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I don't see anything special in Go for commanding the GC, can you point me to these APIs?

by Locke1689 14y ago

enneff 14y ago
It's not about commanding the GC. It's about avoiding unnecessary allocations.
- Locke1689 14y ago
  OK, can you be specific how this is done? What is the technical reason here?
  - enneff 14y ago
    For instance, in Go everything is passed by value. If you want to pass a reference, you must pass a reference type (like a pointer). This means that you can pass around structs without having to put them on the heap. In JVM-land, nearly everything is passed by reference, and the data must go on the heap. Relatedly, in Go if you allocate an array of n values of type T, the size of that array in memory is n * sizeof(T), and the items are laid out sequentially in memory. There are no other bookkeeping data structures and no indirection. Here's a story about one project's struggle with data structures and Java: http://marc.info/?l=git&m=124111702609723 http://marc.info/?l=git&m=124111702609723 Relevant quote: JGit struggles with not having an efficient way to represent a SHA-1. C can just say "unsigned char[20]" and have it inline into the container's memory allocation. A byte[20] in Java will cost an *additional* 16 bytes of memory, and be slower to access because the bytes themselves are in a different area of memory from the container object. We try to work around it by converting from a byte[20] to 5 ints, but that costs us machine instructions. In Go, you would just write [20]byte. If you wanted a struct with a field of [20]byte you can do so, and all it costs is the 20 bytes.
    - Locke1689 14y ago
      The JVM does not support this, but the CLR does.
zaphar 14y ago
Go's memory model has similarities to C. For instance you can use the heap or the stack. Go figures out which one an allocation should live in by doing escape analysis. If it never leaves the function it goes on the stack. If it does it goes on the heap. GC ensures anything on the heap gets cleaned up. It's not an API.
- pcwalton 14y ago
  To be fair, Java does escape analysis for this too. This isn't a difference between Java and Go.
- Locke1689 14y ago
  Both the JVM and the CLR do this. This is not a Go technology. In fact, in certain cases the JVM can detect type information and remove the allocation entirely by hoisting integer fields and the like into registers.
- ericbb 14y ago
  It's not about escape analysis. The programmer explicitly specifies when to use pointers. Whenever pointers are not used, the object is stored in-place (for example, on the stack, in the array or slice, or in the struct).

I don't see anything special in Go for commanding the GC, can you point me to these APIs?

enneff 14y ago

It's not about commanding the GC. It's about avoiding unnecessary allocations.

Locke1689 14y ago
OK, can you be specific how this is done? What is the technical reason here?
- enneff 14y ago
  For instance, in Go everything is passed by value. If you want to pass a reference, you must pass a reference type (like a pointer). This means that you can pass around structs without having to put them on the heap. In JVM-land, nearly everything is passed by reference, and the data must go on the heap. Relatedly, in Go if you allocate an array of n values of type T, the size of that array in memory is n * sizeof(T), and the items are laid out sequentially in memory. There are no other bookkeeping data structures and no indirection. Here's a story about one project's struggle with data structures and Java: http://marc.info/?l=git&m=124111702609723 http://marc.info/?l=git&m=124111702609723 Relevant quote: JGit struggles with not having an efficient way to represent a SHA-1. C can just say "unsigned char[20]" and have it inline into the container's memory allocation. A byte[20] in Java will cost an *additional* 16 bytes of memory, and be slower to access because the bytes themselves are in a different area of memory from the container object. We try to work around it by converting from a byte[20] to 5 ints, but that costs us machine instructions. In Go, you would just write [20]byte. If you wanted a struct with a field of [20]byte you can do so, and all it costs is the 20 bytes.
  - Locke1689 14y ago
    The JVM does not support this, but the CLR does.

zaphar 14y ago

Go's memory model has similarities to C. For instance you can use the heap or the stack. Go figures out which one an allocation should live in by doing escape analysis. If it never leaves the function it goes on the stack. If it does it goes on the heap. GC ensures anything on the heap gets cleaned up. It's not an API.

pcwalton 14y ago
To be fair, Java does escape analysis for this too. This isn't a difference between Java and Go.
Locke1689 14y ago
Both the JVM and the CLR do this. This is not a Go technology. In fact, in certain cases the JVM can detect type information and remove the allocation entirely by hoisting integer fields and the like into registers.
ericbb 14y ago
It's not about escape analysis. The programmer explicitly specifies when to use pointers. Whenever pointers are not used, the object is stored in-place (for example, on the stack, in the array or slice, or in the struct).