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The fundamental services and primitives ofthe OS X kernel are based on Mach3.0. Apple has modified and extended Mach to better meet OS X functional and performance goals.
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Mach 3.0 was originally conceived as a simple, extensible,communications microkernel. It iscapable of running as a stand–alone kernel, with other traditionaloperating-system services such as I/O, file systems, and networkingstacks running as user-mode servers.
However, in OS X, Mach is linked with other kernel componentsinto a single kernel address space. This is primarily for performance;it is much faster to make a direct call between linked componentsthan it is to send messages or do remote procedure calls (RPC) betweenseparate tasks. This modular structure results in a more robustand extensible system than a monolithic kernel would allow, withoutthe performance penalty of a pure microkernel.
Thus in OS X, Mach is not primarily a communication hubbetween clients and servers. Instead, its value consists of itsabstractions, its extensibility, and its flexibility. In particular,Mach provides
object-basedAPIs with communication channels (for example, ports) as object references
highly parallel execution, including preemptively scheduled threads and support for SMP
a flexible scheduling framework, with support for real-time usage
a complete set of IPC primitives, including messaging, RPC, synchronization, and notification
support for large virtual address spaces, shared memoryregions, and memory objects backed by persistent store
proven extensibility and portability, for example across instructionset architectures and in distributed environments
security and resource management as a fundamental principleof design; all resources are virtualized
Mach Kernel Abstractions
Mach provides a small set of abstractions that have been designedto be both simple and powerful. These are the main kernel abstractions:
Tasks. Theunits of resource ownership; each task consists of a virtual addressspace, a portrightnamespace, and one or more threads.(Similar to a process.)
Threads. The units of CPU execution withina task.
Addressspace. In conjunction with memory managers, Mach implementsthe notion of a sparse virtual address space and shared memory.
Memoryobjects. The internal units of memory management. Memoryobjects include named entries and regions; they are representationsof potentially persistent data that may be mapped into address spaces.
Ports.Secure, simplex communication channels, accessible only via sendand receive capabilities (known as port rights).
IPC.Message queues, remote procedure calls, notifications, semaphores,and lock sets.
Time.Clocks, timers, and waiting.
At the trap level, the interface to most Mach abstractionsconsists of messages sent to and from kernel ports representingthose objects. The trap-level interfaces (such as mach_msg_overwrite_trap)and message formats are themselves abstracted in normal usage bythe Mach Interface Generator(MIG).MIG is used to compile procedural interfaces to the message-basedAPIs, based on descriptions of those APIs.
Tasks and Threads
OS X processes and POSIXthreads (pthreads)are implemented on top of Mach tasks and threads, respectively.A thread is a point of control flow in a task. A task exists to provideresources for the threads it contains. This split is made to providefor parallelism and resource sharing.
A thread
is a pointof control flow in a task.
has access to all of the elements of the containing task.
executes (potentially) in parallel with other threads, eventhreads within the same task.
has minimal state information for low overhead.
A task
is a collectionof system resources. These resources, with the exception of theaddress space, are referenced by ports. These resources may be sharedwith other tasks if rights to the ports are so distributed.
provides a large, potentially sparse address space, referencedby virtual address. Portions of this space may be shared throughinheritance or external memory management.
contains some number of threads.
Note that a task has no life of its own—only threads executeinstructions. When it is said that “task Y does X,” what isreally meant is that “a thread contained within task Y does X.”
A task is a fairly expensive entity. It exists to be a collectionof resources. All of the threads in a task share everything. Twotasks share nothing without an explicit action (although the actionis often simple) and some resources (such as port receive rights) cannotbe shared between two tasks at all.
A thread is a fairly lightweight entity. It is fairly cheapto create and has low overhead to operate. This is true becausea thread has little state information (mostly its register state). Itsowning task bears the burdenof resource management. On a multiprocessor computer, it is possiblefor multiple threads in a task to execute in parallel. Even whenparallelism is not the goal, multiple threads have an advantagein that each threadcan use a synchronous programming style, instead of attempting asynchronousprogramming with a single thread attempting to provide multipleservices.
A threadis the basic computational entity. A thread belongs to one and onlyone task that defines its virtual address space. To affect the structureof the address space or to reference any resource other than theaddress space, the thread must execute a special trap instructionthat causes the kernel to perform operations on behalf of the threador to send a message to some agent on behalf of the thread. In general,these traps manipulate resources associated with the task containingthe thread. Requests can be made of the kernel to manipulate theseentities: to create them, delete them, and affect their state.
Mach provides a flexible framework for thread–schedulingpolicies. Early versions of OS X support both time-sharing and fixed-priority policies.A time-sharing thread’s priority is raised and lowered to balanceits resource consumption against other time-sharing threads.
Fixed-priority threads execute for a certain quantum of time, and then areput at the end of the queue of threads of equal priority. Settinga fixed priority thread’s quantum level to infinity allows thethread to run until it blocks, or until it is preempted by a threadof higher priority. High priority real-time threads are usuallyfixed priority.
OS X also provides time constraint scheduling for real-timeperformance. This scheduling allows you to specify that your threadmust get a certain time quantum within a certain period of time.
Mach scheduling is described further in Mach Scheduling and Thread Interfaces.
Ports, Port Rights, Port Sets,and Port Namespaces
With the exception of the task’s virtual address space,all other Mach resources are accessed through a level of indirectionknown as a port.A port is an endpoint of a unidirectional communication channelbetween a client who requests a service and a server who providesthe service. If a reply is to be provided to such a service request,a second port must be used. This is comparable to a (unidirectional)pipe in UNIX parlance.
In most cases, the resource that is accessed by the port (thatis, named by it) is referred to as an object. Most objects namedby a port have a single receiver and (potentially) multiple senders.That is, there is exactly one receive port, and at least one sendingport, for a typical object such as a message queue.
The service to be provided by an object is determined by themanager that receives the request sent to the object. It followsthat the kernel is the receiver for ports associated with kernel-providedobjects and that the receiver for ports associated with task-provided objectsis the task providing those objects.
For ports that name task-provided objects, it is possibleto change the receiver of requests for that port to a differenttask, for example by passing the port to that task in a message. Asingle task may have multiple ports that refer to resources it supports.For that matter, any given entity can have multiple ports that representit, each implying different sets of permissible operations. Forexample, many objects have a name port anda controlport (sometimes called the privileged port).Access to the control port allows the object to be manipulated;access to the name port simply names the object so that you canobtain information about it or perform other non-privileged operationsagainst it.
Tasks have permissions to access ports in certain ways (send,receive, send-once); these are called port rights. A port can be accessed only via a right. Ports are often usedto grant clients access to objects within Mach. Having the rightto send to the object’s IPC port denotes the right to manipulatethe object in prescribed ways. As such, port right ownership isthe fundamental security mechanismwithin Mach. Having a right to an object is to have a capabilityto access or manipulate that object.
Port rights can be copied and moved between tasks via IPC. Doing so,in effect, passes capabilities to some object or server.
One type of object referred to by a port is a port set.As the name suggests, a port set is a set of port rights that canbe treated as a single unit when receiving a message or event fromany of the members of the set. Port sets permit one thread to waiton a number of message and event sources, for example in work loops.
Traditionally in Mach, the communication channel denoted bya port was always a queue of messages.However, OS X supports additional types of communication channels, andthese new types of IPC object are also represented by ports andport rights. See the section Interprocess Communication (IPC),for more details about messages and other IPC types.
Ports and port rights do not have systemwide names that allowarbitrary ports or rights to be manipulated directly. Ports canbe manipulated by a task only if the task has a port right in itsport namespace. A port right is specified by a port name, an integerindex into a 32-bit portnamespace. Each task has associated with it a single port namespace.
Tasks acquire port rights when another task explicitly insertsthem into its namespace, when they receive rights in messages, bycreating objects that return a right to the object, and via Machcalls for certain special ports (mach_thread_self, mach_task_self,and mach_reply_port.)
Memory Management
As with most modern operating systems, Mach provides addressingto large, sparse, virtual address spaces. Runtime access is madevia virtual addresses that may not correspond to locations in physicalmemory at the initial time of the attempted access. Mach is responsiblefor taking a requested virtual address and assigning it a correspondinglocation in physical memory. It does so through demand paging.
A range of a virtual address space is populated with datawhen a memory object is mapped into that range. All data in an addressspace is ultimately provided through memory objects. Mach asks theowner of a memory object (apager)for the contents of a page when establishing it in physical memoryand returns the possibly modified data to the pager before reclaimingthe page. OS X includes two built-in pagers—the defaultpager and the vnode pager.
The default pager handles nonpersistent memory, known as anonymousmemory. Anonymous memory is zero-initialized, and it existsonly during the life of a task. The vnode pager maps files intomemory objects. Mach exports an interface to memory objects to allowtheir contents to be contributed by user-mode tasks. This interfaceis known as the External Memory Management Interface, or EMMI.
The memory management subsystem exports virtual memory handlesknown as named entries or namedmemory entries. Like most kernel resources, these aredenoted by ports. Having a named memory entry handle allows theowner to map the underlying virtual memory object or to pass theright to map the underlying object to others. Mapping a named entryin two different tasks results in a shared memory window betweenthe two tasks, thus providing a flexible method for establishingshared memory.
Beginning in OS X v10.1, the EMMI systemwas enhanced to support “portless” EMMI. In traditional EMMI,two Mach ports were created for each memory region, and likewise twoports for each cached vnode. Portless EMMI, in its initial implementation,replaces this with direct memory references (basically pointers).In a future release, ports will be used for communication with pagersoutside the kernel, while using direct references for communicationwith pagers that reside in kernel space. The net result of thesechanges is that early versions of portless EMMI do not support pagersrunning outside of kernel space. This support is expected to bereinstated in a future release.
Address ranges of virtual memory space may also be populatedthrough direct allocation (using vm_allocate).The underlying virtual memory object is anonymous and backed by thedefault pager. Shared ranges of an address space may also be setup via inheritance. When new tasks are created, they are clonedfrom a parent. This cloning pertains to the underlying memory addressspace as well. Mapped portions of objects may be inherited as acopy, or as shared, or not at all, based on attributes associatedwith the mappings. Mach practices a form of delayed copy known as copy-on-write tooptimize the performance of inherited copies on task creation.
Rather than directly copying the range, a copy-on-write optimization is accomplishedby protected sharing. The two tasks share the memory to be copied,but with read-only access. When either task attempts to modify aportion of the range, that portion is copied at that time. Thislazy evaluation of memory copies is an important optimization thatpermits simplifications in several areas, notably the messaging APIs.
One other form of sharing is provided by Mach, through theexport of namedregions. A named region is a form of a named entry, butinstead of being backed by a virtual memory object, it is backedby a virtual map fragment. This fragment may hold mappings to numerousvirtual memory objects. It is mappable into other virtual maps,providing a way of inheriting not only a group of virtual memoryobjects but also their existing mapping relationships. This featureoffers significant optimization in task setup, for example when sharinga complex region of the address space used for shared libraries.
Interprocess Communication(IPC)
Communication between tasks is an important element of theMach philosophy. Mach supports a client/server system structurein which tasks (clients) access services by making requests of othertasks (servers) via messages sent over a communication channel.
The endpoints of these communication channels in Mach arecalled ports, while port rights denote permission to use the channel.The forms of IPC provided by Mach include
messagequeues
semaphores
notifications
lock sets
remote procedure calls (RPCs)
The type of IPC object denoted by the port determines theoperations permissible on that port, and how (and whether) datatransfer occurs.
Important: The IPCfacilities in OS X are in a state of transition. In early versionsof the system, not all of these IPC types may be implemented.
There are two fundamentally different Mach APIs for raw manipulationof ports—the mach_ipc familyand the mach_msg family.Within reason, both families may be used with any IPC object; however,the mach_ipc calls arepreferred in new code. The mach_ipc calls maintainstate information where appropriate in order to support the notionof a transaction. The mach_msg callsare supported for legacy code but deprecated; they are stateless.
IPC Transactions and EventDispatching
When a thread calls mach_ipc_dispatch,it repeatedly processes events coming in on the registered portset. These events could be an argument block from an RPCobject (as the results of a client’s call), a lock object beingtaken (as a result of some other thread’s releasing the lock),a notification or semaphore being posted, or a message coming infrom a traditional message queue.
These events are handled via callouts from mach_msg_dispatch.Some events imply a transaction during the lifetime of the callout.In the case of a lock, the state is the ownership of the lock. Whenthe callout returns, the lock is released. In the case of remoteprocedure calls, the state is the client’s identity, the argumentblock, and the reply port. When the callout returns, the reply issent.
When the callout returns, the transaction (if any) is completed,and the thread waits for the next event. The mach_ipc_dispatch facilityis intended to support work loops.
Message Queues
Originally, the sole style of interprocess communication inMach was the messagequeue. Only one task can hold the receive right for a port denotinga message queue. This one task is allowed to receive (read) messagesfrom the port queue. Multiple tasks can hold rights to the portthat allow them to send (write) messages into the queue.
A task communicates with another task by building a data structurethat contains a set of data elements and then performing a message-sendoperation on a port for which it holds send rights. At some latertime, the task with receive rights to that port will perform a message-receiveoperation.
A message may consist of some or all of the following:
pure data
copies of memory ranges
port rights
kernel implicit attributes, such as the sender’s security token
The message transfer is an asynchronous operation. The messageis logically copied into the receiving task, possibly with copy-on-writeoptimizations. Multiple threads within the receiving task can beattempting to receive messages from a given port, but only one thread canreceive any given message.
Semaphores
Semaphore IPC objects support wait, post, and post all operations.These are counting semaphores, in that posts are saved (counted)if there are no threads currently waiting in that semaphore’swait queue. A post all operation wakes up all currently waitingthreads.
Notifications
Like semaphores, notification objects also support post andwait operations, but with the addition of a state field. The stateis a fixed-size, fixed-format field that is defined when the notificationobject is created. Each post updates the state field; there is asingle state that is overwritten by each post.
Locks
A lock is an object that provides mutually exclusive accessto a critical section. The primary interfaces to locks are transactionoriented (see IPC Transactions and Event Dispatching). During the transaction,the thread holds the lock. When it returns from the transaction,the lock is released.
Remote Procedure Call (RPC) Objects
As the name implies, an RPC object is designed to facilitateand optimize remote procedure calls. The primary interfaces to RPCobjects are transaction oriented (see IPC Transactions and Event Dispatching)
When an RPC object is created, a set of argument block formatsis defined. When an RPC (a send on the object) is made by a client,it causes a message in one of the predefined formats to be createdand queued on the object, then eventually passed to the server (the receiver).When the server returns from the transaction, the reply is returnedto the sender. Mach tries to optimize the transaction by executingthe server using the client’s resources; this is called threadmigration.
Time Management
The traditional abstraction of time in Mach is the clock, which provides a setof asynchronous alarm services based on mach_timespec_t.There are one or more clock objects, each defining a monotonicallyincreasing time value expressed in nanoseconds. The real-time clockis built in, and is the most important, but there may be other clocksfor other notions of time in the system. Clocks support operationsto get the current time, sleep for a given period, set an alarm(a notification that is sent at a given time), and so forth.
The mach_timespec_t API is deprecatedin OS X. The newer and preferred API is based on timer objectsthat in turn use AbsoluteTime asthe basic data type. AbsoluteTime isa machine-dependent type, typically based on the platform-nativetime base. Routines are provided to convert AbsoluteTime valuesto and from other data types, such as nanoseconds. Timer objectssupport asynchronous, drift-free notification, cancellation, andpremature alarms. They are more efficient and permit higher resolutionthan clocks.
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Super Sons Vol. 3: The Parent Trap
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