Why Doesn't Go 1.3's Garbage Collector Always Return Memory to the Operating System?

Go 1.3 Garbage Collector Leaks Memory on Server

Question:

Why is the Go 1.3 garbage collector failing to release server memory back to the operating system?

Answer:

The Go runtime periodically runs a garbage collection process that is responsible for deallocating memory that is no longer in use. However, there are a few reasons why the garbage collector may not release all of the memory that is requested back to the OS:

• The heap is freed, but the virtual address space is not: On Unix-based platforms, Go uses a system call to tell the operating system that it can reclaim unused parts of the heap. This facility is not available on Windows platforms.
• Memory is not marked as free: Slices and other data structures may not be marked as freed, but instead in use. This prevents them from being returned to the OS.
• Not all allocated system memory is 'real' memory: Memory that is allocated by the runtime but not actually in use by the program may still be reported as allocated memory by Go's runtime.Stats command. The OS may still have use of this memory internally.

While Go doesn't always shrink its memory space, there are a few possible solutions to mitigate the problem:

• Force garbage collection: Call the runtime.GC() function to manually trigger garbage collection.
• Force OS memory return: Call the runtime.FreeOSMemory function to explicitly return memory to the OS. This will only work after the GC has been run.

It's important to note that not all memory occupied by goroutines is released by the GC.

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