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Topic: Priority ceiling

	Priority inversion - Wikipedia, the free encyclopedia
		If the high priority task is left starved of the resources, it might lead to a system malfunction or the triggering of pre-defined corrective measures, such as a watch dog timer resetting the entire system.
		Low priority tasks usually have a low priority because it is not important for them to finish promptly (for example, they might be a batch job or another non-interactive activity).
		With priority ceilings, the shared mutex process (that runs the operating system code) has a characteristic (high) priority of its own, which is assigned to the task locking the mutex.
	en.wikipedia.org /wiki/Priority_inversion (650 words)

	Priority inversion -- Facts, Info, and Encyclopedia article (Site not responding. Last check: 2007-11-01)
		If the high priority task is left (additional info and facts about starved) starved of the resources, it might lead to a system malfunction or the triggering of pre-defined corrective measures, such as a (additional info and facts about watch dog) watch dog timer resetting the entire system.
		Low priority tasks usually have a low priority because it is not important for them to finish promptly (for example, they might be a (additional info and facts about batch job) batch job or another non-interactive activity).
		In the second method, the shared mutex process has a characteristic (high) priority of its own, which is assigned to the task locking the (additional info and facts about mutex) mutex.
	www.absoluteastronomy.com /encyclopedia/p/pr/priority_inversion.htm (494 words)

	How to use priority inheritance (Site not responding. Last check: 2007-11-01)
		Priority inversion occurs when a high-priority task is forced to wait for the release of a shared resource owned by a lower-priority task.
		Bounded priority inversion, shown in Figure 1, occurs when low-priority Task L acquires a lock on a shared resource, but before releasing the resource is preempted by high-priority Task H. Task H attempts to acquire the resource but is forced to wait for Task L to finish its critical section.
		The priority ceiling must be higher than the highest priority of all tasks that can access the resource, thereby ensuring that a task owning a shared resource won't be preempted by any other task attempting to access the same resource.
	www.embedded.com /shared/printableArticle.jhtml?articleID=20600062 (3717 words)

	[No title]
		The ability to dynamically change protected object ceiling priorities is especially required in situations where dynamic task priority change occurs, or where a library containing POs with inadequate ceilings is used by an application-specific set of tasks and interrupt handlers.
		The use of ceiling of ceilings implies that a task running in a particular mode can be blocked by another one unnecessarily, due to an inflated ceiling priority resulting from the requirements of a different mode of operation.
		The effect of the call to P'Set_Priority is to amend the ceiling priority of the enclosing protected object (or current instance of the enclosing protected type) to X, at completion of the protected action in which it is executed.
	www.ada-auth.org /cgi-bin/cvsweb.cgi/AIs/AI-00327.TXT?rev=1.6 (2514 words)

	Priority Ceiling Locking
		The ceiling priority of a protected object (or ceiling, for short) is an upper bound on the active priority a task can have when it calls protected operations of that protected object.
		Priority, Interrupt_Priority, Interrupt_Handler, or Attach_Handler is specified in the
		18 The ceiling priority of a protected object has to be in the Interrupt_Priority range if one of its procedures is to be used as an interrupt handler (see C.3).
	www.adaic.org /standards/95aarm/html/AA-D-3.html (842 words)

	SEMAPHORE
		Priority inheritance is an algorithm that calls for the lower priority task holding a resource to have its priority increased to that of the highest priority task blocked waiting for that resource.
		Priority ceiling is an algorithm that calls for the lower priority task holding a resource to have its priority increased to that of the highest priority task which will EVER block waiting for that resource.
		Although the priority ceiling algorithm is more efficient than the priority inheritance algorithm with respect to the maximum number of task priority changes which may occur while a task holds a particular semaphore, the priority inheritance algorithm is more forgiving in that it does not require this apriori information.
	www.infres.enst.fr /~domas/astre/sem.html (3141 words)

	[No title]
		For a given system of threads and their assigned priorities, the schedulable utilization is a function of the number of assigned priorities, the number of system priority levels, and the method used to map assigned priorities to system priority levels.
		Because threads of priority 31 are scheduled on a round robin basis by NT 4.0, a thread holding a resource may be delayed by these threads, and the length of this delay is theoretically unbounded.
		Priority inversion occurs when a work-thread executing on behalf of a high real-time priority requester is preempted by a thread at a lower real-time priority.
	www.cs.bu.edu /fs/fs/IEEE-RTTC/newsletter/1999.08.05 (6951 words)

	D. Kalinsky Associates - Whitepaper "Mutexes Battle Priority Inversions"
		ceiling priority for the mutex is the priority of this task.
		ceiling should simply be the priority of the highest-priority task which might ever request to lock the mutex.
		Priority inheritance mutexes are prone to mutual deadlocks.
	www.kalinskyassociates.com /Wpaper2.html (3220 words)

	Priority Inheritance: The Real Story
		In contrast, priority ceiling emulation will pay the cost of changing a thread's priority twice regardless of whether there is contention for the lock or not, resulting in higher overhead and many unnecessary context switches and blocking in unrelated tasks.
		Priority ceiling emulation is an attractive choice when the set of threads contending for the lock is known, when there may be nested locks, and when worst-case behavior is the only concern.
		Priority inheritance and priority ceiling emulation are both effective and powerful techniques to prevent uncontrolled priority inversion when locks are used to protect critical sections in a real-time system.
	www.linuxdevices.com /articles/AT5698775833.html (2089 words)

	ITworld.com - Solaris 8 threads attributes
		Priority inversion describes a scenario in which a higher-priority thread is prevented from running because a lower-priority thread is holding a resource the higher-priority thread requires to execute.
		locks, implements a priority ceiling attribute for the lock, so that the lock owner has its priority elevated to the ceiling value of the lock if the thread's current priority is less than the ceiling.
		That is, if a thread has a priority of 50 and the lock's ceiling value is 60, the thread will have its priority promoted to 60 for as long as it owns the lock, after which its priority is reset to 50.
	www.itworld.com /AppDev/1170/swol-1218-insidesolaris/pfindex.html (1725 words)

	Introduction to Priority Inversion - real-time operating system, RTOS, preemption, fixed priority scheduling
		Priority inversion is the term for a scenario in which the highest-priority ready task fails to run when it should.
		This priority change should take place as soon as the high-priority task begins to pend; it should end when the resource is released.
		A beneficial feature of the priority ceiling solution is that tasks can share resources simply by changing their priorities, thus eliminating the need for semaphores.
	www.netrino.com /Publications/Glossary/PriorityInversion.html (1055 words)

	LSU EE 4770 Lecture Notes (Site not responding. Last check: 2007-11-01)
		18-10 * * 18-10 Priority Inheritance Implementation of Priority Inheritance The dynamic priority of a task locking a resource : : : : : :is set to the maximum of: : : :its own priority, : : :and the priority of tasks blocked on the resource.
		That is, a task in a critical region "inherits" the priority of waiting tasks.
		Modified global priority for a task is the global priority determined without in- cluding resources locked by task (but including resources locked by other tasks).
	www.ece.lsu.edu /ee4770/1998/lsli18.html (1433 words)

	[No title]
		In priority ceiling protocol, a lower priority task can block the same higher priority task at most once, but it potentially can block more than one higher priority tasks.
		Due to the problem of priority inversion (i.e., without using the priority ceiling protocol), a higher priority task can be indirectly blocked some lower priority tasks.
		One practical issue regarding priority ceiling protocol is that we need to know the priority ceiling values for all the semaphores in advance.
	www.cs.ucdavis.edu /~wu/ecs150/ecs150_final_fall_2003_new.txt (1534 words)

	Change list RSTJ 1 (Site not responding. Last check: 2007-11-01)
		With this policy, a monitor is given a priority ceiling when it is created, which is the highest priority of any thread that could attempt to enter the monitor.
		As soon as a thread enters synchronized code, its priority is raised to the monitor?s ceiling priority.
		As soon as a thread enters synchronized code, its (active) priority is raised to the monitor?s ceiling priority.
	www.rtsj.org /specjavadoc/ChangeLog.html (2763 words)

	docs.sun.com: Multithreaded Programming Guide (Site not responding. Last check: 2007-11-01)
		A thread's priority and scheduling are not affected by the mutex ownership.
		Priority inversion occurs when a low-priority thread holds a lock that a higher-priority thread wants.
		The ceiling defines the minimum priority level at which the critical section guarded by the mutex is executed.
	docs.sun.com /app/docs/doc/806-5257/6je9h032o?a=view (1932 words)

	Design Patterns for Real-Time Systems: Resource Patterns (Site not responding. Last check: 2007-11-01)
		The priority ceiling is just greater than the priority of the highest-priority client of the resource—this is known at design time in a static priority scheme.
		When the resource is locked, the priority of the locking task is augmented to the priority ceiling of the resource.
		The priority ceiling can be determined at design time for each resource by examining the clients of a given resource and identifying to which active object they belong and selecting the highest from among those.
	www.informit.com /articles/article.asp?p=30188&seqNum=4 (1833 words)

	Ada 95 RM - D.3 Priority Ceiling Locking
		The expression of a Priority or Interrupt_Priority pragma is evaluated as part of the creation of the corresponding protected object and converted to the subtype System.Any_Priority or System.Interrupt_Priority, respectively.
		The value of the expression is the ceiling priority of the corresponding protected object.
		Since implementations are allowed to place restrictions on code that runs at an interrupt-level active priority (see C.3.1 and D.2.1), the implementation may implement a language feature in terms of a protected object with an implementation-defined ceiling, but the ceiling shall be no less than Priority'Last.
	lglwww.epfl.ch /docs/ada/rm95html-1.0/rm9x-D-03.html (746 words)

	Rate Monotonic Analysis Paper
		J uses its assigned priority, unless it is in its critical section and blocks higher priority jobs.
		When the priority inheritance and ceiling priority protocols are followed, the theorems in Section 1 may be modified to allow for the blocking time introduced by task synchronization [5, p.
		This protocol preserves the ceiling priority protocol's properties of freedom from deadlock and the guarantee that a higher priority task may be blocked by a lower priority task at most once.
	www.heidmann.com /paul/rma/PAPER.htm (4893 words)

	Dynamic Priorities
		When an entry call is cancelled, it is a bounded error if the priority of the calling task is higher than the ceiling priority of the corresponding protected object.
		That is, setting the base priority of a task causes the priority of a queued entry call from that task to be updated and the call to be removed and then reinserted in the entry queue at the new priority (see D.4), unless the call originated from the
		For this to work reliably, it should be done within a protected operation that has high enough ceiling priority to guarantee that the operation completes without being preempted by any of the affected tasks.
	www.adaic.org /standards/95lrm/html/RM-D-5.html (511 words)

	[No title]
		The ceiling priority of a protected object (PO) would be dynamically changed by giving access to a new attribute of a protected object.
		When the locking policy in effect defines priorities for protected objects, 'Priority denotes the priority of the designated protected object.
		!discussion Both the old and new values of the ceiling must be retained by the implementation, as an external call needs to have its priority checked against the old value until the new value takes effect, at the end of the protected action.
	www.ada-auth.org /cgi-bin/cvsweb.cgi/AIs/AI-00327.TXT?rev=1.8 (1560 words)

	[No title] (Site not responding. Last check: 2007-11-01)
		Allow the application of the priority ceiling emulation algorithm to individual objects.
		If threads with the same priority are possible under the active scheduling policy such threads are queued in FIFO order.
		Priority inheritance and priority ceiling emulation are both popular priority inversion control mechanisms; however, priority inheritance is more widely implemented in real-time operating systems and so is the default mechanism in this specification.
	www.rtj.org /public/src/javax/realtime/synchronization.html (411 words)

	Man Page: pthread_mutexattr_getprioceiling(3thr)
		The pthread_mutexattr_getprioceiling and pthread_mutexattr_setprioceiling functions, respectively, get and set the priority ceiling attribute of a mutex attribute object pointed to by attr, which was previously created by the pthread_mutexattr_init function.
		In order to avoid priority inversion, the priority ceiling of the mutex must be set to a priority higher than or equal to the highest priority of all the threads that may lock that mutex.
		The semantics of the actual ceiling value are determined by the existing priority range for the SCHED_FIFO policy, as returned by the sched_get_priority_min and sched_get_priority_max functions (see sched_get_priority_min(3RT)) when called on the version of Solaris on which the ceiling value is being utilized.
	www.cse.msu.edu /cgi-bin/man2html?pthread_mutexattr_getprioceiling?3THR?/usr/man (385 words)

	[linux-audio-dev] Re: desktop and multimedia as an afterthought? (Site not responding. Last check: 2007-11-01)
		Priority inheritance or some other protocol for priority inversion _is_ needed for realtime applications that have threads with different priorities accessing common data.
		One could increase the priority of the low priority thread before taking the mutex and release it afterwards (as in priority ceiling), but I doubt that's optimal.
		Mutexes are part of the POSIX realtime threading extensions; you can use a semaphore when priority inversion is not an issue.
	www.music.columbia.edu /pipermail/linux-audio-dev/2004-July/008996.html (328 words)

	EETimes.com - RTOSes, 'mutexes' fight priority inversion (Site not responding. Last check: 2007-11-01)
		When using priority inheritance, the operating system dynamically changes the priority of the task that "owns" a mutex, depending on the priorities of other tasks that attempt to lock the mutex.
		In other words, when a high-priority task attempts to lock a mutex that is already owned by a lower-priority task, the priority of the owner is raised to the high priority of the other task.
		For those applications, the priority ceiling should simply be the priority of the highest-priority task that may request to lock the mutex.
	www.eetimes.com /story/OEG20010406S0053 (1424 words)

	manquery - -s 3thr pthread_mutex_getprioceiling @ Eastern Illinois University (Site not responding. Last check: 2007-11-01)
		The pthread_mutex_setprioceiling() function either locks the mutex if it is unlocked, or blocks until it can successfully lock the mutex, then it changes the mutex's priority ceiling and releases the mutex.
		When the change is successful, the previous value of the priority ceiling is returned in old_ceiling.
		The ceiling value should not be treated as a persistent value resident in a pthread_mutex_t that is valid across upgrades of Solaris.
	www.eiu.edu /cgi-bin/manquery?pthread_mutex_getprioceiling(3thr) (513 words)

	system.task_primitives.operations
		-- For Lock, Prio is the ceiling priority associated with the lock.
		This would mean there is no -- ceiling violation, but we would end up with priority inversions -- inside the runtime system, resulting in failure to satisfy the -- Ada priority rules, and possible missed validation tests.
		-- The priority set is what would correspond to the Ada concept of -- "base priority" in the terms of the lower layer system, but -- the operation may be used by the upper layer to implement -- changes in "active priority" that are not due to lock effects.
	www.seas.gwu.edu /~adagroup/adalib_html/system-html/s-taprop.html (2003 words)

	ceiling.ca - Priority ceiling protocol (Site not responding. Last check: 2007-11-01)
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	www.ceiling.ca /Priority-ceiling-protocol/reference/fullview/wikipedia... (182 words)

	Text errata for Programming in the OSEK/VDX Environment (Site not responding. Last check: 2007-11-01)
		However, if resources are nested, which is allowed in OSEK/VDX, the Priority Ceiling Protocol may work differently for the nested priorities.
		Step 3 - When the task locks a nested resource, the priority of the task is raised to the ceiling priority of the newly locked resource if and only if the new ceiling priority is greater than the current ceiling priority of the task.
		Step 4 - When the task unlocks a nested resource, the priority of the task is set to the highest ceiling priority of all resources that are still locked by the task.
	mywebpages.comcast.net /osekbook/text_errata.html (265 words)

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