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Algorithm: {{ algorithmShortLabel }} Requests: {{ resultsReady ? computation.values.request_count : '—' }} Average move: {{ resultsReady ? formatMovement(computation.values.average_movement, 2) : '—' }}
Run one classic educational policy over the current queue.
Use 1–12 cylinders, for example: 98, 183, 37, 122, 14, 124, 65, 67.
cylinder
Inclusive upper endpoint from 1 to 9,999.
cylinder
Initial head position inside the selected cylinder range.
Used by the four directional policies; retained but ignored for FCFS and SSTF.
{{ playbackSpeedLabel }}
Presentation-only timing; the neutral default is 2×.

The chart renderer is unavailable. The exact path remains available in the head trace and movement ledger.

{{ algorithmLabel }} trace

Inspect every service, physical endpoint, and circular wrap in the canonical path.

Trace position{{ traceCursor }} / {{ traceRows.length }}
Movement shown{{ formatMovement(visibleMovement) }}
Total movement{{ formatMovement(computation.values.total_movement) }}
Requests serviced{{ visibleServiceCount }} / {{ computation.values.request_count }}
Current decision

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Complete service order

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Educational model: cylinder distance approximates seek movement for classic magnetic-disk examples. It excludes arrival times, rotational latency, transfer time, firmware reordering, and modern device-specific schedulers.

StepKindMovementDistanceRequestDecisionCopy
{{ row.step }}{{ row.kindLabel }}{{ row.from }} → {{ row.to }}{{ row.distance }} cylinders{{ row.requestId || '—' }}{{ row.reason }}
CPU Scheduling Algorithm Simulator Compare CPU scheduling policies through dispatch traces and ready-queue decisions, including context-switch overhead and per-process timing metrics.
Page Replacement Algorithm Simulator Compare four page-replacement policies with step-by-step frame traces and see how fault counts, hit ratios and frame capacity change.
Memory Allocation Algorithm Simulator Compare contiguous memory fit policies through stepwise placements, coalescing and failure reasons alongside external fragmentation metrics.
Banker's Algorithm Deadlock Avoidance Simulator Test Banker’s algorithm states and resource requests with step-by-step safety traces, safe-sequence witnesses, and rollback decisions.

On a moving-head magnetic disk, serving requests in a different order changes how far the head travels between cylinders. A short path can reduce seek work, but the shortest-looking next move is not automatically the fairest policy for requests that have waited longer.

Classic disk-scheduling algorithms expose that tradeoff by applying different ordering rules to the same pending queue.

Policy Service rule Main lesson
FCFS Keep arrival order. Simple and predictable, but the head may cross the disk repeatedly.
SSTF Choose the nearest pending cylinder. Often reduces movement, but a steady stream of nearby requests can delay distant ones.
SCAN Sweep in one direction to the disk endpoint, then reverse. Orders service like an elevator and avoids repeated local choices.
C-SCAN Sweep one way, wrap at the endpoint, and resume in the same direction. Creates a more uniform directional service pattern.
LOOK Reverse at the last pending request rather than the physical endpoint. Avoids travel through an empty tail of the cylinder range.
C-LOOK Wrap from the last request in one direction to the farthest pending request on the other side. Combines circular service with request-bounded travel.

Head movement is a teaching proxy for seek distance, not a complete latency model. Rotational delay, transfer time, request priorities, caching, controller remapping, and newly arriving requests can change real performance. Solid-state storage has no moving head, and current operating systems use schedulers that address workloads and device behavior beyond these textbook policies.

A fair comparison keeps the queue, starting cylinder, cylinder range, and initial direction fixed. Changing any of them changes the path independently of the selected algorithm.

How to Use This Tool:

Use one request queue as a controlled experiment, then switch policies to compare service order and movement.

  1. Select FCFS, SSTF, SCAN, C-SCAN, LOOK, or C-LOOK.
  2. Enter 1 to 12 request cylinders separated by commas, spaces, or semicolons. Repeated cylinders remain separate requests in their original order.
  3. Set the inclusive Maximum cylinder from 1 to 9,999 and place Starting head between 0 and that maximum.
  4. Choose the initial direction for SCAN, C-SCAN, LOOK, or C-LOOK. FCFS and SSTF ignore that choice.
  5. Replay or step through the trace. Compare Total head movement, service order, and endpoint or wrap events before changing algorithms.

Interpreting Results:

Total head movement adds every service, endpoint, and wrap distance. Average movement divides that total by the number of requests, so it is comparable only when the queue and cylinder scale stay fixed.

  • A lower total means less movement for this fixed queue; it does not establish lower real I/O latency or better fairness.
  • Endpoint rows in SCAN and C-SCAN count travel to cylinder 0 or the selected maximum even when no request is there.
  • Wrap rows in C-SCAN and C-LOOK count distance but do not service a request at the destination until the next service event.
  • When requests are equally near, SSTF keeps their original queue order. Directional policies also keep source order for duplicate cylinders.

Technical Details:

A trace is an ordered series of head positions. Each event records the absolute difference between its starting and ending cylinder; zero-distance requests at the current head still count as serviced requests.

Formula Core:

For head positions h0 through hm, total movement is the sum of absolute consecutive differences. The displayed average uses the request count n, not the number of trace events.

TotalMovement = ∑j=1m|hj−hj−1| AverageMovement = TotalMovementn

Movement is reported in cylinder units. Total movement is an integer; the average retains full division precision until display formatting.

Mechanism Core:

Algorithm Ordered mechanism Tie or turn rule
FCFS Visit requests in source order. No reordering.
SSTF Recompute the nearest pending request after every service. Equal distance keeps source order.
SCAN Service sorted requests in the initial direction, visit the endpoint when work remains beyond the head, then reverse. Duplicate cylinders keep source order.
C-SCAN Service in one direction, visit the endpoint, wrap to the opposite endpoint, and continue in the same direction. The wrap is a non-service movement.
LOOK Service in one direction and reverse at the last pending request. No physical-endpoint event is added.
C-LOOK Service in one direction, wrap to the farthest pending request on the other side, then continue. The wrapped-to request is serviced in the following event.

References: