System exploration of next generation flash devices
de Ij René Jong · 2012
Nowadays the memory system is a major factor in determining the performance and the power consumption of embedded, as well as server systems. Typically the memory latency is significantly higher than the cycle time of the processor. Even modern out-of-order CPUs are not able to hide the stalls caused by interactions with the memory system. In mobile devices flash memory is nowadays mostly used as non-volatile memory. Flash differs from standard memory by its asymmetric access latencies and its maintenance functions which are needed to keep the flash effectively usable for an as long as possible time. Mobile systems have become more complex; different layers of abstraction obscure the transaction flow of the disk accesses, making it very hard for the application to predict the location where the data will eventually end up, making performance optimizations very hard. There is therefore a need to explore system configurations and simulate the designed solution to evaluate their impact. Currently there is no timing accurate full system simulator that has a flash disk model. To improve the system performance a new interface, UFS has been introduced by JEDEC. This new interface has not yet been evaluated on a system level, and it is therefore not sure what the impact will be when a complete system is considered. At the moment of writing, no UFS device is commercially available. This work will introduce a UFS flash disk model for gem5. The Flash model is a major contribution of this thesis, programmed in the language C++ as part of the open source BSD simulator gem5. It enables full system analysis of a mobile system that embeds a UFS device. The UFS device model allows the analysis of the UFS protocol itself, and reasoning about the best usage of the protocol. The UFSA, the association which promotes the UFS protocol, claims that this protocol is most effective when it is used with a few intensive data transfers, rather than a lot of small transactions. This claim has been evaluated in this work. This work demonstrates the model, shows it relevance to the current research fields and proposes an improvement to the software system in order to effectively implement UFS in mobile devices.