Software lockout


In multiprocessor 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 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 smaller substructures

Contents

  • 1 Kernel-level critical sections
  • 2 Analytical studies
  • 3 Software lockout mitigation
  • 4 See also
  • 5 Notes
  • 6 References
  • 7 Further reading

Kernel-level critical sectionsedit

In most multiprocessor systems, each processor schedules and controls itself, therefore there's no "supervisor" processor,1 and kernel data structures 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 modularity1 Examples of such kernel data structure are ready list and communication channels

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 data structure a memory portion, while other CPUs trying to access at the same time are locked-out, waiting in idle status12

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 operation The idle wait is not necessary but convenient in the case of a critical section for synchronization/IPC operations, which require less time than a context switch executing another process to avoid idle wait Idle wait is instead not convenient in case of a kernel critical section for device management, present in monolithic kernels only A microkernel 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 scalability and relative efficiency

Analytical studiesedit

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,1 the ratio L/E is crucial in evaluating software lockout

Typical values for L/E range from 001 to 013 In a system with a L/E ratio of 005, for instance, if there are 15 CPUs, it is expected that on average 1 CPU will always be idle;3 with 21 CPUs, 28 will be idle;4 with 40 CPUs, 19 will be idle; with 41 CPUs, 20 will be idle3 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 mitigationedit

To reduce the performance degradation of software lockout to reasonable levels L/E between 005 and 01, 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 by setting a lock at any access to "supervisor state", L/E would easily be greater than 1,3 resulting in a system with the same bandwidth of the uniprocessor despite the number of CPUs

See alsoedit

  • Amdahl's law
  • Dependency issues on Superscalar architectures
  • Concurrency control § Concurrency control mechanisms
  • Schedule computer science § Serializable
  • Serializability

Notesedit

  1. ^ a b c d Madnick 1968, p19
  2. ^ Saltzer, Jerome Traffic control in a multiplexed computer system MIT Project MAC MAC-TR-30 June 1966
  3. ^ a b c d Madnick 1968, p20
  4. ^ Raynor 76, p62

Referencesedit

  • Madnick, Stuart Elliot 1 1968 Multi-processor software lockout2 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, JrMinimization of supervisor conflict for multiprocessor computer systems ACM SIGSIM Simulation Digest Volume 7, Issue 4 July 1976 pp 61 – 69

Further readingedit

  • Rodgers, David P 1985 Improvements in multiprocessor system design ACM SIGARCH Computer Architecture News archive Volume 13, Issue 3 June 1985 table of contents Special Issue: Proceedings of the 12th annual International Symposium on Computer Architecture ISCA '85 Pages: 225 - 231 Year of Publication: 1985 ISSN 0163-5964 Also published in International Symposium on Computer Architecture, Proceedings of the 12th annual international symposium on Computer architecture, 1985, Boston, Massachusetts, United States
  • Jörg Cordsen, Wolfgang Schröder-Preikschat Towards a Scalable Kernel Architecture In: Proceedings of the Autumn 1992 Openforum Technical Conference pp 15-33, Utrecht, The Netherlands, November 23-27, 1992


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