berkil/optimizing-memory-by-ordering-struct-by-type

How to organize the go struct, in order to save memory.

In this article I want to explain you how the memory works internally in a 64 bit architecture, when you instantiate a struct. (You can play with the code https://play.golang.org/p/cUgB54yCpL )

How memory works internally :

When you have a struct like this one :

type myStruct struct {
 myBool   bool    // 1 byte
 myFloat float64  // 8 bytes
 myInt  int32     // 4 bytes
}

As you see a boolean takes 1 byte, a float64 8 bytes, and an int32 4 bytes. But the memory allocates consecutive packet of 8 bytes. So instead of taking 1 + 8 + 4 = 13 bytes. This struct will takes : 24 bytes

a := myStruct{}
fmt.Println(unsafe.Sizeof(a)) // 24 bytes

Because in memory we will have :

So 8 + 8 + 8 = 24 bytes

How to optimize :

It’s possible to optimize, ordering the struct by bytes taken :

type myStructOptimized struct {
 myFloat float64 // 8 bytes
 myBool  bool    // 1 byte
 myInt   int32   // 4 bytes
}

This new struct ordered will take now :

b := myStructOptimized{}
fmt.Println(unsafe.Sizeof(b)) // ordered 16 bytes

16 bytes, because in memory it will be allocated like that (https://play.golang.org/p/gmkrt6X7aM) :

8 + 8 = 16 bytes

As you can see, there are a padding to respect the data alignment, (https://en.wikipedia.org/wiki/Data_structure_alignment), So it means that in some case the data is not optimally word-aligned (depending on many things processor, cache memory, virtual memory) and it can sometimes slower the application, so benchmark everything :D

Links :

https://play.golang.org/p/cUgB54yCpL https://github.com/Tkanos/optimizing-memory-by-ordering-struct-by-type http://www.catb.org/esr/structure-packing/ https://en.wikipedia.org/wiki/Data_structure_alignment