocation of the communication medium for each data transfer between tasks. This part consists in defining the protocol of wavelength allocation. Indeed, the optical wavelengths are a shared resource between all the electrical computing clusters and are allocated at run time according to application needs and quality of service.
- The last point concerns the definition of techniques allowing to reduce the power consumption of on-chip optical communications. The power of each Laser can be dynamically tuned in the optical/electrical interface at run time for a given targeted bit-error-rate. Due to the relatively high power consumption of such integrated Laser, we study how to define adequate policies able to adapt the laser power to the signal losses.
Optical Network Interface design
We have designed an Optical-Network-Interface (ONI) to connect a cluster of several processors to the optical communication medium. This interface, constrained by the 10 Gb/s data-rate of the Lasers, integrates Error Correcting Codes (ECC) and a communication manager. This manager can select, at run-time, the communication mode to use depending on timing or power constraints. Indeed, as the use of ECC is based on redundant bits, it increases the transmission time, but saves power for a given Bit Error Rate (BER). Moreover, our ONI allows for data to be sent using several wavelengths in parallel, hence increasing transmission bandwidth. From the design of this interface, estimation in terms of power consumption and execution time have been obtained, as well as the energy per bit of each communication.
Wavelength Division Multiplexing for ONoC
The optical medium can support multiple transactions at the same time on different wavelengths by using Wavelength Division Multiplexing (WDM). Moreover, multiple wavelengths can be gathered as high- bandwidth channel to reduce transmission time. However, multiple signals sharing simultaneously a waveg- uide lead to inter-channel crosstalk noise. This problem impacts the Signal to Noise Ratio (SNR) of the optical signal, which increases the Bit Error Rate (BER) at the receiver side. We have formulated the crosstalk noise and execution time models and then proposed a Wavelength Allocation (WA) method in a ring-based WDM ONoC to reach performance and energy trade-offs based on the application constraints. We showed that for a 16-core ONoC architecture using 12 wavelengths, more than 105 allocation solutions exist and only 51 are on a Pareto front giving a tradeoff between execution time and energy per bit (derived from the BER). These optimized solutions reduce the execution time by 37% or the energy from 7.6fJ/bit to 4.4fJ/bit.
Laser power selection
We also proposed to explore the selection of laser power for each communication. This approach reduces the global power consumption by ensuring the targeted Bit Error Rate for each communication. To support laser power selection, we have also studied, designed and evaluated at transistor level different configurable laser drivers using a 28NM FDSOI technology.