How do allocations work and how do you prevent them?

The go test tool has a profiler which can tell you the amount of allocations you did inside the code.
However, seeing libraries such as this one:
https://github.com/valyala/fasthttp
stating "Zero memory allocations in hot paths"... what does that mean? and how do you achieve this in Go?

I personally don't like their use of language as it sounds like something a marketer would say... All they mean to say is that no allocations will occur in that code because a buffer has been allocated in advance for use there.

So to be clear, they mean 'in this limited scope no allocations will occur'. How do you achieve this? By allocating a sufficiently large buffer in advance of that and then leveraging it in the scope.

The intent of packages author(s) is to speed up request handling by allocating up front at the cost of using more memory (or having a more constant hold on memory at least, in theory the buffer could be the same size as what would need to be allocated).

If you're curious about the implementation details take a look in files like byte_buffer.go and args.go and you'll find that there is a pool of buffer objects allocated in advance so that your handler code doesn't have to do an allocation for the response body ect. Instead you obtain a buffer from the pool (already allocated) and write the response data to it and then when you're done it's released back into the pool for reuse. In the standard scenario you'd instead allocate space for the response body and after the response is returned that object would leave scope and the memory would be freed. As I mentioned in the paragraph above, moving all of this upfront means when your service starts up it will obtain and hold a larger amount of memory than a similar service which used net/http since it will instead obtain and release memory on an as needed basis.