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There is one common situation where not using mutable globals is a recipe for complexity and a serious code smell. That case is when your internal structures di

There is one common situation where not using mutable globals is a recipe for complexity and a serious code smell. That case is when your internal structures directly control the physical resources of the machine. There is no amount of window dressing that can make these anything but global structures because that is what they literally are. That gets hidden a bit if you delegate resource management to the OS but you can’t do that if you care about performance. And if you are creating and scheduling all of your concurrency in user space, which is common for some types of server software, then passing what are effectively global objects down the call stack becomes a real mess and introduces a number of suboptimal behaviors in the code gen. I’ve seen people try to design database engines, the high-performance kind that directly manage all the resources they use, that don’t use globals in a misguided attempt to adhere to this heuristic. The end result was a convoluted mess of indirection that just obscured the reality that all of those objects were mutable globals. If you care about performance then I/O isn’t very abstract; the code knows exactly what kind of device it is dealing with. I don’t like mutable globals as a general rule, but for some types of software they are unambiguously the correct engineering choice and not using them would be a design defect.

forrestthewoods 5y ago
Maaaaybe. There are definitely resources which are globally unique. But that does not necessarily follow that access should also be global. Rust has some elegant patterns when working with embedded devices. For example GPIO pins could totally be stateful globals. But instead their passed around as types and Rust’s type system + borrow checker ensure correctness. It’s pretty neat. I’ll assume you’re right for databases. My expertise is real-time VR video game type stuff. Which is also high-performance, but of a different variety. I strongly agree that layers of abstraction compound into convoluted and inscrutable garbage. I loathe web development for this very reason.
- jandrewrogers 5y ago
  Rust struggles with software where most of the address space is used for DMA, like many database engines, because it requires ownership and mutability to be observable at compile-time. Also, DMA often does not respect object boundaries as a compiler sees it because DMA does not understand objects. In modern C++ it is straightforward to write wrappers in the style of unique_ptr that safely hide the DMA and life cycle mechanics, which means the average dev using them doesn’t need to know how it works, but someone has to write that code and it is necessarily global heavy because it references physical devices that have their own behavior in your address space. Under the hood, if a physical device is stomping on address space your code accesses, you need a way to both detect that an object is effectively owned by a particular DMA engine before touching it and immediately de-schedule the thread of execution until such a time as there is no concurrent DMA operation that might conflict with the code execution. This happens within a single thread, so no blocking or OS context switching. A big part of database kernel internals is coordination and management of physical resources, which are global by nature.

forrestthewoods 5y ago

Maaaaybe. There are definitely resources which are globally unique. But that does not necessarily follow that access should also be global. Rust has some elegant patterns when working with embedded devices. For example GPIO pins could totally be stateful globals. But instead their passed around as types and Rust’s type system + borrow checker ensure correctness. It’s pretty neat. I’ll assume you’re right for databases. My expertise is real-time VR video game type stuff. Which is also high-performance, but of a different variety. I strongly agree that layers of abstraction compound into convoluted and inscrutable garbage. I loathe web development for this very reason.

jandrewrogers 5y ago
Rust struggles with software where most of the address space is used for DMA, like many database engines, because it requires ownership and mutability to be observable at compile-time. Also, DMA often does not respect object boundaries as a compiler sees it because DMA does not understand objects. In modern C++ it is straightforward to write wrappers in the style of unique_ptr that safely hide the DMA and life cycle mechanics, which means the average dev using them doesn’t need to know how it works, but someone has to write that code and it is necessarily global heavy because it references physical devices that have their own behavior in your address space. Under the hood, if a physical device is stomping on address space your code accesses, you need a way to both detect that an object is effectively owned by a particular DMA engine before touching it and immediately de-schedule the thread of execution until such a time as there is no concurrent DMA operation that might conflict with the code execution. This happens within a single thread, so no blocking or OS context switching. A big part of database kernel internals is coordination and management of physical resources, which are global by nature.