Comment by cogman10
3 hours ago
Well, let me just circle back to the start of this comment chain.
> Many programs in GC language end up fighting the GC by allocating a large buffer and managing it by hand
That's the primary thing I'm contending with. This is a strategy for fighting the GC, but it's also generally a bad strategy. One that I think gets pulled more because someone heard of the suggestion and less because it's a good way to make things faster.
That guy I'm talking about did a lot of "performance optimizations" based on gut feelings and not data. I've observed that a lot of engineers operate that way.
But I've further observed that when it comes to optimizing for the GC, a large amount of problems don't need such an extreme measure like building your own memory buffer and managing it directly. In fact, that sort of a measure is generally counter productive in a GC environment as it makes major collections more costly. It isn't a "never do this" thing, but it's also not something that "many programs" should be doing.
I agree that many programs with a GC will probably need to change their algorithms to minimize allocations. I disagree that "allocating a large buffer and managing it by hand" is a technique that almost any program or library needs to engage in to minimize GCs.
This is a strategy for fighting the GC, but it's also generally a bad strategy.
Allocating a large buffer is literally what an array or vector is. A heap uses a heap structure and hops around in memory for every allocation and free. It gets worse the more allocations there are. The allocations are fragmented and in different parts of memory.
Allocating a large buffer takes care of all this if it is possible to anything else. It doesn't make sense to make lots of heap allocations when what you want is multiple items next to each other in memory and one heap allocation.
That guy I'm talking about did a lot of "performance optimizations" based on gut feelings and not data.
You need to let this go, that guy has nothing to do with what works when optimizing memory usage and allocation.
But I've further observed that when it comes to optimizing for the GC, a large amount of problems don't need such an extreme measure like building your own memory buffer and managing it directly.
Making an array of contiguous items is not an "extreme strategy", it's the most efficient and simplest way for a program to run. Other memory allocations can just be an extension of this.
I agree that many programs with a GC will probably need to change their algorithms to minimize allocations. I disagree that "allocating a large buffer and managing it by hand"
If you need the same amount of memory but need to minimize allocations how do you think that is done? You make larger allocations and split them up. You keep saying "managing it by hand" as if there is something that has to be tricky or difficult. Using indices of an array is not difficult and neither is handing out indices or ranges to in small sections.
> A heap uses a heap structure and hops around in memory for every allocation and free.
Not in the JVM. And maybe this is ultimately what we are butting up against. After all, the JVM isn't all GCed languages, it's just one of many.
In the JVM, heap allocations are done via bump allocation. When a region is filled, the JVM performs a garbage collection which moves objects in the heap (it compacts the memory). It's not an actual heap structure for the JVM.
> It doesn't make sense to make lots of heap allocations when what you want is multiple items next to each other in memory and one heap allocation.
That is (currently) not possible to do in the JVM, barring primitives. When I create a `new Foo[128]` in the JVM, that creates an array big enough to hold 128 references of Foo, not 128 Foo objects. Those have to be allocated onto the heap separately. This is part of the reason why managing such an object pool is pointless in the JVM. You have to make the allocations anyways and you are paying for the management cost of that pool.
The object pool is also particularly bad in the JVM because it stops the JVM from performing optimizations like scalarization. That's where the JVM can completely avoid a heap allocation all together and instead pulls out the internal fields of the allocated object to hand off to a calling function. In order for that optimization to occur, and object can't escape the current scope.
I get why this isn't the same story if you are talking about another language like C# or go. There are still the negative consequences of needing to manage the buffer, especially if the intent is to track allocations of items in the buffer and to reassign them. But there is a gain in the locality that's nice.
> Using indices of an array is not difficult and neither is handing out indices or ranges to in small sections.
Easy to do? Sure. Easy to do fast? Well, no. That's entirely the reason why C++ has multiple allocators. It's the crux of the problem an allocator is trying to solve in the first place "How can I efficiently give a chunk of memory back to the application".
Obviously, it'll matter what your usage pattern is, but if it's at all complex, you'll run into the same problems that the general allocator hits.