RPC has been remarkably successful. Most distributed applications built today use an RPC runtime such as gRPC [3] or Apache Thrift [2]. The key behind RPC's success is the simple but powerful semantics of its programming model. In particular, RPC has no shared state: arguments and return values are passed by value between processes, meaning that they must be copied into the request or reply. Thus, arguments and return values are inherently immutable. These simple semantics facilitate highly efficient and reliable implementations, as no distributed coordination is required, while remaining useful for a general set of distributed applications. The generality of RPC also enables interoperability: any application that speaks RPC can communicate with another application that understands RPC.

Applications composed of multiple parallel libraries perform poorly when those libraries interfere with one another by obliviously using the same physical cores, leading to destructive resource oversubscription. This paper presents the design and implementation of Lithe, a low-level substrate that provides the basic primitives and a standard interface for composing parallel codes efficiently. Lithe can be inserted underneath the runtimes of legacy parallel libraries to provide bolt-on composability without needing to change existing application code. Lithe can also serve as the foundation for building new parallel abstractions and libraries that automatically interoperate with one another.