How is that related to memory allocation costs? We are talking about the cost of obtaining a chunk of memory. The cost of actually creating an object is allowed to be much higher because constructors are allowed to execute any arbitrary code.

Just think about how expensive it would be to allocate a 3d vector consisting of 3 floats with malloc() 20000 times and then later deallocate it. Nobody is worrying about the cost of writing 3 floats to RAM. Everyone is worrying about the cost of malloc traversing a free list and it causing memory fragmentation in the process. Meanwhile the arena allocator would be at least as efficient as using an array of 3d vectors.

> How is that related to memory allocation costs?

It's a cost that has to be paid when using bump pointer allocation.

> We are talking about the cost of obtaining a chunk of memory. The cost of actually creating an object is allowed to be much higher because constructors are allowed to execute any arbitrary code.

Accessing main memory is about two orders of magnitude slower than accessing L1. For that time you can run a lot of arbitrary code that accesses data in L1 and registers.

> Just think about how expensive it would be to allocate a 3d vector consisting of 3 floats with malloc() 20000 times and then later deallocate it. Nobody is worrying about the cost of writing 3 floats to RAM. Everyone is worrying about the cost of malloc traversing a free list and it causing memory fragmentation in the process. Meanwhile the arena allocator would be at least as efficient as using an array of 3d vectors.

malloc doesn't mandate free lists, other implementations exist. It's not about relative costs. OP claimed bump pointer allocation to be a "no-op" when it's clearly not.