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Topic: Priority inversion

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In the News (Sun 29 Nov 09)

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Marvin Harrison

Peter Chernin

Gary Locke

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Mickey Rourke

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Danny Boyle

Hugh Jackman

	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.
		Many priority inversions are innocuous or, at most, briefly delay a task that should run right away.
		This priority change should take place as soon as the high-priority task begins to pend; it should end when the resource is released.
	www.netrino.com /Publications/Glossary/PriorityInversion.php (1076 words)

	Priority inversion - Wikipedia, the free encyclopedia
		In scheduling, priority inversion is the scenario where a low priority task holds a shared resource that is required by a high priority task.
		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 (666 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 reqires 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 (295 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.
		Granted, this is still a priority inversion (in that L is now running at a higher priority than what it should have been running at), but it prevents a worse priority inversion from happening, namely, it prevents H from being blocked.
		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.
	www.parse.com /~rk/articles/priority.html (2218 words)

	Lab 2: Read/Write Locks with Priority Inheritance
		Priority inversion occurs when a high priority thread is blocked waiting on a lock (or a semaphore) held by a lower priority thread.
		The priority inheritance protocol solves the problem of priority inversion by increasing the priority of the low priority process holding the lock to the priority of the high priority process waiting on the lock.
		Release: On releasing a lock, the priority of the process has to be reset to the maximum priority of all the processes in the wait queues of all the locks still held by the process.
	www.cs.purdue.edu /homes/cs503/lab2/lab2.html (1981 words)

	[No title]
		Priority Inheritance (TBD) The temporary changing of priority of some schedulable entities due to the blocking of one schedulable entity by a lower priority schedulable entity to minimize priority inversion 6.
		Priority inversion Unwanted, unjustified use of a resource by a lower priority task when a higher priority task is ready to use the resource 7.
		Fixed Priority Scheduling priority scheduling in which the priorities are changeable by the application but not by the scheduler (except for priority inheritance).
	www.omg.org /docs/realtime/97-05-03.txt (803 words)

	[No title]
		In general, the priority inversion problem in client-server communication is more serious one, since the length of priority inversion tends to be much longer than that of synchronization.
		Priority Hand-off Since the sender and the receiver threads are independent scheduling entities, there should be a mechanism to maintain a priority of receiver when the receiver receives a message.
		This is caused by the overhead of priority inheritance.
	www.usenix.org /publications/library/proceedings/micro93/full_papers/kitayama.txt (4951 words)

	Priority Inversion and Windows NT Scheduler (Site not responding. Last check: 2007-10-09)
		The remainder of this article talks about the scheduling of threads that are not real-time priority class and how the system solves the problem of priority inversion.
		Priority inversion occurs when two (or more) threads with different priorities are in contention to be scheduled.
		Consider a simple case with three threads: Thread 1 is high priority and becomes ready to be scheduled, while thread 2, a low-priority thread, is executing in a critical section.
	support.microsoft.com /default.aspx?scid=kb;en-us;Q96418 (806 words)

	SoC drawer: Shared resource management
		Priority preemptive scheduling of multiple service threads or tasks is often used by programmers to provide low-latency response to service requests and to handle errors and exceptions.
		Least locker protocol: L's priority is amplified to the maximum priority of all of the threads involved in the critical section.
		I'll have to admit, given a choice between priority inheritance and priority ceiling, I'd agree that ceiling seems to be an easier mechanism to implement, but also agree with Locke's assessment that, in the end, the issue of unbounded inversion must be understood and that avoidance mechanisms shouldn't be considered panaceas to be used carelessly.
	www.ibm.com /developerworks/power/library/pa-soc5?ca=dgr-lnxw02aSocResource (3335 words)

	Avoiding Shared Resources and Priority Inversions for Deterministic Applications - Tutorial - Developer Zone - National ...
		This conflict is an inversion of priorities or “priority inversion”; the lower priority thread has suddenly become more important than the time-critical thread, because it must finish its work and relinquish the shared resource before the time-critical thread can proceed.
		If a higher priority thread is forced to wait on a lower priority thread because of a protected resource, the RTOS under LabVIEW Real-Time uses a method called priority inheritance to resolve the priority inversion as quickly as possible.
		Priority inheritance allows the lower priority thread to temporarily “inherit” the time-critical priority setting, long enough to finish using the shared resource and to remove the protection.
	zone.ni.com /devzone/conceptd.nsf/webmain/3EF39F7BC420236B862569A80078B9ED?opendocument&node=dz00000_us (1027 words)

	Priority Inversion historical notes (Site not responding. Last check: 2007-10-09)
		You know, I must rain on this parade a bit -- priority inversion was well known in the embedded realtime community when I arrived in the mid 1970s, but it had neither a name nor a literature back then.
		The idea of a process inheriting the max priority of members of the wait list was implemented in the Tandem system in 1983 as a solution to the problem (and we even documented it in the literature).
		By that time we had learned the term priority inversion and used it to describe the problem and the solution.
	research.microsoft.com /~mbj/Mars_Pathfinder/Priority_Inversion.html (364 words)

	Priority Inversion (Windows CE 3.0 Kernel Services) (Site not responding. Last check: 2007-10-09)
		Priority inversion occurs when a high priority thread is blocked from running because a lower priority thread owns a kernel object, such as a mutex or critical section, that the high priority thread needs.
		In contrast, Windows CE 3.0 guarantees the handling of priority inversion only to a depth of one level for case (2).
		Thread 2 and 3 will be scheduled based on a normal scheduling algorithm In a real-time system priority inversion cases are to be avoided for the basic reason that you loose control on the scheduling of your threads.
	msdn.microsoft.com /library/en-us/wcekern/html/wcekernPriorityInversion.asp?frame=true (259 words)

	Yodaiken: Against Priority Inheritance
		Priority inversion refers to the situation when a scheduled task must wait for a lower priority task to complete.
		Priority inheritance is intended to allow "a task that is blocked waiting for a resource [to pass] its priority down to the owner.
		The low priority task is [thus] considered to be acting on behalf of the highest priority blocked task and inheritance prevents intermediate priority tasks from interfering," says Yodaiken.
	www.linuxdevices.com /articles/AT7168794919.html (495 words)

	Priority Inversion and the Mars Pathfinder (Site not responding. Last check: 2007-10-09)
		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.
		This scenario is a classic case of priority inversion.
		Interestingly enough, the JPL engineers actually created a priority inversion situation during testing, but did not manage to analyze their recorded data well enough so to conclude that priority inversion is indeed a bug in their system.
	www.time-rover.com /Priority.html (512 words)

	Design Patterns for Real-Time Systems: Resource Patterns > Introduction (Site not responding. Last check: 2007-10-09)
		The priority of the task determines which tasks will run preferentially when multiple tasks are ready to run—the highest-priority task that is ready.
		Unbounded priority inversion is a problem that is addressed by the first three patterns in this chapter.
		Note that priority inversion is a necessary consequence of resource sharing with mutual exclusion locking, but it can be bounded using these patterns.
	www.informit.com /articles/article.asp?p=30188 (1562 words)

	SWAP: A Scheduler With Automatic Process Dependency Detection
		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.
		Sommer notes the difficulty of addressing priority inversion for non-mutex resources when it is difficult if not impossible to determine precisely on which process a high priority dependent process is waiting.
	www.cs.columbia.edu /~hzheng/swap (10007 words)

	Preemption-Threshold
		Priority inversion takes place when a higher-priority thread is suspended because a lower-priority thread has a needed resource.
		However, if a thread of an intermediate priority becomes active during this situation, the priority inversion time is no longer deterministic and could result in total system failure.
		By disabling preemption between the executing lower-priority thread and the higher-priority thread, priority inversion is now guaranteed to be bounded by the time the lower-priority thread holds the shared resource.
	www.rtos.com /wppreemption.asp (977 words)

	P415 readings
		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.
	www.cs.indiana.edu /classes/p515/hw/MarsPathfinder.htm (3648 words)

	Q: What about priority inversion/priority inheritance? (Site not responding. Last check: 2007-10-09)
		When a lower-priority task owns a resource that a higher priority task wants, mutual exclusion algorithms make the higher priority task wait -- priorities are "inverted." The correct solution to this problem is to make clean use of mutual exclusion mechanisms, by making sure, for example, that operations on shared resources are simple and fast.
		In priority inheritance, when a high-priority task blocks, it raises the priority of the task holding the semaphore, allowing it to complete.
		Priority inheritance tries to cover up a design problem with a complicated, slow and failure-prone hack.
	euler.aero.iitb.ac.in /docs/rtlinux-3.0/doc/html/FAQ/x162.html (269 words)

	Design Patterns for Real-Time Systems: Resource Patterns > Priority Inheritance Pattern (Site not responding. Last check: 2007-10-09)
		The Priority Inheritance Pattern is a simple, low-overhead solution for limiting the priority inversion to at most a single level—that is, at most, a task will only be blocked by a single, lower-priority task owning a needed resource.
		The Priority Inheritance Pattern handles well the problem of priority inversion when at most a single resource is locked at any given time and prevents unbounded priority inversion in this case.
		It runs at its nominal priority, which is LOW (the priority of the thread is shown inside square brackets in the Running state mark—again, not quite standard UML, but parsimonious).
	www.informit.com /articles/article.asp?p=30188&seqNum=3 (1919 words)

	Priority Inversion (Site not responding. Last check: 2007-10-09)
		The high-priority thread is made to wait while a thread of lower priority (the middle-priority thread) executes.
		To avoid priority inversion, associate a priority (at least as high as the highest priority thread that will use it) with each resource and force any thread using that object to first raise its priority to that associated with the object.
		The Default (throughput) scheduling policy prevents priority inversion from causing a complete blockage of the high-priority thread, because the low-priority thread is eventually permitted to execute and release the resource.
	www.cs.arizona.edu /computer.help/policy/DIGITAL_unix/AA-Q2DPC-TKT1_html/thrd0064.html (147 words)

	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/page_1.html (1764 words)

	Re: Priority Inversion and early Unix (Site not responding. Last check: 2007-10-09)
		To protect other sections of code whose outcome may be affected by the handling of certain interrupts, the processor priority is raised temporarily high enough before the critical section is entered to delay such interrupts until it is safe, when the processor priority is reduced again.
		There was no attempt to set the priority to the lowest necessary, as the lack of different levels (0, 4, 5, 6, 7 for device interrupts) meant that 7 was almost always the lowest such level, although in some cases spl6() was used analogously, that being the clock interrupt priority.
		All "normal" code ran at a processor priority level of 0, with a separate software priority and context switching mechanism.) Clearly Thompson and Ritchie had thought about and found a way to solve the priority inversion problem for the limited case of Unix on a single CPU.
	www.panix.com /~dictum/risks/zzunclassified/unix-inversion.html (301 words)

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