Why Doesn't Go's `unsafe.Sizeof()` Reflect the Actual Memory Usage of Maps with Strings?

Memory Consumption of Strings in Go

When optimizing code that utilizes map[string]string with values limited to "A" or "B," one might assume that a map[string]bool would be more efficient due to its smaller value size. However, upon testing, it was observed that the memory usage for both maps remained unchanged. This discrepancy warrants further exploration.

Go's Internal String Representation

In Go, strings are not stored as contiguous bytes in memory but as a header containing a pointer to the actual data and its length. The unsafe.Sizeof() function, used to determine the size of variables, only retrieves the size of this header, which remains constant regardless of the string's length.

Memory Usage of Maps

Similarly, Go's maps are implemented as pointers, meaning that unsafe.Sizeof() reports the size of the pointer rather than the contents of the map. Therefore, the reported memory usage of both the map[string]string and map[string]bool only reflects the size of their respective pointers.

Determining Actual Memory Requirements

To calculate the actual memory consumption of a map, it is necessary to consider the size of its underlying data structure, including the key-value pairs and any allocated memory. For strings, their memory requirement can be estimated as the sum of their byte length and the header size. However, it is important to note that even if a string is sliced or modified, the underlying backing array may still be retained in memory.

Conclusion

In Go, the unsafe.Sizeof() function does not provide a comprehensive representation of memory usage, particularly for data structures such as maps and strings. When optimizing memory consumption, it is crucial to consider the actual memory requirements of the data structure and its contents.

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