Comment by cogman10
5 hours ago
> 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.
In the JVM, heap allocations are done via bump allocation.
If that were true then they wouldn't be heap allocations.
https://www.digitalocean.com/community/tutorials/java-jvm-me...
https://docs.oracle.com/en/java/javase/21/core/heap-and-heap...
not possible to do in the JVM, barring primitives
Then you make data structures out of arrays of primitives.
Easy to do? Sure. Easy to do fast? Well, no. That's entirely the reason why C++ has multiple allocators.
I don't know what this means. Vectors are trivial and if you hand out ranges of memory in an arena allocator you allocate it once and free it once which solves the heavy allocation problem. The allocator parameter in templates don't factor in to this.
> If that were true then they wouldn't be heap allocations.
"Heap" is a misnomer. It's not called that due to the classic CS "heap" datastructure. It's called that for the same reason it's called a heap allocation in C++. Modern C++ allocators don't use a heap structure either.
How the JVM does allocations for all it's collectors is in fact a bump allocator in the heap space. There are some weedsy details (for example, threads in the JVM have their own heap space for doing allocation to avoid contention in allocation) but suffice it to say it ultimately translates into a region check then pointer bump. This is why the JVM is so fast at allocation, much faster than C++ can be. [1] [2]
> I don't know what this means.
JVM allocations are typically pointer bumps, adding a number to a register. There's really nothing faster than it. If you are implementing an arena then you've already lost in terms of performance.
[1] https://www.datadoghq.com/blog/understanding-java-gc/#memory...
[2] https://inside.java/2020/06/25/compact-forwarding/