Compiler support for lightweight context switching

Abstract

We propose a new language-neutral primitive for the LLVM compiler, which provides efficient context switching and message passing between lightweight threads of control. The primitive, called SWAPSTACK, can be used by any language implementation based on LLVM to build higher-level language structures such as continuations, co-routines, and lightweight threads. As part of adding the primitives to LLVM, we have also added compiler support for passing parameters across context switches. Our modified LLVM compiler produces highly efficient code through a combination of exposing the context switching code to existing compiler optimizations, and adding novel compiler optimizations to further reduce the cost of context switches. To demonstrate the generality and efficiency of our primitives, we add one-shot continuations to C++, and provide a simple fiber library that allows millions of fibers to run on multiple cores, with a work-stealing scheduler and fast inter-fiber sychronization. We argue that compiler-supported lightweight context switching can be significantly faster than using a library to switch between contexts, and provide experimental evidence to support the position.

Compiling with Continuations and LLVM

“…Dolan et al proposed the SWAPSTACK mechanism for LLVM to enable lightweight context switching [10]. To capture stack-allocated one-shot continuations in LLVM, a mechanism like SWAPSTACK can be used in conjunction with runtime system support.…”

Hop, Skip, & Jump

“…Swapstack effectively provides the abstraction of symmetric coroutines. The term Swapstack was first used by Dolan et al [5] to specifically refer to their efficient language-neutral compiler-assisted mechanism in LLVM for context-switching and message passing between lightweight threads of control ('green threads'). However, as we describe below, the ideas underlying Swapstack have been implemented before, including in the Self virtual machine [11,12].…”

“…In this paper, we use the term Swapstack 4 in a broader sense-it is an abstract operation that rebinds the current thread to a different stack, regardless of its implementation. 5 Swapstack is an integral part of Mu. It not only supports coroutines and green threads, but is also a foundation for many VM mechanisms, such as thread creation, trap handling, and OSR.…”

“…The first contribution of this paper is the design of a practical, novel, platform-independent OSR API, allowing implementors to readily use OSR as a building block in language implementation. Key to the API is the insight that the Swapstack primitive [5] can be used to isolate the executing thread from the stack it is manipulating, eliminating the need to use tailored assembly code during OSR [12]. The API operates on a platform-independent, abstract view of stacks, so that the language implementation views the stack data in the abstract type system of the VM instead of byte-level stack layouts.…”

High-Throughput Maps on Message-Passing Manycore Architectures: Partitioning versus Replication

“…Second, even if interrupt handling on the TILE-Gx is rather efficient, its overhead remains high compared to other cpu costs. Solutions have been proposed to save and restore an execution context very efficiently using different architectural and compilation techniques [23,30,14,25]: More specifically in [23], a solution with a constant 4 cycles cost is presented. Hence the second feature we consider is efficient interrupt handling with a cost of 4 cycles.…”