What counts toward an app’s memory limit: why is reserved memory free, a mapped file cheap, and the same file read into a buffer expensive?

Predict first

How much does each way of holding 200 MB change the footprint?

import Foundation
import Darwin

func footprintMB() -> Int {
    var info = task_vm_info_data_t()
    var count = mach_msg_type_number_t(MemoryLayout<task_vm_info_data_t>.size / MemoryLayout<natural_t>.size)
    _ = withUnsafeMutablePointer(to: &info) { $0.withMemoryRebound(to: integer_t.self, capacity: Int(count)) { task_info(mach_task_self_, task_flavor_t(TASK_VM_INFO), $0, &count) } }
    return Int(info.phys_footprint / 1_048_576)
}
func touch(_ p: UnsafeMutableRawPointer, _ size: Int) { var i = 0; while i < size { p.storeBytes(of: 1, toByteOffset: i, as: UInt8.self); i += 4096 } }
func touchRead(_ p: UnsafeRawPointer, _ size: Int) -> Int { var s = 0; var i = 0; while i < size { s &+= Int(p.load(fromByteOffset: i, as: UInt8.self)); i += 4096 }; return s }
let size = 200 * 1_048_576

let base = footprintMB()
let anon = mmap(nil, size, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, -1, 0)!
print("reserved 200 MB, untouched: +\(footprintMB() - base) MB")
touch(anon, size)
print("every page written: +\(footprintMB() - base) MB")
munmap(anon, size)
print("after munmap: +\(footprintMB() - base) MB")

let path = NSTemporaryDirectory() + "big-\(getpid()).bin"
FileManager.default.createFile(atPath: path, contents: Data((0..<size).lazy.map { UInt8(truncatingIfNeeded: $0 &* 31) }))
let b2 = footprintMB()
let fd = open(path, O_RDONLY)
let mapped = mmap(nil, size, PROT_READ, MAP_PRIVATE, fd, 0)!
_ = touchRead(mapped, size)
print("file mapped read-only, every page read: +\(footprintMB() - b2) MB")
munmap(mapped, size)
let b3 = footprintMB()
let buf = mmap(nil, size, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, -1, 0)!
_ = read(fd, buf, size)
print("same file read(2) into memory: +\(footprintMB() - b3) MB")
munmap(buf, size); close(fd)
try? FileManager.default.removeItem(atPath: path)

• reserved 0 / written +200 / munmap 0 / mapped file read 0 / read(2) copy +200
• Every step adds 200 MB
• Mapping adds 200 MB but read(2) adds 0