Title: A Case for Richer Cross-layer Abstractions: Bridging the Semantic Gap with Expressive Memory

Abstract:
This paper makes a case for a new cross-layer interface, Expressive Memory (XMem), 
to communicate higher-level program semantics from the application to the system 
software and hardware architecture. XMem provides (i) a flexible and extensible 
abstraction, called an Atom, enabling the application to express key program semantics 
in terms of how the program accesses data and the attributes of the data itself, and (ii) 
new cross-layer interfaces to make the expressed higher-level information available to 
the underlying OS and architecture. By providing key information that is otherwise unavailable, 
XMem exposes a new, rich view of the program data to the OS and the different architectural 
components that optimize memory system performance (e.g., caches, memory controllers). By 
bridging the semantic gap between the application and the underlying memory resources, XMem 
provides two key benefits. First, it enables architectural/system-level techniques to leverage 
key program semantics that are challenging to predict or infer. Second, it improves the efficacy 
and portability of software optimizations by alleviating the need to tune code for specific hardware 
resources (e.g., cache space). While XMem is designed to enhance and enable a wide range of memory 
optimizations, we demonstrate the benefits of XMem using two use cases: (i) improving the performance 
portability of software-based cache optimization by expressing the semantics of data locality in the 
optimization and (ii) improving the performance of OS-based page placement in DRAM by leveraging the 
semantics of data structures and their access properties.