Question

到此，我们获知哪些内存块可被释放。至于如何释放其物理内存，则因操作系统而异。

UNIX-like 系统以 madvise 建议内核解除物理内存映射。从而在保留虚拟地址的情况下，达到释放物理内存的目的。

当这些内存被重新使用时，引发缺页异常，由内核自动补齐所需物理内存。

请注意，madvise 仅是建议，内核未必执行或立即执行。当物理内存充足时，某些内核会忽略该建议，或延后执行。

有鉴于此，相关统计数据未必准确，仅做参考。

// mem_linux.go

var adviseUnused = uint32(_MADV_FREE)

func sysUnused(v unsafe.Pointer, n uintptr) {

	var advise uint32
	if debug.madvdontneed != 0 {
		advise = _MADV_DONTNEED
	} else {
		advise = atomic.Load(&adviseUnused)
	}
    
	if errno := madvise(v, n, int32(advise)); advise == _MADV_FREE && errno != 0 {
		// MADV_FREE was added in Linux 4.5. Fall back to MADV_DONTNEED if it is
		// not supported.
		atomic.Store(&adviseUnused, _MADV_DONTNEED)
		madvise(v, n, _MADV_DONTNEED)
	}
}

func sysUsed(v unsafe.Pointer, n uintptr) {
	sysHugePage(v, n)
}

使用 MADV_FREE 代替 MADV_DONTNEED 以获得更好的提升。

可用 GODEBUG=madvdontneed=1 设置为 MADV_DONTNEED。

// mem_darwin.go

func sysUnused(v unsafe.Pointer, n uintptr) {
	// MADV_FREE_REUSABLE is like MADV_FREE except it also propagates
	// accounting information about the process to task_info.
	madvise(v, n, _MADV_FREE_REUSABLE)
}

func sysUsed(v unsafe.Pointer, n uintptr) {
	madvise(v, n, _MADV_FREE_REUSE)
}

// mem_windows.go

func sysUnused(v unsafe.Pointer, n uintptr) {
	r := stdcall3(_VirtualFree, uintptr(v), n, _MEM_DECOMMIT)
}

func sysUsed(v unsafe.Pointer, n uintptr) {
	p := stdcall4(_VirtualAlloc, uintptr(v), n, _MEM_COMMIT, _PAGE_READWRITE)
}