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Topic: Priority inheritance

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	ITworld.com - Solaris 8 threads attributes
		We discussed the problem of priority inversion and the solution, priority inheritance, as it's implemented in the kernel in August 1999's column.
		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.
		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 (1780 words)

	[No title]
		Inheritance due to activation and rendezvous should be treated the same, and for rendezvous, it shouldn't make a difference whether the call is the trigger of an ATC or not.
		Having a task inherit the priority of the caller after the rendezvous has started really falls under transitive priority inheritance since the event that may cause the priority change is asynchronous to teh executing task.
		I guess the underlined phrases are meant to mean "asynchronously dynamic transitive priority inheritance", meaning that asynchronous changes to the active priority of a task may have a transitive effect via any task-to-task inheritance relations that are in effect.
	www.ada-auth.org /cgi-bin/cvsweb.cgi/AIs/AI-00092.TXT?rev=1.8 (6222 words)

	Priority inversion - Wikipedia, the free encyclopedia
		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.
		When priority is inherited, the low priority task inherits the priority of the high priority task, thus stopping the medium priority task from pre-empting the low priority task.
	en.wikipedia.org /wiki/Priority_inversion (675 words)

	Priority Inheritance: The Real Story
		In this case his hypothesis is that priority inheritance is blindly used by engineers as a panacea for all priority inversion problems, and, because it has drawbacks under certain conditions, it is therefore dangerous and should never be used.
		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 (2146 words)

	Against Priority Inheritance - fsmlabs.com (Site not responding. Last check: 2007-10-23)
		The limitations, dangers, and performance costs of the “priority inheritance” scheme for managing priority inversion are not widely appreciated.
		The classical nightmare case here is when a low priority task owns a resource, a high priority task is blocked waiting for the resource, and intermediate priority tasks keep preempting the low priority task so it cannot make progress towards releasing the resource.
		The low priority task is considered to be acting on behalf of the highest priority blocked task and the priority promotion prevents intermediate priority tasks from interfering.
	www.fsmlabs.com /against-priority-inheritance.html (379 words)

	Task Priorities
		For a Priority pragma that appears in the declarative_part of a subprogram_body, the expression shall be static, and its value shall be in the range of System.Priority.
		{task priority} {priority} {priority inheritance} {base priority} {active priority} A task priority is an integer value that indicates a degree of urgency and is the basis for resolving competing demands of tasks for resources.
		For a Priority pragma, the value of the expression is converted to the subtype Priority; for an Interrupt_Priority pragma, this value is converted to the subtype Any_Priority.
	www.adaic.org /standards/05aarm/html/AA-D-1.html (1022 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] (Site not responding. Last check: 2007-10-23)
		Note that when we talk about the effect of changing priorities, we refer to the activity which takes place in order to make the system consistent with respect to priorities, including: reevaluating and restructuring queues, preempting or dispatching new tasks, and performing all other active priorities adjustments when priority inheritance is in effect.
		For example, if a task becomes the highest priority task in an entry queue (as a result of changing its base priority), then the active priority of the acceptor will be elevated as well, even though the two are not currently engaged in a rendezvous.
		This priority should be set by the user to the maximum of the active priorities of all of the tasks or interrupt handlers (IH's) that will ever attempt to acquire the lock.
	archive.adaic.com /standards/95lsn/lsn-002.txt (2036 words)

	Priority inheritance - Wikipedia, the free encyclopedia
		Basic idea of Priority Inheritance Protocol is that when a job j blocks one or more high priority jobs, it ignores its original priority assignment and executes its critical section at the highest priority level of all the jobs it blocks.
		The priority inheritance protocol requires that the job j3 executes its critical section at the priority of j1.
		Priority Inheritance Protocols: By Sha, Rajkumar and Lehoczky.
	en.wikipedia.org /wiki/Priority_inheritance (362 words)

	Rob Krten's Articles and Publications: Device Drivers, Realtime Systems, Fractals, USENET News, Priority Inversion, ...
		Priority inversion is defined as an interference between two threads at different priorities, with the thread at the lower priority preventing the thread at the higher priority from getting CPU time.
		In the mutex case, above, the priority of L is boosted to match the priority of the highest priority thread that's waiting for the mutex (H in this case).
		The priority inversion problem stems from the fact that M is lower priority than X, and a lower priority thread should not prevent a higher priority thread from running.
	parse.com /~rk/articles/priority.html (2237 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)

	NewsForge \| Yodaiken: Against priority inheritance
		Priority inversion refers to the situation when a scheduled task must wait for a lower priority task to complete.
		Priority inversions have always been an issue in real-time operating systems, and the problem is far from solved.
		VxWorks skirts the issue by making preserving the thread's highest priority until all locks are released to manage the issue of priority inheritance changing the priority of threads in queue at the risk of unbounded inversion.
	www.newsforge.com /newsvac/02/07/10/1949248.shtml (357 words)

	Embedded.com - How to use priority inheritance
		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.
		Priority inheritance is difficult to implement, with many complicated scenarios arising when two or more tasks attempt to access the same resources.
	www.embedded.com /showArticle.jhtml?articleID=20600062 (3951 words)

	Solaris 10 Scheduling
		The inherited priority is normally zero unless the thread is sitting on a resource that is required by a higher priority thread.
		When it sees threads that are a lower priority, those threads inherit the priority of the blocked thread.
		Priority inheritance is an attempt to=20 deal with this problem, but some types of syncronization do not use inheritance.
	www.princeton.edu /~unix/Solaris/troubleshoot/schedule.html (1708 words)

	The Observation Deck (Site not responding. Last check: 2007-10-23)
		For those unfamiliar with the problem of priority inversion, it is this: given three threads at three different priorities, if the highest priority thread blocks on a synchronization object held by the lowest priority thread, the middling priority thread could (in a pure priority preemptive system running on a uniprocessor) run in perpetuity.
		Under priority inheritance, when one thread is going to block on a lower priority thread, the higher priority thread wills its priority to the lower priority thread.
		That is, the lower priority thread inherits the higher priority for the duration of the critical section.
	blogs.sun.com /bmc/entry/opensolaris_sewer_tour (2638 words)

	[No title]
		An important issue for mutex locks is the handling of priority inversion, where a high priority thread is prevented from running because it needs a lock owned by a lower priority thread.
		If priority inversion is a theoretical possibility, then the application developer may still want to ignore it because the application has been designed in such a way that the problem cannot arise in practice.
		If priority inversion can occur legally, then there are three main ways of handling it: (1) Priority ceilings, (2) priority inheritance, and (3) ignoring the inversion.
	www.redhat.com /support/wpapers/cygnus_ecos/limits.html (1087 words)

	D. Kalinsky Associates - Whitepaper "Mutexes Battle Priority Inversions"
		priority of the “owner” is raised to the high priority of the other task.
		ceiling priority for the mutex is the priority of this task.
		Priority inheritance mutexes are prone to mutual deadlocks.
	www.kalinskyassociates.com /Wpaper2.html (3220 words)

	What really happened on Mars?
		Tasks on the Pathfinder spacecraft were executed as threads with priorities that were assigned in the usual manner reflecting the relative urgency of these tasks.
		However, very infrequently it was possible for an interrupt to occur that caused the (medium priority) communications task to be scheduled during the short interval while the (high priority) information bus thread was blocked waiting for the (low priority) meteorological data thread.
		The mutex in question had been initialized with the parameter off; had it been on, the low-priority meteorological thread would have inherited the priority of the high-priority data bus thread blocked on it while it held the mutex, causing it be scheduled with higher priority than the medium-priority communications task, thus preventing the priority inversion.
	research.microsoft.com /~mbj/Mars_Pathfinder/Mars_Pathfinder.html (1186 words)

	LWN: Priority inheritance in the kernel
		The priority inheritance method is simple in concept: when a process holds a lock, it should run at (at least) the priority of the highest-priority process waiting for the lock.
		Most priority inheritance schemes have shown a tendency to complicate and slow down the locking code, and they can be used to paper over poor application designs.
		The priority inheritance is chained, so that, if the holding process is blocked on a second futex, the boosted priority will propagate to the holder of that second futex.
	lwn.net /Articles/178253 (1332 words)

	EETimes.com - RTOSes, 'mutexes' fight priority inversion (Site not responding. Last check: 2007-10-23)
		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 (1383 words)

	USENIX-97 Dreams in a Nutshell (Site not responding. Last check: 2007-10-23)
		The priority scheduler, with priority inheritance, ensures that the correct thread is scheduled each time the real-time thread blocks or makes a subsystem call.
		Significantly more effort would have been required to implement priority inheritance in a single tiered scheduler in which one of the priority queues was being used to schedule threads in an EDF manner.
		After implementing priority inheritance, we found that a number of very poor scheduling behaviors were removed from the system.
	www.employees.org /~ssommer/usenix97.html (2775 words)

	SWAP: A Scheduler With Automatic Process Dependency Detection
		Using priority inheritance, a process holding a resource inherits the highest priority of any higher priority processes blocked waiting on the resource so that it can run, release the resource, and get out of the way of the higher priority processes.
		Priority inheritance assumes that priorities are static while they are inherited because recalculating the inherited priority due to dynamic priority changes is too complex.
		Priority inheritance addresses the priority inversion problem assuming the resource dependency is known; it does not address the underlying issue of determining resource dependencies.
	www1.cs.columbia.edu /~hzheng/swap (10007 words)

	Citations: On Using Priority Inheritance in Real-Time Databases - Huang, Stankovic, Ramamritham, Towsley, Purimetla ... (Site not responding. Last check: 2007-10-23)
		All of these protocols adopt the priority abort scheme [1] in which a critical section can be aborted by any job that has a higher priority than the job executing the critical section.
		For example, in [7] priority restart is used as the conflict resolution mechanism in the....
		, priority restart is used as the conflict resolution mechanism in the early stages of a transaction s execution, wherease priority inheritance is used in the later stages.
	citeseer.ist.psu.edu /context/1109/315657 (2969 words)

	Rate Monotonic Analysis Paper (Site not responding. Last check: 2007-10-23)
		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)

	Nested Transactions (Site not responding. Last check: 2007-10-23)
		Priorities in such systems are usually assigned according to the given deadlines of individual transactions.
		We have proposed two priority assignment schemes to derive CPU usage and data conflict resolution priorities that are based on either top-level transactions’ deadlines or individual subtransactions’ work amounts.
		In order to prevent this problem, two resolution protocols, priority abort and priority inheritance, are most widely used for flat transactions.
	www.cs.ou.edu /~database/nested.htm (378 words)

	Priority Inheritance (Site not responding. Last check: 2007-10-23)
		With PriorityInheritance, the low priority task has its priority elevated to that of the highest-priority task pending on the resource--it inherits the priority of the task that is waiting for it.
		The correct thing to do would be for the priority analysis to be re-run, and the low-priority task having the highest priority of the remaining blocked tasks.
		A common approximation found in many RTOS's (such as VxWorks) is to not reduce the inherited priority of a low-priority task until it releases all of its resources.
	c2.com /cgi/wiki?PriorityInheritance (287 words)

	Priority inheritance in the kernel [LWN.net]
		The priority inheritance method is simple in concept: when a process holds a lock, it should run at (at least) the priority of the highest-priority process waiting for the lock.
		Most priority inheritance schemes have shown a tendency to complicate and slow down the locking code, and they can be used to paper over poor application designs.
		The priority inheritance is chained, so that, if the holding process is blocked on a second futex, the boosted priority will propagate to the holder of that second futex.
	www.lwn.net /Articles/178253 (1328 words)

	Partitioning Operating Systems Versus Process-based Operating Systems
		For example, in an ARINC 653 partitioning system with just three partitions and a total major allocation of 100ms per cycle, a fixed cyclic scheduler could be set to run the first partition for 20 ms, then the second partition for 30 ms, and then the third for 50 ms.
		LynxOS-178 is an ARINC 653 partitioning operating system that supports multiple POSIX processes and multiple address spaces; priority scheduling; priority inheritance; and priority ceilings within each partition.
		The performance of process-based systems is improved if the operating system handles priority inheritance efficiently and if it incorporates preemptible kernel technology.
	www.lynuxworks.com /products/whitepapers/partition.php (1058 words)

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