multiprocessor Multiprocessing (MP) is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. The ...

computer systems, software lockout is the issue of performance degradation due to the idle wait times spent by the CPUs in kernel-level critical sections. Software lockout is the major cause of

scalability Scalability is the property of a system to handle a growing amount of work. One definition for software systems specifies that this may be done by adding resources to the system. In an economic context, a scalable business model implies that ...

degradation in a multiprocessor system, posing a limit on the maximum useful number of processors. To mitigate the phenomenon, the kernel must be designed to have its critical sections as short as possible, therefore decomposing each

data structure In computer science, a data structure is a data organization and storage format that is usually chosen for Efficiency, efficient Data access, access to data. More precisely, a data structure is a collection of data values, the relationships amo ...

in smaller substructures.

Kernel-level critical sections

In most multiprocessor systems, each processor schedules and controls itself, therefore there's no "supervisor" processor, and kernel

s are globally shared; sections of code that access those shared data structures are critical sections. This design choice is made to improve scaling, reliability and modularity. Examples of such kernel data structure are ready list and

communication channel A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for infor ...

s. A "conflict" happens when more than one processor is trying to access the same resource (a memory portion) at the same time. To prevent critical races and inconsistency, only one processor ( CPU) at a given time is allowed to access a particular

(a memory portion), while other CPUs trying to access at the same time are locked-out, waiting in idle status.Madnick 1968, p.19 Three cases can be distinguished when this idle wait is either necessary, convenient, or not convenient. The idle wait is necessary when the access is to a ready list for a low level

scheduling A schedule (, ) or a timetable, as a basic time-management tool, consists of a list of times at which possible tasks, events, or actions are intended to take place, or of a sequence of events in the chronological order in which such things ...

operation. The idle wait is not necessary but convenient in the case of a critical section for

synchronization Synchronization is the coordination of events to operate a system in unison. For example, the Conductor (music), conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are sa ...

/ IPC operations, which require less time than a

context switch In computing, a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point, and then restoring a different, previously saved, state. This allows multiple processes ...

(executing another

process A process is a series or set of activities that interact to produce a result; it may occur once-only or be recurrent or periodic. Things called a process include: Business and management * Business process, activities that produce a specific s ...

to avoid idle wait). Idle wait is instead not convenient in case of a kernel critical section for

device management Mobile device management (MDM) is the administration of mobile devices, such as smartphones, tablet computers, and laptops. MDM is usually implemented with the use of a third-party product that has management features for particular vendors of ...

, present in

monolithic kernel A monolithic kernel is an operating system software architecture, architecture with the entire operating system running in kernel space. The monolithic model differs from other architectures such as the microkernel in that it alone defines a high ...

s only. A

microkernel In computer science, a microkernel (often abbreviated as μ-kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, ...

instead falls on just the first two of the above cases. In a multiprocessor system, most of the conflicts are kernel-level conflicts, due to the access to the kernel level critical sections, and thus the idle wait periods generated by them have a major impact in performance degradation. This idle wait time increases the average number of idle processors and thus decreases

and

relative efficiency In statistics, efficiency is a measure of quality of an estimator, of an experimental design, or of a hypothesis testing procedure. Essentially, a more efficient estimator needs fewer input data or observations than a less efficient one to achiev ...

Analytical studies

Taking as parameters the average time interval spent by a processor in kernel level critical sections (''L'', for time in locked state), and the average time interval spent by a processor in tasks outside critical sections (''E''), the ratio ''L/E'' is crucial in evaluating software lockout. Typical values for ''L/E'' range from 0.01 to 0.1. In a system with a ''L/E'' ratio of 0.05, for instance, if there are 15 CPUs, it is expected that on average 1 CPU will always be idle;Madnick 1968, p.20 with 21 CPUs, 2.8 will be idle;Raynor 76, p.62 with 40 CPUs, 19 will be idle; with 41 CPUs, 20 will be idle. Therefore, adding more than 40 CPUs to that system would be useless. In general, for each ''L/E'' value, there's a threshold for the maximum number of useful CPUs.

Software lockout mitigation

To reduce the performance degradation of software lockout to reasonable levels (''L/E'' between 0.05 and 0.1), the kernel and/or the operating system must be designed accordingly. Conceptually, the most valid solution is to decompose each kernel data structure in smaller independent substructures, having each a shorter elaboration time. This allows more than one CPU to access the original data structure. Many uniprocessor systems with hierarchical protection domains have been estimated to spend up to 50% of the time performing "supervisor mode" operations. If such systems were adapted for

multiprocessing Multiprocessing (MP) is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. The ...

by setting a lock at any access to "supervisor state", ''L/E'' would easily be greater than 1, resulting in a system with the same throughput as the uniprocessor despite the number of CPUs.

Notes

References

Madnick, Stuart Elliot
(1968)
Multi-processor software lockout
Proceedings of the 1968 23rd ACM national conference, pp. 19 – 24 * M. Dubois, F. Briggs
The run-time efficiency of parallel asynchronous algorithms
' IEEE Transactions on Computers, November 1991 (Vol. 40, No. 11) pp. 1260–1266 * Randy J. Raynor, John M. Gwynn, Jr.
Minimization of supervisor conflict for multiprocessor computer systems
' ACM SIGSIM Simulation Digest. Volume 7, Issue 4 (July 1976). pp. 61 – 69

Kernel-level critical sections

Analytical studies

Software lockout mitigation

See also

Notes

References

Further reading