Editing Mach (kernel) (section)

===Potential solutions===
IPC overhead is a major issue for Mach 3 systems. However, the concept of a ''multi-server operating system'' is still promising, though it still requires some research. The developers have to be careful to isolate code into modules that do not call from server to server. For instance, the majority of the networking code would be placed in a single server, thereby minimizing IPC for normal networking tasks.

Most developers instead stuck with the original POE concept of a single large server providing the operating system functionality.<ref name="c6CLw">{{cite web|url=https://www.cs.cmu.edu/afs/cs/project/mach/public/doc/unpublished/poe_notes.ps|title=A Brief Description of the POE server|author=Mary Thompson|date=April 14, 1994}}</ref> In order to ease development, they allowed the operating system server to run either in user-space or kernel-space. This allowed them to develop in user-space and have all the advantages of the original Mach idea, and then move the debugged server into kernel-space in order to get better performance. Several operating systems have since been constructed using this method, known as ''co-location'', among them [[Lites]], [[MkLinux]], [[OSF/1]], and NeXTSTEP/OPENSTEP/macOS. The [[ChorusOS|Chorus microkernel]] made this a feature of the basic system, allowing servers to be raised into the kernel space using built-in mechanisms.

Mach 4 attempted to address these problems with a more radical set of upgrades. In particular, it was found that program code was typically not writable, so potential hits due to copy-on-write were rare. Thus it made sense to not map the memory between programs for IPC, but instead migrate the program code being used into the local space of the program. This led to the concept of "shuttles" and it seemed performance had improved, but the developers moved on with the system in a semi-usable state. Mach 4 also introduced built-in co-location primitives, making it a part of the kernel.

By the mid-1990s, work on microkernel systems was largely stagnant, although the market [[Tanenbaum–Torvalds debate|had generally believed]] that all modern operating systems would be microkernel based by the 1990s. The primary remaining widespread uses of the Mach kernel are Apple's macOS and its sibling iOS, which run atop a heavily modified [[hybrid kernel|hybrid]] Open Software Foundation Mach Kernel (OSFMK 7.3) called "[[XNU]]"<ref name="wwdc-2000-session-106">{{cite AV media|url=https://www.youtube.com/watch?v=ggnFoDqzGMU |archive-url=https://ghostarchive.org/varchive/youtube/20211211/ggnFoDqzGMU| archive-date=December 11, 2021 |url-status=live|author=Jim Magee|title=WWDC 2000 Session 106 - Mac OS X: Kernel|minutes=14}}{{cbignore}}</ref> also used in OSF/1.<ref name=":5">{{cite conference |url=https://pdfs.semanticscholar.org/03ac/1296f530719497b49d7580b55a2d9b8353ab.pdf|archive-url=https://web.archive.org/web/20170822053715/https://pdfs.semanticscholar.org/03ac/1296f530719497b49d7580b55a2d9b8353ab.pdf|archive-date=August 22, 2017|url-status=dead|title=A Trusted, Scalable, Real-Time Operating System Environment |conference=1994 IEEE Dual-Use Technologies and Applications Conference |author=Douglas M. Wells|year=1994 |s2cid=5205380}}</ref> In XNU, the file systems, networking stacks, and process and memory management functions are implemented in the kernel; and file system, networking, and some process and memory management functions are invoked from user mode via ordinary [[system call]]s rather than message passing;<ref name="m9l5s">{{cite web|url=https://developer.apple.com/library/mac/documentation/Darwin/Conceptual/KernelProgramming/Architecture/Architecture.html#//apple_ref/doc/uid/TP30000905-CH1g-CACDAEDC|title=Kernel Architecture Overview|work=Kernel Programming Guide|publisher=[[Apple Inc.]]|date=August 8, 2013|access-date=March 3, 2015}}</ref><ref name="HytGhy">{{cite web|url=https://developer.apple.com/library/mac/documentation/Darwin/Conceptual/KernelProgramming/boundaries/boundaries.html#//apple_ref/doc/uid/TP30000905-CH217-BEHJDFCA|title=Boundary Crossings|work=Kernel Programming Guide|publisher=Apple Inc.|date=August 8, 2013|access-date=March 3, 2015}}</ref> XNU's Mach messages are used for communication between user-mode processes, and for some requests from user-mode code to the kernel and from the kernel to user-mode servers.