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Parallel

The Case for D


Memory Model and Manycores

Given that D can call C functions directly, it may seem that D builds straight on C's memory model. That might be good news if it weren't for the pink elephant in the room dangerously close to that Ming-dynasty vase: manycores -- massively parallel architectures that throw processing power at you like it's going out of style, if only you could use it. Manycores are here, and C's way of dealing with them is extremely pedestrian and error prone. Other procedural and object-oriented languages made only little improvements, a state of affairs that marked a recrudescence of functional languages that rely on immutability to elegantly sidestep many parallelism-related problems.

Being a relative newcomer, D is in the enviable position of placing a much more informed bet when it comes to threading. And D bets the farm on a memory model that is in a certain way radically different from many others. You see, old-school threads worked like this: you call a primitive to start a new thread and the new thread instantly sees and can touch any data in the program. Optionally and with obscure OS-dependent means, a thread could also acquire the so-called thread-private data for its own use. In short, memory is by default shared across all threads. This has caused problems yesterday, and it makes today a living hell. Yesterday's problems were caused by the unwieldy nature of concurrent updates: it's very hard to properly track and synchronize things such that data stays in good shape throughout. But people were putting up with it because the notion of shared memory was a close model to the reality in hardware, and as such was efficient when gotten right. Now is where we're getting to the "living hell" part -- nowadays, memory is in fact increasingly less shared. Today's reality in hardware is that processors communicate with memory through a deep memory hierarchy, a large part of which is private to each core. So not only shared memory is hard to work with, it turns out to be quickly becoming the slower way of doing things because it is increasingly removed from physical reality.

While traditional languages were wrestling with all of these problems, functional languages took a principled position stemming from mathematical purity: we're not interested in modeling hardware, they said, but we'd like to model true math. And math for the most part does not have mutation and is time-invariant, which makes it an ideal candidate for parallel computing. (Imagine the moment when one of those first mathematicians-turned-programmers has heard about parallel computing -- they must have slapped their forehead: 'Wait a minute!. . . ') It was well noted in functional programming circles that such a computational model does inherently favor out-of-order, parallel execution, but that potential was more of a latent energy than a realized goal until recent times. Today, it becomes increasingly clear that a functional, mutation-free style of writing programs will be highly relevant for at least parts of a serious application that wants to tap into parallel processing.

So where's D positioning itself in all this? There's one essential concept forming the basis of D's approach to parallelism:

Memory is thread-private by default, shared on demand.

In D, all memory is by default private to the thread using it; even unwitting globals are allocated per-thread. When sharing is desired, objects can be qualified with shared, which means that they are visible from several threads at once. Crucially, the type system knows about shared data and limits what can be done with it to ensure that proper synchronization mechanisms are used throughout. This model avoids very elegantly a host of thorny problems related to synchronization of access in default-shared threaded languages. In those languages, the type system has no idea which data is supposed to be shared and which isn't so it often relies on the honor system -- the programmer is trusted to annotate shared data appropriately. Then complicated rules explain what happens in various scenarios involving unshared data, shared annotated data, data that's not annotated yet still shared, and combinations of the above -- in a very clear manner so all five people who can understand them will understand them, and everybody calls it a day.

Support for manycores is a very active area of research and development, and a good model has not been found yet. Starting with the solid foundation of a default-private memory model, D is incrementally deploying amenities that don't restrict its options: pure functions, lock-free primitives, good old lock-based programming, message queues (planned), and more. More advanced features such as ownership types are being discussed.


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