From 3641f37e289175e94d9148ad21e797976a3746a5 Mon Sep 17 00:00:00 2001 From: prosolis <5590409+prosolis@users.noreply.github.com> Date: Sun, 23 Aug 2026 15:28:29 -0700 Subject: [PATCH] The bus is 4x idle while the CPU is pinned: price the trade The codec was designed when bytes were scarce, so every decision in it trades cycles to save bytes. That is now backwards: sasi spends 110 KB/s of a 488 KB/s pipe while missing 31% of frames on CPU. The cheapest thing a 68000 can be handed is the most expensive thing to store. Measured, per pixel: row-linear copy from word-expanded memory 9.08 cycles, block-order 12.98, V1 codebook 18.74, RAW byte literals 25.03. So the 1024-byte stride costs 43% and unpacking bytes to words costs more than the write itself. Pricing one new mode -- a per-row span of word-expanded literals movem.l'd straight from the stream buffer -- against the UNCHANGED mode maps: sasi median 74.4% -> 43.0%, worst 136.2% -> 106.2%, misses 37 -> 8/120, 101.7 -> 453.2 KB/s scsi median 94.9% -> 69.4%, misses 51 -> 18/120, 272 -> 479.7 KB/s scsi gains less precisely because it has less idle bandwidth left to trade. Two consequences worth flagging. A word-expanded literal block derives to ~240 cycles, cheaper than V1's measured 299.9 and pixel-exact -- so every codebook mode is CPU-dominated by a literal, and the codebook is a byte optimisation that now costs cycles. And 28.5's "a scene cut cannot fit at 12fps" reopens: CPU needs >=19% of the frame as spans, the bus allows <=39%, and that interval is not empty. DERIVED, NOT MEASURED, and labelled as such everywhere. The 9.08 cycles/pixel is real but was measured at full row width with 12-register bursts, so short spans are flattered. Measuring one span on the 68000 is now step 0 of the next session, ahead of the cost-aware mode decision, because it changes the mode set that decision optimises over. FINDINGS 29. tools/analysis/12_span_tradeoff.py. Claude-Session: https://claude.ai/code/session_01194oWYW8DQXK1SZ2DnChW6 --- docs/FINDINGS.md | 111 +++++++++++++++++++++++++++++ docs/STATUS.md | 67 ++++++++++++----- tools/analysis/12_span_tradeoff.py | 111 +++++++++++++++++++++++++++++ 3 files changed, 272 insertions(+), 17 deletions(-) create mode 100644 tools/analysis/12_span_tradeoff.py diff --git a/docs/FINDINGS.md b/docs/FINDINGS.md index 0a55a29..e902dc2 100644 --- a/docs/FINDINGS.md +++ b/docs/FINDINGS.md @@ -1423,3 +1423,114 @@ Caveat: this ordering is a property of *this* decoder, not of the codec. V4's cost is four indexed `movem.l` lookups; pairing sub-block rows into `movem.l d0/d2,(a4)` would save ~16 of 448 cycles, which narrows the gap to RAW without closing it. + +--- + +## 29. Trading bytes for cycles: the bus has 4x the headroom the CPU has (session 7) + +> **STATUS: DERIVED, NOT MEASURED.** No 68000 has executed a span decoder. The +> per-pixel figure it rests on *is* measured (FINDINGS 24 V1) but at full row +> width; the per-span overhead is hand-derived. Treat every number below as a +> hypothesis with a test attached, not as a result. FINDINGS 4 is why. + +FINDINGS 28 leaves the project CPU-bound while the **bus sits 4x idle**: `sasi` +spends 110 KB/s of a 488 KB/s pipe. That asymmetry is exploitable, because the +codec was designed when bytes were the scarce thing and every one of its +decisions trades cycles to save them. + +### 29.1 The decoder pays per changed PIXEL; the disk pays per BYTE +Per-pixel costs, all measured: + +| what | cycles/pixel | source | +|---|---:|---| +| write-only floor (no source read) | 4.59 | FINDINGS 24 V3 | +| **row-linear copy from word-expanded RAM** | **9.08** | FINDINGS 24 V1 | +| block-order copy, same bytes | 12.98 | FINDINGS 24 V4 | +| V1 codebook block | 18.74 | FINDINGS 28.2 | +| RAW, byte literals unpacked to words | 25.03 | FINDINGS 28.2 | +| naive per-pixel byte expansion | 26.13 | FINDINGS 24 V2 | + +Two structural facts fall out. **The 1024-byte stride costs 43%** — the same +bytes cost 12.98 cycles/px in 4x4 block order against 9.08 row-linear, because +the stride breaks the `movem.l` burst. And **unpacking bytes to words costs more +than the write itself**: 25.03 against 9.08. + +So the two cheapest things a decoder can be handed are *word-expanded* pixels +in *row-linear runs* — and both cost bytes on disc, which is what we have. + +### 29.2 Codebooks are a byte optimisation that now costs cycles +A word-expanded literal 4x4 block, `movem.l (a0)+,d0-d7` straight from the +stream buffer into GVRAM, derives to **~240 cycles** — cheaper than V1's +measured 299.9, and pixel-exact. V1 is dearer *because* it is compressed: it +pays an index decode and an indexed `movem.l` that a literal does not, and then +does exactly the same four writes. It buys 31 bytes and spends 60 cycles. + +**Every codebook mode is CPU-dominated by a literal.** V4 was already dominated +by RAW (28.8); with word-expanded literals available, so is V1. The VQ codebook +earns its place only while bytes are scarce. + +### 29.3 Row-linear literal spans, priced against the real mode maps +Replace the per-block escape with a per-row **span**: `(x, count, word-expanded +pixels)`, decoded with `movem.l` bursts. A run of L horizontally adjacent dirty +blocks becomes 4 spans of 4L pixels, deriving to `4 * (50 + 4L * 9.08)` cycles +against `300L` for V1 — **cheaper for any run of 2 blocks or more**, at 32 bytes +per block instead of 1. + +Applied greedily (buy the best cycles-saved-per-byte until the bus budget is +gone) to the *unchanged* mode maps of the `sasi` Singe window: + +| | today | + literal spans | +|---|---:|---:| +| median frame | 74.4% | **43.0%** | +| p90 frame | 115.1% | **83.6%** | +| worst frame | 136.2% | **106.2%** | +| frames missing the budget | **37/120** | **8/120** | +| bitrate | 101.7 KB/s | 453.2 KB/s (bus 488) | + +And the fit is structural rather than lucky: **spans get cheaper exactly where +blocks get expensive.** A span amortises its overhead over a long run, and long +runs are what a high-change frame is made of. The frames that miss today are the +frames spans help most. + +### 29.4 This reopens 28.5, which said a scene cut cannot fit +28.5 concluded that no mode assignment fits a 100%-changed frame at 12fps, +because the cheapest full redraw available — all-V1 — is 110.5%. That was true +of *the mode set the codec has*. Adding a byte-expensive, cycle-cheap mode +changes the arithmetic: mixing a fraction `x` of the frame as spans against V1 +for the rest, + +- CPU needs `x >= 0.19` +- the 40,977 B/frame bus budget allows `x <= 0.39` + +**The interval is not empty.** A scene cut fits at 12fps if roughly a quarter to +a third of it arrives as word-expanded row-linear literals. 28.5's "structural +ceiling" was a ceiling of the bitstream, not of the machine. + +### 29.5 What has to be measured before any of this is believed +1. **Span cost on the 68000.** The 50-cycle per-span overhead is derived, and + the 9.08 cycles/px is measured at *full row width* with 12-register bursts — + a short or oddly-aligned span cannot burst as well, so short spans are + flattered here. Extend `tools/bench/blit.s` with a span variant and measure + it against run length. **This is the load-bearing number.** +2. **Re-run the ring-buffer simulation at ~450 KB/s.** FINDINGS 21's zero + required prefill was established at 110 and 280 KB/s against a 488 KB/s pipe. + At 453 the margin is a tenth of what it was, and 21's own caveat was that the + test is cumulative — it needs redoing, not extrapolating. +3. **Confirm the 4 Mbps figure**, which is user-supplied with no recorded + provenance and which this design would run at 93% of. It has been a "would be + nice" since session 1; a design that leans on it makes it load-bearing. +4. **Confirm DMA, not PIO** (STATUS priority 5). At 453 KB/s a PIO fallback puts + the transfer cost on the CPU we are trying to relieve. Cheapest check + available and now the most consequential. + +### 29.6 The other lever, not yet costed: let the DMAC do the copy +The X68000 has an HD63450 DMAC (4 channels, `x68k.cpp:1046`). Channel 3 is +ADPCM — confirmed, `adpcm_drq_tick` asserts `drq3_w` — but memory-to-memory +transfer on a free channel would take the GVRAM copy off the CPU entirely, +leaving it only the parsing. This is the one idea here that could move the +budget without spending a single extra byte. + +It cannot be settled in MAME: like the SCSI/SASI devices (BENCHMARK.md), the +HD63450 is a functional model, so a timing number out of it would measure the +emulator's scheduler. It needs hand-derivation against the datasheet plus real +hardware — the same three-tier approach the disk benchmark already documents. diff --git a/docs/STATUS.md b/docs/STATUS.md index b81ecda..10527d7 100644 --- a/docs/STATUS.md +++ b/docs/STATUS.md @@ -1,16 +1,42 @@ # Status & next-session handoff — end of session 7 (2026-08-23) -## NEXT SESSION: make the mode decision cost-aware +## NEXT SESSION: measure a span, then make the mode decision cost-aware The decoder exists, it is pixel-exact, and **it does not fit**. On the worst -sustained window at the shipping `sasi` profile it costs a mean of **81.7% of a -12fps frame budget** and **31% of frames exceed 100%** (`scsi`: 94.9% median, -42% of frames miss). FINDINGS 28. CPU is now the binding constraint — the first -time in this project that it has been. +sustained window at `sasi` it costs a mean of **81.7% of a 12fps frame** and +**31% of frames exceed 100%** (`scsi`: 94.9% median, 42% miss). FINDINGS 28. +CPU is the binding constraint now — the first time in this project. -The fix is not assembly micro-optimisation. It is that **`vq_hybrid.decide()` -minimises `D + lam*R` — distortion against BYTES — on a machine where the -binding budget is CYCLES**, and the two are not proportional: +**Two levers, and the cheap one has to be measured first.** + +*Lever A — spend bandwidth to buy cycles.* The bus sits 4x idle: `sasi` uses 110 +KB/s of 488. Every codec decision was made when bytes were scarce, so each one +trades cycles to save them, and the cheapest thing a 68000 can be handed is the +most expensive thing to store — **word-expanded pixels in row-linear runs**. +Adding one mode, a per-row span of literal words `movem.l`-ed straight from the +stream buffer into GVRAM, prices out at (FINDINGS 29, `12_span_tradeoff.py`): + +| | today | + literal spans | +|---|---:|---:| +| median frame | 74.4% | **43.0%** | +| worst frame | 136.2% | **106.2%** | +| frames missing | **37/120** | **8/120** | +| bitrate | 101.7 KB/s | 453.2 KB/s (bus 488) | + +**This is DERIVED, not measured, and it is load-bearing — so measure it first.** +Extend `tools/bench/blit.s` with a span variant and time it against run length. +The 9.08 cycles/pixel it rests on is real (FINDINGS 24 V1) but was measured at +full row width with 12-register bursts; short and oddly-aligned spans cannot +burst as well and are flattered by the model. If spans come in near the derived +figure, the whole mode set changes and lever B optimises over different modes — +which is exactly why this goes first. FINDINGS 29.5 lists the other three things +that have to hold, of which **confirming DMA vs PIO is the cheapest and now the +most consequential**: at 453 KB/s a PIO fallback puts the transfer back on the +CPU this is trying to relieve. + +*Lever B — stop buying modes the CPU cannot afford.* `vq_hybrid.decide()` +minimises `D + lam*R` — distortion against BYTES — on a machine whose binding +budget is CYCLES, and the two are not proportional: | mode | payload bytes | measured cycles | cycles per byte | |---|---:|---:|---:| @@ -18,13 +44,18 @@ binding budget is CYCLES**, and the two are not proportional: | V1 | 1 | 300 | 300 | | V4 | 4 | 448 | 112 | | RAW | 16 | 400 | 25 | +| *word-expanded literal block* | *32* | *~240 (derived)* | *7.5* | V4 is **25% of blocks and 50% of the cycles**. The lagrangian charges it 4x a V1 -block; the CPU charges it 1.49x. So the encoder currently buys V4 whenever it is -worth 4 bytes, with no idea what it costs to draw. +block; the CPU charges it 1.49x. Note the last row: a literal block is cheaper +than **every** codebook mode, and pixel-exact — the codebook is a byte +optimisation that now costs cycles (FINDINGS 29.2). **The work, in order:** +0. **Measure the span cost on the 68000** (lever A above). Cheap, and everything + below optimises over whatever mode set it leaves. + 1. **Add a cycle term to the mode decision.** `decide()` already builds a cost matrix of `error + lam * bytes` per mode per block; add `+ mu * cycles`, with the per-mode cycles measured in FINDINGS 28.2. @@ -66,13 +97,15 @@ worth 4 bytes, with no idea what it costs to draw. `sasi` cannot afford it, so expect the cycle ceiling to cost `sasi` more quality even though it costs `sasi` fewer cycles. FINDINGS 28.8. -4. **28.5 may not be solvable by the encoder at all.** An all-V1 frame — the - cheapest possible full redraw — is **110.5%** of the budget. A scene cut - changes 100% of the screen, so *no* mode assignment fits one at 12fps. Decide - deliberately: allow one late frame at a cut (the outgoing content is - unrelated, so it may be invisible), spread a cut over two frame times, or - drop to 10fps where an all-V1 frame fits. This is a design decision, not a - measurement, and it needs the user. +4. **Scene cuts: 28.5 said impossible, 29.4 reopened it.** An all-V1 frame — + the cheapest full redraw the *current* mode set allows — is 110.5% of budget, + so no mode assignment fits a 100%-changed frame. With literal spans the + arithmetic changes: CPU needs at least 19% of the frame sent as spans, the + bus allows up to 39%, **and that interval is not empty**. So 28.5 was a + ceiling of the bitstream, not of the machine — *if* lever A measures out. + If it does not, this is still a design decision that needs the user: one late + frame at each cut (the outgoing content is unrelated, so it may be + invisible), a cut spread over two frame times, or 10fps. **Do not start by hand-optimising `decode.s`.** The hand-derived timings agree with the measurements to 0.5% on V1 and 1% on RAW (FINDINGS 28.4), so the diff --git a/tools/analysis/12_span_tradeoff.py b/tools/analysis/12_span_tradeoff.py new file mode 100644 index 0000000..a37d67a --- /dev/null +++ b/tools/analysis/12_span_tradeoff.py @@ -0,0 +1,111 @@ +#!/usr/bin/env python3 +"""What does spending the idle bus bandwidth buy back in CPU cycles? + + python3 tools/analysis/12_span_tradeoff.py [container.dlx] [--bus 488] + +FINDINGS 28 leaves the decoder CPU-bound at 110 KB/s on a 488 KB/s pipe. Every +codec decision was made when bytes were scarce, so each one trades cycles to +save them -- and the cheapest thing a 68000 can be handed is the most expensive +thing to store: word-expanded pixels in row-linear runs. + +This prices ONE new mode against the real mode maps: a per-row SPAN of +word-expanded literals, `movem.l`-ed straight from the stream buffer into GVRAM. +A run of L horizontally adjacent dirty blocks becomes 4 spans of 4L pixels. + +DERIVED, NOT MEASURED (FINDINGS 29). The 9.08 cycles/pixel is measured +(FINDINGS 24 V1) but at full row width with 12-register bursts; SPAN_OVERHEAD is +hand-derived. Short spans are therefore flattered. Measure before believing -- +FINDINGS 29.5 item 1. + +The mode maps are NOT re-optimised: this only re-codes regions the encoder +already chose to redraw, so it is a lower bound on what a cost-aware encoder +would find. +""" +import sys, os, argparse +sys.path.insert(0, "tools/encoder") +import numpy as np +from dlx import DLX + +FRAME_CYC = 833333.0 # 12fps at 10 MHz +AUDIO_KBPS = 7.8 + +CYC_PX_ROWLIN = 446286 / 49152. # 9.08, FINDINGS 24 V1 (measured) +C_V1, C_V4, C_RAW = 299.9, 448.2, 400.4 # FINDINGS 28.2 (measured) +C_SKIP_CLUSTERED, C_SKIP_MIXED = 13.25, 45.0 +SPAN_OVERHEAD = 50.0 # per span, DERIVED +SPAN_BYTES_PX = 2 # word-expanded: 1 pixel = 1 word +SPAN_HDR = 3 # x, count, and a byte of slack + +ap = argparse.ArgumentParser() +ap.add_argument("container", nargs="?", + default="tmp/rc_fr_singe_sasi_rcprofile.dlx") +ap.add_argument("--bus", type=float, default=488.0, + help="sustained KB/s the pipe delivers (FINDINGS 21)") +ap.add_argument("--fps", type=float, default=12.0) +a = ap.parse_args() +if not os.path.exists(a.container): + sys.exit(f"missing {a.container}") + +BYTE_BUD = (a.bus - AUDIO_KBPS) * 1024 / a.fps +d = DLX(a.container) +BLK_C = {1: C_V1, 2: C_V4, 3: C_RAW} +BLK_B = {1: 1, 2: 4, 3: 16} + +rows = [] +for f in range(d.nframes): + mode = d.modes(f) + g = mode.reshape(-1, 4) + allskip = (g == 0).all(1) + base = allskip.sum() * 4 * C_SKIP_CLUSTERED + mm = g[~allskip] + base += (mm == 0).sum() * C_SKIP_MIXED + for k, c in BLK_C.items(): + base += (mm == k).sum() * c + base_b = d.mode_bytes + sum(BLK_B.get(int(x), 0) for x in mode) + + m = mode.reshape(d.nby, d.nbx) + cand = [] + for by in range(d.nby): + dirty = m[by] != 0 + i = 0 + while i < d.nbx: + if not dirty[i]: + i += 1 + continue + j = i + while j < d.nbx and dirty[j]: + j += 1 + L = j - i + cur_c = sum(BLK_C[int(b)] for b in m[by][i:j]) + cur_b = sum(BLK_B[int(b)] for b in m[by][i:j]) + span_c = 4 * (SPAN_OVERHEAD + 4 * L * CYC_PX_ROWLIN) + span_b = 4 * (SPAN_HDR + 4 * L * SPAN_BYTES_PX) + if span_c < cur_c: + cand.append((cur_c - span_c, span_b - cur_b, L)) + i = j + + cand.sort(key=lambda s: -(s[0] / max(s[1], 1))) # best cycles per byte + cyc, byt, taken = base, base_b, 0 + for dc, db, L in cand: + if byt + db <= BYTE_BUD: + cyc -= dc; byt += db; taken += 1 + rows.append((base, cyc, base_b, byt, len(cand), taken)) + +base, new, bb, nb, ncand, ntaken = map(np.array, list(zip(*rows))) +pc = lambda v: 100 * v / FRAME_CYC + +print(f"{a.container}: {d.nframes} frames") +print(f"bus {a.bus:.0f} KB/s - {AUDIO_KBPS} audio -> {BYTE_BUD:,.0f} B/frame " + f"at {a.fps:g}fps\n") +print(f"{'':<26}{'today':>12}{'+ literal spans':>18}") +for label, fn in (("median frame", np.median), + ("p90 frame", lambda v: np.percentile(v, 90)), + ("worst frame", np.max)): + print(f" {label:<24}{pc(fn(base)):>11.1f}%{pc(fn(new)):>17.1f}%") +print(f" {'frames missing budget':<24}{int((base>FRAME_CYC).sum()):>8}/{d.nframes}" + f"{int((new>FRAME_CYC).sum()):>14}/{d.nframes}") +print(f" {'bitrate':<24}{bb.mean()*a.fps/1024:>10.1f} KB/s" + f"{nb.mean()*a.fps/1024:>13.1f} KB/s") +print(f"\nspans taken: {ntaken.sum()} of {ncand.sum()} candidate runs " + f"({100*ntaken.sum()/max(ncand.sum(),1):.0f}%) -- the rest priced out by the bus") +print("\nDERIVED, NOT MEASURED: see FINDINGS 29.5 before acting on this.")