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Concurrency in the D Programming Language

Andrei Alexandrescu explains recent hardware changes allowing concurrency and how the D programming languages addresses these possibilities.
This chapter is from the book

This chapter is from the book

Convergence of various factors in the hardware industry has led to qualitative changes in the way we are able to access computing resources, which in turn prompts profound changes in the ways we approach computing and in the language abstractions we use. Concurrency is now virtually everywhere, and it is software's responsibility to tap into it.

Although the software industry as a whole does not yet have ultimate responses to the challenges brought about by the concurrency revolution, D's youth allowed its creators to make informed decisions regarding concurrency without being tied down by obsoleted past choices or large legacy code bases. A major break with the mold of concurrent imperative languages is that D does not foster sharing of data between threads; by default, concurrent threads are virtually isolated by language mechanisms. Data sharing is allowed but only in limited, controlled ways that offer the compiler the ability to provide strong global guarantees.

At the same time, D remains at heart a systems programming language, so it does allow you to use a variety of low-level, maverick approaches to concurrency. (Some of these mechanisms are not, however, allowed in safe programs.)

In brief, here's how D's concurrency offering is layered:

  • The flagship approach to concurrency is to use isolated threads or processes that communicate via messages. This paradigm, known as message passing, leads to safe and modular programs that are easy to understand and maintain. A variety of languages and libraries have used message passing successfully. Historically message passing has been slower than approaches based on memory sharing—which explains why it was not unanimously adopted—but that trend has recently undergone a definite and lasting reversal. Concurrent D programs are encouraged to use message passing, a paradigm that benefits from extensive infrastructure support.
  • D also provides support for old-style synchronization based on critical sections protected by mutexes and event variables. This approach to concurrency has recently come under heavy criticism because of its failure to scale well to today's and tomorrow's highly parallel architectures. D imposes strict control over data sharing, which in turn curbs lock-based programming styles. Such restrictions may seem quite harsh at first, but they cure lock-based code of its worst enemy: low-level data races. Data sharing remains, however, the most efficient means to pass large quantities of data across threads, so it should not be neglected.
  • In the tradition of system-level languages, D programs not marked as @safe may use casts to obtain hot, bubbly, unchecked data sharing. The correctness of such programs becomes largely your responsibility.
  • If that level of control is insufficient for you, you can use asm statements for ultimate control of your machine's resources. To go any lower-level than that, you'd need a miniature soldering iron and a very, very steady hand.

Before getting into the thick of these topics, let's take a brief detour in order to gain a better understanding of the hardware developments that have shaken our world.

13.1 Concurrentgate

When it comes to concurrency, we are living in the proverbial interesting times more than ever before. Interesting times come in the form of a mix of good and bad news that contributes to a complex landscape of trade-offs, forces, and trends.

The good news is that density of integration is still increasing by Moore's law; with what we know and what we can reasonably project right now, that trend will continue for at least one more decade after the time of this writing. Increased miniaturization begets increased computing power density because more transistors can be put to work together per area unit. Since components are closer together, connections are also shorter, which means faster local interconnectivity. It's an efficiency bonanza.

Unfortunately, there are a number of sentences starting with "unfortunately" that curb the enthusiasm around increased computational density. For one, connectivity is not only local—it forms a hierarchy [16]: closely connected components form units that must connect to other units, forming larger units. In turn, the larger units also connect to other larger units, forming even larger functional blocks, and so on. Connectivity-wise, such larger blocks remain "far away" from each other. Worse, increased complexity of each block increases the complexity of connectivity between blocks, which is achieved by reducing the thickness of wires and the distance between them. That means an increase of resistance, capacity, and crosstalk. Resistance and capacity worsen propagation speed in the wire. Crosstalk is the propensity of the signal in one wire to propagate to a nearby wire by (in this case) electromagnetic field. At high frequencies, a wire is just an antenna and crosstalk becomes so unbearable that serial communication increasingly replaces parallel communication (a somewhat counterintuitive phenomenon visible at all scales—USB replaced the parallel port, SATA replaced PATA as the disk data connector, and serial buses are replacing parallel buses in memory subsystems, all because of crosstalk. Where are the days when parallel was fast and serial was slow?).

Also, the speed gap between processing elements and memory is also increasing. Whereas memory density has been increasing at predictably the same rate as general integration density, its access speed is increasingly lagging behind computation speed for a variety of physical, technological, and market-related reasons [22]. It is unclear at this time how the speed gap could be significantly reduced, and it is only growing. Hundreds of cycles may separate the processor from a word in memory; only a few years ago, you could buy "zero wait states" memory chips accessible in one clock cycle.

The existence of a spectrum of memory architectures that navigate different trade-offs among density, price, and speed, has caused an increased sophistication of memory hierarchies; accessing one memory word has become a detective investigation that involves questioning several cache levels, starting with precious on-chip static RAM and going possibly all the way to mass storage. Conversely, a given datum could be found replicated in a number of places throughout the cache hierarchy, which in turn influences programming models. We can't afford anymore to think of memory as a big, monolithic chunk comfortably shared by all processors in a system: caches foster local memory traffic and make shared data an illusion that is increasingly difficult to maintain [37].

In related, late-breaking news, the speed of light has obstinately decided to stay constant (immutable if you wish) at about 300,000,000 meters per second. The speed of light in silicon oxide (relevant to signal propagation inside today's chips) is about half that, and the speed we can achieve today for transmitting actual data is significantly below that theoretical limit. That spells more trouble for global interconnectivity at high frequencies. If we wanted to build a 10GHz chip, under ideal conditions it would take three cycles just to transport a bit across a 4.5-centimeter-wide chip while essentially performing no computation.

In brief, we are converging toward processors of very high density and huge computational power that are, however, becoming increasingly isolated and difficult to reach and use because of limits dictated by interconnectivity, signal propagation speed, and memory access speed.

The computing industry is naturally flowing around these barriers. One phenomenon has been the implosion of the size and energy required for a given computational power; today's addictive portable digital assistants could not have been fabricated at the same size and capabilities with technology only five years old. Today's trends, however, don't help traditional computers that want to achieve increased computational power at about the same size. For those, chip makers decided to give up the battle for faster clock rates and instead decided to offer computing power packaged in already known ways: several identical central processing unit (CPUs) connected to each other and to memory via buses. Thus, in a matter of a few short years, the responsibility for making computers faster has largely shifted from the hardware crowd to the software crowd. More CPUs may seem like an advantageous proposition, but for regular desktop computer workloads it becomes tenuous to gainfully employ more than around eight processors. Future trends project an exponential expansion of the number of available CPUs well into the dozens, hundreds, and thousands. To speed up one given program, a lot of hard programming work is needed to put those CPUs to good use.

The computing industry has always had moves and shakes caused by various technological and human factors, but this time around we seem to be at the end of the rope. Since only a short time ago, taking a vacation is not an option for increasing the speed of your program. It's a scandal. It's an outrage. It's Concurrentgate.

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