Node-Local Encoding Cost Probe

Measured: 2026-09-07. This is a component experiment, not a media-policy decision. Node-local adaptive reuse owns the design model. Full measurement data includes both rounds, phase counts, timings, keyframe sizes and measured source digests.

Method

Windows 11 build 26200, Ryzen 7 9700X, 16 logical CPUs; existing libvpx 1.17.0 and unmodified Vp8Encoder, four threads per encoder, realtime CBR, no GPU.
Synthetic NV12 texture at 30 fps: mostly static with a small changing patch, or scrolling detail. Each size uses the same deterministic scene. Input preparation, scaling, decoding, capture and network are excluded.
Fixture outputs are 1920x1080 at 5 Mbps, 960x540 at 1.25 Mbps and 480x270 at 0.312 Mbps. These half/quarter-size fixtures are not a production ladder.
Steady arms run four seconds, excluding the first second. Serial scheduling isolates encoding work; two passes reverse scenario order. This is not a comparison against actual Browser WebRTC senders or hardware simulcast.
Trace arms replay two direct children for two seconds per phase: H/H -> H/L -> L/L -> M/M -> H/H -> H/L -> H/H. A new requested output needs an encoded keyframe, even if that layer was already being encoded.
envelope encodes all outputs at or below the maximum scripted demand. exact-cold encodes only demanded outputs and destroys inactive codecs. exact-warm retains at most three codec objects through the 14-second trace, without encoding inactive outputs. No LiveKit BWE or debounce is emulated.
Elapsed work uses a monotonic clock. Whole-window process CPU cycles include worker activity. GetProcessTimes is also retained but varied considerably in these short windows; conclusions below use cycles and elapsed work, not its apparent CPU percentage. Resident-memory deltas are not leak/peak-memory proof.

Steady Results

Means across two rounds. CPU work is relative process cycles per source frame, normalized to one high encode within the same scene, not whole-machine usage.

Active encodings	Mostly static ms/frame	Motion ms/frame	Relative CPU work, static / motion
High	4.26	6.52	1.00 / 1.00
Two independent high encodes	8.47	12.93	1.95 / 1.95
High plus low	4.68	7.11	1.09 / 1.09
High, middle and low	5.84	8.71	1.35 / 1.32
Middle plus low	1.69	2.35	0.39 / 0.36
Low	0.45	0.65	0.09 / 0.10

Three outputs cost about 31-32% fewer CPU cycles than two full-size outputs in this fixture, but 32-35% more than one. Lower outputs are not free. The three sizes contain 1.3125 times the high output's pixels, so this does not establish special multi-resolution work sharing: the current library build has CONFIG_MULTI_RES_ENCODING=0 and uses separate encoder contexts.

Steady high output produced 90 measured frames and the same total output bytes across the high-containing arms. This is a count/size check, not a bitwise or perceptual-quality proof. Two-high motion had one work sample over 33.3 ms in each round; the other steady arms had none. This is not a Viewer freeze count. Motion resident-memory deltas were about 70 MiB for high, 93 MiB for three outputs and 138 MiB for two high encodes, excluding the prebuilt input allocation.

Demand And Resume

The envelope used about 26% more CPU cycles than exact-cold on the scripted trace. The extra output counts were visible: low/middle/high encoded 420/360/300 frames versus 180/60/300 for exact demand. Two children requesting the same low output did not double its encode count. Stopping high output in the all-low phase did not discard the input fixture.

Naive warm retention saved only about 5 ms of total codec initialization across 14 seconds, compared with cold recreation, and retained about 23 MiB more resident memory at trace end. It also changed resumed output substantially:

High resumes at frame 240	Keyframe bytes, both rounds identical	Ready time, two rounds
Fresh codec after pause	32,268	22.79 / 19.76 ms
Retained codec, original source timestamps	228,469	21.79 / 21.56 ms
Retained codec, timestamp-gap ablation	31,079	13.24 / 12.90 ms

The ablation changes only the encoder PTS to count active encoded frames while retaining the same input pictures. It isolates the timestamp-gap effect; it is not an accepted replacement for source timestamps, RTP timing or A/V sync. No network was measured, so the larger keyframe is evidence of burst risk, not a demonstrated network stall.

The pinned libvpx source derives an internal frame rate from input timestamps. Its multi-resolution path explicitly uses the lowest active layer's frame rate to avoid a paused upper layer misreading the timestamp gap. That is a mature reuse lead, not a reason to implement an application clock workaround. The current probe did not enable or validate that multi-resolution implementation.

Decision And Next Evidence

Keep the maximum-demand envelope as a serious, simpler candidate. This result does not prove that exact demand is worth a custom retention controller, nor that the envelope switches receivers faster. Serial high-first scheduling and keyframe work still affect its local readiness time. Do not add a warm pool or timing workaround to production from this experiment.

Next compare the library's coordinated multi-resolution behavior, then measure H264 hardware cost and actual decode/forward switching under bounded congestion. Relay decode, scaling, surface copies, output reuse and inherited-low-input recovery are separate remaining costs. Blindly starting lower encoders at every healthy forwarding relay would violate the one-encode reuse objective.

Reproduction

Use the existing pinned libvpx build produced by the Windows App build. From a Visual Studio x64 developer shell, with VPX_INCLUDE and VPX_LIBRARY pointing to that build:

cl /nologo /std:c++20 /EHsc /O2 /MT /W4 /WX /external:I "%VPX_INCLUDE%" /external:W0 native/capture/windows/encoded_variants.probe.cpp native/capture/windows/vp8_encoder.cpp /Fo:build/embedded-media/ /Fe:build/embedded-media/encoded-variants.probe.exe "%VPX_LIBRARY%" /link /LTCG psapi.lib
node scripts/encoded-variants-probe.mjs

Create the build/embedded-media directory first. The Node entry runs 32 bounded processes sequentially with frame/count checks and writes its report under that directory. No default CI, application dependency, capture, room or production configuration was changed. All generated executables stay in this stable build path. The clock-ablation arm exists only to reproduce the observed pause effect.

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