wiki:LightweightConcurrency

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Lightweight Concurrency in GHC

This page documents the effort to move GHC's concurrency support from its current location in the C part of the runtime system (RTS) to Haskell. This works builds on Peng Li's earlier work (http://community.haskell.org/~simonmar/papers/conc-substrate.pdf). This page contains information about the design, implementation, problems and potential solutions for building user-level concurrency primitives in GHC. Currently, the focus is on user-level implementation of non-deterministic parallelism in GHC (Control.Concurrent).

Introduction

GHC has a rich support for concurrency (forkIO, MVars, STM, Asynchronous exceptions, bound threads, safe FFI, transparent scaling on multicores, etc.) and a fast and robust runtime system. However, the concurrency support is implemented in C and baked into the RTS. The concurrency primitives non-trivially interact among each other, and along with the lightweight thread scheduler, through a cascade of locks and condition variables. Often times, the invariants on which RTS fields can be accessed when are expressed as comments, and enforced through assertions (See here for one such fascinating example). This policy of enforcing through assertions keeps the overheads low, but makes the task of modifying and extending the runtime cumbersome.

But, why would we be interested in modifying GHC's concurrency environment? There are several good reasons to believe that a particular concurrent programming model, or a scheduling policy would not suit every application. With the emergence of many-core processors, we see NUMA effects becoming more prominent, and applications might benefit from NUMA aware scheduling and load balancing policies. Moreover, an application might have a better knowledge of the scheduling requirements -- a thread involved in user-interaction is expected to be given more priority over threads performing background processing. We might want to experiment with various work-stealing or work-sharing policies. More ambitiously, we might choose to build X10 style async-finish or Cilk style spawn-sync task parallel abstractions. Ideally, we would like allow the programmer to write an application that can seamlessly combine all of these different programming abstractions, with pluggable scheduling and load balancing policies.

While we want to provide flexibility to the Haskell programmer, this should not come at a cost of added complexity and decreased performance. This idea reflects in the synchronization abstractions exposed to the programmer - Primitive Transactional Memory(PTM)), and our decision to keep certain pieces of the concurrency puzzle in the RTS (Safe FFI,Blackholes). One would think lifting parts of the runtime system to Haskell, and retaining other parts in C, would complicate the interactions between the concurrency primitives and schedulers. We abstract the scheduler interface using PTM monads, which simplifies the interactions. The figure below captures the key design principles of the proposed system.

key_idea

Although implementing concurrency primitives as a library is hardly a novel idea, the aim of this work is to bring it to the GHC programmer, without having to give up any of the existing concurrency features in return.

Background - GHC's Concurrency RTS

This section focuses on the concurrency part of the runtime system, and is intended to set a stage for subsequent discussions. This section also provides would-be GHC RTS hackers a summary of different parts of the system.

Related Work

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