The Mach system was especially designed for distributed systems, which we discuss in Chapter 17,
but Mach was shown to be suitable for systems with fewer processing cores, as evidenced by its inclusion in the Mac OS X system.
The major problem with message systems has generally been poor performance caused by double copying of messages: the message is copied first from the sender to the mailbox and then from the mailbox to the receiver.
The Mach message system attempts to avoid double-copy operations by using virtual-memory-management techniques (Chapter 9).
Essentially, Mach maps the address space containing the sender’s message into the receiver’s address space. The message itself is never actually copied.
This message-management technique provides a large performance boost but works for only intrasystem messages.
The Mach operating system is discussed in more detail in the online Appendix B.