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<div class="mw-indicators"> </div> <div id="siteSub" class="noprint">From Wikipedia, the free encyclopedia</div> </div> <div id="contentSub"><div id="mw-content-subtitle"></div></div> <div id="mw-content-text" class="mw-body-content"><div class="mw-content-ltr mw-parser-output" lang="en" dir="ltr"><div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none">Processing scheduling algorithm</div> <p>In <a href="/wiki/Computer_science" title="Computer science">computer science</a>, a <b>multilevel feedback queue</b> is a <a href="/wiki/Scheduling_(computing)" title="Scheduling (computing)">scheduling</a> algorithm. Scheduling algorithms are designed to have some process running at all times to keep the <a href="/wiki/Central_processing_unit" title="Central processing unit">central processing unit</a> (CPU) busy.<sup id="cite_ref-osc-ch5-p131_1-0" class="reference"><a href="#cite_note-osc-ch5-p131-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> The multilevel feedback queue extends standard algorithms with the following design requirements: </p> <ol><li>Separate processes into multiple <i>ready queues</i> based on their need for the processor.</li> <li>Give preference to processes with short CPU bursts.</li> <li>Give preference to processes with high <a href="/wiki/I/O_bound" title="I/O bound">I/O</a> bursts. (I/O bound processes will sleep in the <i>wait queue</i> to give other processes CPU time.)</li></ol> <p>The multilevel feedback queue was first developed by <a href="/wiki/Fernando_J._Corbat%C3%B3" title="Fernando J. Corbató">Fernando J. Corbató</a> (1962).<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> For this accomplishment, the <a href="/wiki/Association_for_Computing_Machinery" title="Association for Computing Machinery">Association for Computing Machinery</a> awarded Corbató the <a href="/wiki/Turing_Award" title="Turing Award">Turing Award</a>.<sup id="cite_ref-OS_Three_Pieces_3-0" class="reference"><a href="#cite_note-OS_Three_Pieces-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="Process_scheduling">Process scheduling</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Multilevel_feedback_queue&action=edit&section=1" title="Edit section: Process scheduling"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Whereas the <a href="/wiki/Multilevel_queue" title="Multilevel queue">multilevel queue</a> algorithm keeps processes permanently assigned to their initial queue assignments, the multilevel feedback queue shifts processes between queues.<sup id="cite_ref-osc-ch5-p147_4-0" class="reference"><a href="#cite_note-osc-ch5-p147-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> The shift is dependent upon the CPU bursts of prior <a href="/wiki/Preemption_(computing)#Time_slice" title="Preemption (computing)">time-slices</a>.<sup id="cite_ref-osc-ch5-p148_5-0" class="reference"><a href="#cite_note-osc-ch5-p148-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> </p> <ul><li>If a process uses too much CPU time, it will be moved to a lower-priority queue.</li> <li>If a process is I/O-bound or an interactive process, it will be moved to a higher-priority queue.</li> <li>If a process is waiting too long in a low-priority queue and <a href="/wiki/Starvation_(computer_science)" title="Starvation (computer science)">starving</a>, it will be <a href="/wiki/Aging_(scheduling)" title="Aging (scheduling)">aged</a> to a higher-priority queue.</li></ul> <div class="mw-heading mw-heading3"><h3 id="Algorithm">Algorithm</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Multilevel_feedback_queue&action=edit&section=2" title="Edit section: Algorithm"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Multiple <a href="/wiki/FIFO_(computing_and_electronics)" title="FIFO (computing and electronics)">FIFO</a> queues are used and the operation is as follows: </p> <ol><li>A new process is inserted at the end (tail) of the top-level <a href="/wiki/FIFO_(computing_and_electronics)" title="FIFO (computing and electronics)">FIFO</a> queue.</li> <li>At some stage the process reaches the head of the queue and is assigned the <a href="/wiki/Central_processing_unit" title="Central processing unit">CPU</a>.</li> <li>If the process is completed within the time slice of the given queue, it leaves the system.</li> <li>If the process voluntarily relinquishes control of the CPU, it leaves the queuing network, and when the process becomes ready again it is inserted at the tail of the same queue which it relinquished earlier.</li> <li>If the process uses all the quantum time, it is <a href="/wiki/Preemption_(computing)" title="Preemption (computing)">pre-empted</a> and inserted at the end of the next lower-level queue. This next lower-level queue will have a time quantum that is more than that of the previous higher-level queue.</li> <li>This scheme will continue until the process completes or it reaches the base-level queue.</li></ol> <dl><dd><dl><dd><ul><li>At the base level queue the processes circulate in <a href="/wiki/Round-robin_scheduling" title="Round-robin scheduling">round robin</a> fashion until they complete and leave the system. Processes in the base level queue can also be scheduled on a <a href="/wiki/First-come,_first-served" class="mw-redirect" title="First-come, first-served">first come first served</a> basis.<sup id="cite_ref-OS_Concepts,_8th_edition_6-0" class="reference"><a href="#cite_note-OS_Concepts,_8th_edition-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup></li> <li>Optionally, if a process blocks for I/O, it is <i>promoted</i> one level, and placed at the end of the next-higher queue. This allows I/O bound processes to be favored by the scheduler and allows processes to <i>escape</i> the base-level queue.</li></ul></dd></dl></dd></dl> <p>For scheduling, the scheduler always starts picking up processes from the head of the highest-level queue. Only if the highest-level queue has become empty will the scheduler take up a process from the next lower-level queue. The same policy is implemented for picking up in the subsequent lower-level queues. Meanwhile, if a process comes into any of the higher-level queues, it will preempt a process in the lower-level queue. </p><p>Also, a new process is always inserted at the tail of the top-level queue with the assumption that it will complete in a short amount of time. Long processes will automatically sink to lower-level queues based on their time consumption and interactivity level. In the multilevel feedback queue a process is given just one chance to complete at a given queue level before it is forced down to a lower-level queue. </p> <div class="mw-heading mw-heading3"><h3 id="Scheduling_parameters">Scheduling parameters</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Multilevel_feedback_queue&action=edit&section=3" title="Edit section: Scheduling parameters"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In general, a multilevel feedback queue scheduler is defined by the following parameters:<sup id="cite_ref-OS_Concepts,_8th_edition_6-1" class="reference"><a href="#cite_note-OS_Concepts,_8th_edition-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </p> <ul><li>The number of queues.</li> <li>The scheduling algorithm for each queue which can be different from FIFO.</li> <li>The method used to determine when to promote a process to a higher priority queue.</li> <li>The method used to determine when to demote a process to a lower-priority queue.</li> <li>The method used to determine which queue a process will enter when that process needs service.</li></ul> <div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Multilevel_feedback_queue&action=edit&section=4" title="Edit section: External links"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a rel="nofollow" class="external text" href="https://pages.cs.wisc.edu/~remzi/solaris-notes.pdf">Multilevel Feedback Queue Schedulers — Solaris 2.6 Time-Sharing</a></li> <li><a rel="nofollow" class="external text" href="https://www.lk.cs.ucla.edu/data/files/Kleinrock/Processor%20Sharing%20Queueing%20Models%20of%20Mixed.pdf">Processor Sharing Queueing Models of Mixed Scheduling Disciplines for Time Shared System</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Multilevel_feedback_queue&action=edit&section=5" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Lottery_scheduling" title="Lottery scheduling">Lottery scheduling</a></li> <li><a href="/wiki/Fair-share_scheduling" title="Fair-share scheduling">Fair-share scheduling</a></li> <li><a href="/wiki/Round-robin_scheduling" title="Round-robin scheduling">Round-robin scheduling</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Multilevel_feedback_queue&action=edit&section=6" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> 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<li id="cite_note-osc-ch5-p131-1"><span class="mw-cite-backlink"><b><a href="#cite_ref-osc-ch5-p131_1-0">^</a></b></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFSilberschatz1994" class="citation book cs1">Silberschatz, Abraham (1994). <i>Operating System Concepts, Fourth Edition</i>. Addison-Wesley. p. 131. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-201-50480-4" title="Special:BookSources/978-0-201-50480-4"><bdi>978-0-201-50480-4</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Operating+System+Concepts%2C+Fourth+Edition&rft.pages=131&rft.pub=Addison-Wesley&rft.date=1994&rft.isbn=978-0-201-50480-4&rft.aulast=Silberschatz&rft.aufirst=Abraham&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMultilevel+feedback+queue" class="Z3988"></span></span> </li> <li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222" /><cite id="CITEREFCorbatóMerwin-DaggettDaley1962" class="citation book cs1">Corbató, Fernando J.; Merwin-Daggett, Marjorie; Daley, Robert C. (1962). "An experimental time-sharing system". <i>Proceedings of the May 1-3, 1962, spring joint computer conference on - AIEE-IRE '62 (Spring)</i>. p. 335. <a href="/wiki/Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1145%2F1460833.1460871">10.1145/1460833.1460871</a>. <a href="/wiki/S2CID_(identifier)" class="mw-redirect" title="S2CID (identifier)">S2CID</a> <a rel="nofollow" class="external text" href="https://api.semanticscholar.org/CorpusID:14363753">14363753</a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=bookitem&rft.atitle=An+experimental+time-sharing+system&rft.btitle=Proceedings+of+the+May+1-3%2C+1962%2C+spring+joint+computer+conference+on+-+AIEE-IRE+%2762+%28Spring%29&rft.pages=335&rft.date=1962&rft_id=info%3Adoi%2F10.1145%2F1460833.1460871&rft_id=https%3A%2F%2Fapi.semanticscholar.org%2FCorpusID%3A14363753%23id-name%3DS2CID&rft.aulast=Corbat%C3%B3&rft.aufirst=Fernando+J.&rft.au=Merwin-Daggett%2C+Marjorie&rft.au=Daley%2C+Robert+C.&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMultilevel+feedback+queue" class="Z3988"></span></span> </li> <li id="cite_note-OS_Three_Pieces-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-OS_Three_Pieces_3-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222" /><cite id="CITEREFArpaci-DusseauArpaci-Dusseau2014" class="citation book cs1">Arpaci-Dusseau, Remzi H.; Arpaci-Dusseau, Andrea C. (2014). "Multi-level Feedback Queue". <a rel="nofollow" class="external text" href="https://pages.cs.wisc.edu/~remzi/OSTEP/cpu-sched-mlfq.pdf"><i>Operating Systems: Three Easy Pieces</i></a> <span class="cs1-format">(PDF)</span>. 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Addison-Wesley. p. 147. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-201-50480-4" title="Special:BookSources/978-0-201-50480-4"><bdi>978-0-201-50480-4</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Operating+System+Concepts%2C+Fourth+Edition&rft.pages=147&rft.pub=Addison-Wesley&rft.date=1994&rft.isbn=978-0-201-50480-4&rft.aulast=Silberschatz&rft.aufirst=Abraham&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMultilevel+feedback+queue" class="Z3988"></span></span> </li> <li id="cite_note-osc-ch5-p148-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-osc-ch5-p148_5-0">^</a></b></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222" /><cite id="CITEREFSilberschatz1994" class="citation book cs1">Silberschatz, Abraham (1994). <i>Operating System Concepts, Fourth Edition</i>. Addison-Wesley. p. 148. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0-201-50480-4" title="Special:BookSources/978-0-201-50480-4"><bdi>978-0-201-50480-4</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Operating+System+Concepts%2C+Fourth+Edition&rft.pages=148&rft.pub=Addison-Wesley&rft.date=1994&rft.isbn=978-0-201-50480-4&rft.aulast=Silberschatz&rft.aufirst=Abraham&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMultilevel+feedback+queue" class="Z3988"></span></span> </li> <li id="cite_note-OS_Concepts,_8th_edition-6"><span class="mw-cite-backlink">^ <a href="#cite_ref-OS_Concepts,_8th_edition_6-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-OS_Concepts,_8th_edition_6-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1238218222" /><cite id="CITEREFSilberschatzGalvinGagne2008" class="citation book cs1">Silberschatz, Abraham; Galvin, Peter Baer; Gagne, Greg (2008). <i>Operating system concepts</i> (8th ed.). Hoboken, N.J.: Wiley. p. 198. <a href="/wiki/ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <a href="/wiki/Special:BookSources/978-0470128725" title="Special:BookSources/978-0470128725"><bdi>978-0470128725</bdi></a>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=book&rft.btitle=Operating+system+concepts&rft.place=Hoboken%2C+N.J.&rft.pages=198&rft.edition=8th&rft.pub=Wiley&rft.date=2008&rft.isbn=978-0470128725&rft.aulast=Silberschatz&rft.aufirst=Abraham&rft.au=Galvin%2C+Peter+Baer&rft.au=Gagne%2C+Greg&rfr_id=info%3Asid%2Fen.wikipedia.org%3AMultilevel+feedback+queue" class="Z3988"></span></span> </li> </ol></div></div> <div class="navbox-styles"><style data-mw-deduplicate="TemplateStyles:r1129693374">.mw-parser-output .hlist dl,.mw-parser-output .hlist ol,.mw-parser-output .hlist ul{margin:0;padding:0}.mw-parser-output .hlist dd,.mw-parser-output .hlist dt,.mw-parser-output .hlist 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href="/wiki/Earliest_deadline_first_scheduling" title="Earliest deadline first scheduling">Earliest deadline</a></li> <li><a href="/wiki/Earliest_eligible_virtual_deadline_first_scheduling" title="Earliest eligible virtual deadline first scheduling">Earliest eligible virtual deadline first</a></li> <li><a href="/wiki/Fair-share_scheduling" title="Fair-share scheduling">Fair-share</a></li> <li><a href="/wiki/Fixed-priority_pre-emptive_scheduling" title="Fixed-priority pre-emptive scheduling">Fixed-priority pre-emptive</a></li> <li><a href="/wiki/Foreground-background" title="Foreground-background">Foreground-background</a></li> <li><a href="/wiki/Gang_scheduling" title="Gang scheduling">Gang</a></li> <li><a href="/wiki/Generalized_foreground-background" title="Generalized foreground-background">Generalized foreground-background</a></li> <li><a href="/wiki/Highest_response_ratio_next" title="Highest response ratio next">Highest response ratio next</a></li> <li><a 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scheduling">Stride</a></li> <li><a href="/wiki/Two-level_scheduling" title="Two-level scheduling">Two-level</a></li> <li><a href="/wiki/Windows_NT_processor_scheduling" title="Windows NT processor scheduling">Windows NT</a></li> <li><a href="/wiki/YDS_algorithm" title="YDS algorithm">YDS algorithm</a></li></ul> </div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Linux</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em"> <ul><li><a href="/wiki/Brain_Fuck_Scheduler" title="Brain Fuck Scheduler">Brain Fuck</a></li> <li><a href="/wiki/Completely_Fair_Scheduler" title="Completely Fair Scheduler">Completely Fair</a></li> <li><a href="/wiki/O(1)_scheduler" title="O(1) scheduler">O(1)</a></li> <li><a href="/wiki/O(n)_scheduler" title="O(n) scheduler">O(n)</a></li> <li><a href="/wiki/SCHED_DEADLINE" title="SCHED DEADLINE">SCHED DEADLINE</a></li> <li><a href="/wiki/SCHED_FIFO" class="mw-redirect" 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