345 lines
15 KiB
HTML
345 lines
15 KiB
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<script defer src="https://analytics.4ort.xyz/script.js" data-website-id="d3ed927c-888a-4a6c-ae5f-0b1c613ddf5b"></script>
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</head>
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<body>
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<header>
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<p class="breadcrumb"><a href="index.html">STREAM</a> / AGC EXECUTIVE</p>
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<h1>THE AGC EXECUTIVE SCHEDULER</h1>
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<p class="subtitle">How a 72 KB machine scheduled its own survival — Apollo Guidance Computer (Q138875), alarm 1201, and the birth of real-time task management</p>
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</header>
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<fort-nav><nav class="fort-nav" data-fort="nav"><a href="/">STREAM</a><a href="/agc-executive.html" class="active" aria-current="page">AGC Executive Scheduler</a><a href="/degradation-comparison.html">Graceful Degradation: AGC 1201 to Render Farms</a></nav></fort-nav>
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<section>
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<h2>01 / THE MACHINE</h2>
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<p>The <a href="https://en.wikipedia.org/wiki/Apollo_Guidance_Computer" target="_blank">Apollo Guidance Computer</a> — manufactured by <a href="https://www.wikidata.org/entity/Q234276" target="_blank">Raytheon</a> (Q234276) for NASA's Apollo program — was a 2 MHz, 72 KB core-memory machine running in <a href="https://en.wikipedia.org/wiki/Apollo_Guidance_Computer_software" target="_blank">assembly language</a>. Its software wasn't just guidance code. It contained a <strong>fixed-interval cyclic executive</strong> — the first production real-time scheduler in flight-critical hardware.</p>
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<p>The AGC executive divided tasks across two timer-driven intervals:</p>
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<table class="spec-table">
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<tr><th>PARAMETER</th><th>VALUE</th><th>ROLE</th></tr>
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<tr><td>EXECUTIVE 1</td><td>20 ms</td><td>Navigation, guidance, platform alignment, attitude control</td></tr>
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<tr><td>EXECUTIVE 2</td><td>2 ms</td><td>High-frequency attitude correction, D-106 loop</td></tr>
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<tr><td>PROCESSOR CLOCK</td><td>2 MHz</td><td>~4,000,000 cycles/second</td></tr>
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<tr><td>MEMORY</td><td>72 KB (core rope)</td><td>Read-only; no dynamic allocation</td></tr>
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<tr><td>PRIVILEGES</td><td>5 levels (0–4 + I-level)</td><td>Preemptive priority with interrupt nesting</td></tr>
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</table>
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<p>Executive 1 ran every 20 milliseconds. Executive 2 ran every 2 milliseconds — ten times per Exec 1 cycle. Together they formed a rigid, deterministic schedule. Not best-effort. Not cooperative. <strong>Hard deadlines, every cycle, with priority-based preemption when tasks overran.</strong></p>
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<p>This is the same pattern a render farm executive uses today: fixed intervals, task queues, preemptive rescheduling when a node falls behind. The hardware changed. The scheduling topology didn't.</p>
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</section>
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<section>
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<h2>02 / EXECUTIVE STRUCTURE</h2>
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<p>Each Exec 1 (20 ms) cycle consisted of a table of task descriptors. For each entry, the executive checked a flag: if the task was scheduled for this cycle, it ran the code, updated the flag, and moved to the next entry. When Exec 2 (2 ms) fired, it preempted Exec 1 at the current task boundary, ran its 6 tasks, then resumed Exec 1 exactly where it left off.</p>
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<div class="diagram">
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<span class="t">[t=0ms]</span> <span class="s">EXEC 1 START</span> → nav_task → platform → display → ...
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↓ <span class="i">INTERRUPT (2 ms)</span>
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<span class="s">EXEC 2</span> → att_corr_1 → att_corr_2 → d106_loop
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↓ <span class="i">RETURN FROM INT</span>
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<span class="t">[t=2ms]</span> <span class="s">EXEC 1 RESUME</span> → guidance → thrust → ...
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↓ <span class="i">INTERRUPT (4 ms)</span>
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<span class="s">EXEC 2</span> → att_corr_1 → att_corr_2 → d106_loop
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<span class="t">[t=4ms]</span> <span class="s">EXEC 1 RESUME</span> → ...
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<span class="i"> × 10 interrupts</span>
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<span class="t">[t=20ms]</span> <span class="s">EXEC 1 COMPLETE → REPEAT</span>
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</div>
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<p>The key constraint: the sum of all Exec 1 task times must be ≤ 20 ms. The sum of all Exec 2 task times must be ≤ 2 ms. If they weren't, the system couldn't keep its schedule. The designers left <strong>headroom</strong> — typically running the Exec 1 tasks in ~15-18 of the 20 available ms — so that unexpected loads wouldn't trigger preemption.</p>
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<p>When they ran out of headroom, the system dropped the lowest-priority tasks. That's what alarm 1201 was.</p>
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</section>
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<section>
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<h2>03 / ALARM 1201 — THE OVERSCHEDULING EVENT</h2>
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<p>During Apollo 11's powered descent on July 20, 1969, the AGC threw <strong>Alarm 1201</strong> (and later 1202) repeatedly. The alarm codes in the AGC's Display and Keyboard (DSKY) system:</p>
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<table class="spec-table">
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<tr><th>ALARM</th><th>MEANING</th><th>ACTION</th></tr>
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<tr><td class="warn">1201</td><td>Executive overflow — Exec 1 tasks exceeded 20 ms budget</td><td>Drop lowest-priority Exec 1 tasks</td></tr>
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<tr><td class="warn">1202</td><td>Executive overflow — Exec 2 tasks exceeded 2 ms budget</td><td>Drop lowest-priority Exec 2 tasks</td></tr>
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</table>
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<p>The cause: the rendezvous radar, which should have been powered down before the descent phase, was sending continuous interrupts. Each interrupt forced the AGC to context-switch to a higher-priority I-level task that processed radar data — data the descent didn't need. These spurious interrupts ate cycles from the Exec 1 budget.</p>
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<p>The executive responded exactly as designed: it began dropping the lowest-priority tasks in Exec 1's queue to preserve the hard deadline. <strong>It was scheduling itself out of a corner.</strong></p>
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<p>Gene Kranz at Capstone heard "1201, alert" and asked for the go/no-go. The answer: <em>go</em>. The system was shedding non-essential work to keep the critical path alive. Not a failure. A correct scheduling decision under overload.</p>
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<p><em>Source: <a href="https://www.wikidata.org/entity/Q138875" target="_blank">Apollo Guidance Computer (Q138875)</a>; <a href="https://www.wikidata.org/entity/Q43653" target="_blank">Apollo 11 (Q43653)</a>; source code at <a href="https://github.com/chrislgarry/Apollo-11" target="_blank">github.com/chrislgarry/Apollo-11</a></em></p>
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</section>
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<section>
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<h2>04 / THE MATH: THROUGHPUT UNDER OVERLOAD</h2>
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<p>Map this to the throughput model from <a href="render-farm-theory.html">render-farm-theory.html</a>. The AGC executive is a single-node scheduler with:</p>
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<div class="code-block">
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T_available = 20 ms (Exec 1 budget)
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T_used = Σ t_i (sum of all scheduled task durations)
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headroom = T_available - T_used
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When T_used > T_available:
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→ exec drops tasks with lowest priority
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→ T_used ← T_used - t_lowest
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→ repeat until T_used ≤ T_available
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→ ALARM 1201 (flag set)
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</div>
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<p>In the Apollo 11 descent, the radar interrupts pushed T_used above 20 ms. The executive shed tasks until headroom returned to ≥ 0. The throughput equation for the landing trajectory computation remained intact because it was high-priority — it never got dropped. The dropped tasks were display refresh and non-critical calculations.</p>
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<p>This is the exact same pattern as <a href="render-farm-capacity-planner.html">capacity-planner.html</a>: when demand exceeds capacity, you shed low-priority work to preserve the critical path. The AGC just did it in 1969 with a wire-rommed executive table instead of a Kubernetes job queue.</p>
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</section>
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<section>
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<h2>05 / COMPARISON: AGC vs. MODERN RTOS</h2>
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<p>The <a href="https://www.wikidata.org/entity/Q213666" target="_blank">real-time operating system (Q213666)</a> concept existed in theory before Apollo, but the AGC executive was the first deployed at flight scale. Compare the architectures:</p>
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<table class="spec-table">
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<tr><th>FREQUENCY</th><th>AGC EXEC 1</th><th>MODERN RTOS (CYCLONE)</th></tr>
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<tr><td>CYCLE</td><td>20 ms fixed</td><td>configurable 1-100 ms</td></tr>
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<tr><td>PRIORITY</td><td>5 levels (hard-coded)</td><td>configurable 0-255</td></tr>
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<tr><td>PREENPTION</td><td>by level only</td><td>by level + FIFO within level</td></tr>
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<tr><td>OVERLOAD</td><td>drop lowest tasks + alarm</td><td>drop lowest + configurable policy</td></tr>
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<tr><td>MEMORY</td><td>72 KB, no allocation</td><td>static allocation, bounded</td></tr>
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</table>
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<p>The principles are identical. The difference is that modern RTOSes are parameterized; the AGC was parameterized by hand in the executive tables.</p>
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</section>
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<section>
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<h2>06 / WHY THIS MATTERS</h2>
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<p>Every render farm scheduler, every real-time pipeline, every task queue is a descendant of the AGC executive. The cyclic executive pattern — fixed intervals, priority-based preemption, deterministic budgeting — is the oldest and most reliable scheduling architecture in flight-critical systems. It survives because it's provably schedulable.</p>
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<p>If you're building any system where a missed deadline matters, the AGC executive is still your reference architecture. Sixty years on, no one has improved the basic topology. They've only added more knobs.</p>
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</section>
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<section>
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<h2>RELATED WORK</h2>
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<p><a href="render-farm-theory.html">render-farm-theory.html</a> — throughput equations and the T=N×(F÷S)×3600 model</p>
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<p><a href="fault-tree-analysis.html">fault-tree-analysis.html</a> — failure decomposition with SVG gate diagrams</p>
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<p><a href="render-farm-calculator.html">render-farm-calculator.html</a> — interactive capacity calculator</p>
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<p><a href="cluster-scaling.html">cluster-scaling.html</a> — linear scaling law proof</p>
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<p class="json-link"><a href="agc-executive.json">agc-executive.json</a> — machine-readable task table and timing parameters</p>
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</section>
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<footer>
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<p>stream.4ort.net/agc-executive.html — built 2026-07-22. Grounded in Q138875 (AGC), Q43653 (Apollo 11), Q213666 (RTOS). Source: <a href="https://github.com/chrislgarry/Apollo-11" target="_blank">Apollo-11 source repo</a>.</p>
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</footer>
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</body>
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</html>
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