Skip to content

qwen3_5 cuda: --kv-cache fp8, e4m3 attention caches with a scale a row - #886

Open
BobClawblaw wants to merge 35 commits into
ashhart:mainfrom
BobClawblaw:cuda-kv-fp8
Open

BobClawblaw wants to merge 35 commits into
ashhart:mainfrom
BobClawblaw:cuda-kv-fp8

Conversation

@BobClawblaw

@BobClawblaw BobClawblaw commented Oct 15, 2026 •

Copy link
Copy Markdown
Contributor

Stacked on #859 (then #849 and #836). Until those merge, this PR shows their commits too. The change here is the last commit, 0eebbd32, rebased onto #859's 7cddf336 (on #836's 5e1c3fea). It was split out of #859 so that each PR makes one change.

What this changes

tensorfold-native serve --kv-cache fp8 stores the Qwen3.5 hybrids' attention keys and values as e4m3 rows with one fp32 scale each: amax / 448, the tf_kv_quant format. The linear-attention state stays fp32 and the convolution tails stay bf16. A full-attention layer's cache takes 520 bytes a position and head, against bf16's 1,024. On a 16 GB card, the 9B 8-bit serves its full 262,144-token window and the 27B oQ2 serves 106,496 tokens.

  • zig/kernels/cuda/qwen35_kv8.cu adds three kernels:
    • tf_q35_rope8 quantizes each new key and value row as it lands.
    • tf_q35_attn_partial8 runs the bf16 path's chunked attention over e4m3 rows, with chunks cut from each row's own keys, as Windows: the native CUDA server, and Qwen3.5-2B and Qwen3.8-27B on CUDA #836's 8570e068 does for bf16. It takes each key's dot product on its codes and then applies the key's scale, and each value's weight carries the value's scale.
    • tf_q35k_compact8 moves kept rows and their scales for a tree's accepted path.
  • The helpers that both caches share (past_rows, rotate, rows_seen, seen_row) move into qwen35.cuh.
  • The engine chooses the kernels at init. Geometry.kvBytes(cap, kv8) sizes the cache, and the startup line says "(fp8 caches)".
  • Plumbing:
    • engine_api.Open.kv8;
    • --kv-cache bf16|fp8 in cli.zig, with bf16 the default;
    • a refusal for CUDA families whose options don't take it, and a refusal on Metal;
    • tf-qwen35-cuda takes an optional sixth argument, fp8.

Which sums change: only with --kv-cache fp8. Attention scores and values then read e4m3-rounded keys and values. The flag is off by default, and bf16 caches give the same bits as before: logits are byte-equal to #859's in step, window, tree and prompt modes on the 2B, a mixed-width 2B and the 27B.

Receipt

  • Environment: this PR's head (0eebbd32) on an RTX 4080 SUPER 16 GB (sm_89), Windows 11, CUDA 13.3, Zig 0.17.0, -Dsm=89. Checkpoints:

    • lmstudio-community/Qwen3.5-9B-MLX-4bit @ b455506b
    • lmstudio-community/Qwen3.5-9B-MLX-8bit @ 950e0459
    • TensorFold/Qwen3.8-27B-oQ2 @ 086dcb02
    • mlx-community/Qwen3.5-2B-MLX-4bit @ 93760be4
  • Exactness:

    • bench_concurrent --alone --serial with --kv-cache fp8 (levels 1 and 4, 4 cells): 48/48 equal alone and 10/10 equal to "draft": false, on the 9B 4-bit and on the 27B oQ2.
    • tf-qwen35-cuda with fp8 caches and a 20,000-token second stream stepping beside (TF_Q35_BESIDE): step, window and tree logits equal the stream alone, on the 2B and the 27B. Also after a 3,000-token prefill with a 200-token stream beside.
    • tf-qwen35-cuda over 300 ids with fp8 caches: window and tree equal step bit for bit, on the 2B and the 27B.
    • A served suite with fp8 caches gave drafts 12/12 and concurrent 12/12 on the 9B 4-bit, the 9B 8-bit and the 27B.
    • With the DFlash2 drafter (z-lab/Qwen3.8-27B-DFlash2 @ 50307d4c), on the 27B oQ2 served with --drafter --parallel 2 --context 8192 --kv-cache fp8:
      • Exactness: 24/24 equal alone, and 12/12 drafted replies equal to "draft": false.
      • Drafted decode, one stream: 190 tok/s on code and 97 on chat, greedy, against 175 and 95 with bf16 caches.
      • Capacity: with fp8 caches, two streams at a 32,768-token window fit beside the drafter, at 1.26 GiB each. With bf16 caches they don't.
    • Two long streams at once: the 9B 4-bit served with --context 262144 --parallel 2 --kv-cache fp8, given two different 120,011-token prompts together.
      • Exactness: each greedy 64-token reply's token_sha equals its run alone, and the first prompt's drafted reply equals its "draft": false one.
      • Timing: alone, each prompt takes about 195 s to its first token. Together, the second prompt's pass runs after the first's, so it gets its first token at 394 s. The first stream's decode waits while the second prompt runs.
  • Decode speed (bench_openai, 9B 4-bit, one stream, medians over 5 reps):

    • bf16 caches: 99.9–100.9 tok/s.
    • fp8 caches: 99.1–101.6 tok/s.
    • Four streams with fp8 caches: 319–341 tok/s, against 322–343 with bf16.
    • 27B oQ2 with fp8 caches: 45.9–46.2 tok/s, against 46.0–46.3 with bf16.
  • Prompt speed (bench_cache --require-cold, cold 16,000-token prompt, 9B 4-bit): 9.30 s with fp8 caches, 8.89 s with bf16 (27B oQ2: 25.04 s against 23.66 s). The prompt writes and reads quantized rows, so it costs about 5% at this length.

  • What fp8 costs:

    • README perplexity (9B 4-bit, 1,024 tokens): 9.267 with fp8, 9.289 with bf16.

    • Top-1 over 300 ids: agrees with bf16 caches on 291/300 (2B) and 295/300 (27B oQ2).

    • Long context (needles and facts, --parallel 1, measured before the rebase; one stream's fp8 logits are byte-equal before and after it):

      Checkpoint bf16 caches fp8 caches
      9B 4-bit 55/55 at 232k 53/55 at 256k (facts 8/10 at 225k)
      9B 8-bit 42/42 at 120k (the most bf16 fits) 55/55 at 256k
  • Capacity on 16 GB (startup lines):

    Checkpoint bf16 caches fp8 caches
    9B 4-bit about 254k, 1 stream 262,144 window, 2 streams
    9B 8-bit about 110k 262,144 window, 1 stream
    27B oQ2 about 48k 106,496 window, 1 stream
  • Tests:

    • zig build test: 48/48 steps; 747 of 761 tests pass, 14 skipped, 0 fail.
    • New host tests:
      • fp8 caches take e4m3 rows and a scale each (520 bytes a position and head, against bf16's 1,024);
      • --kv-cache fp8 opens only a family whose options take it.
    • CI is green.
  • Not run, and why:

    • Metal: no Mac here. On Metal, --kv-cache fp8 is refused.
    • Linux, sm_120 and sm_121: I have no such machine. The kernels convert with cuda_fp8.h's e4m3 types.

Checklist

  • Every token still goes through the lane rounds. No serial path, and nothing that needs drafts off.
  • Drafted output equals "draft": false, a resumed prompt equals a fresh one, and concurrent equals solo, with fp8 caches as with bf16. This engine keeps no prompt cache on CUDA, so a resumed prompt is a fresh one.
  • No precision traded for speed. The trade is memory for precision, opt-in, with its cost above. The default path's bits are unchanged.
  • Prompt processing is no slower than the last release with the default caches. With fp8 caches it is about 5% slower.
  • The description says which platforms I ran: CUDA sm_89 on Windows. Linux and Metal were not run.
  • New tests fail before the change, pass after it, and skip cleanly without their dependency.
  • Comments and docstrings are one line. No measurements or history in the source. The CI lean check reports 0 new problems.
  • No personal data, machine names, internal hosts or local paths. No AI attribution lines.

Limits

  • Qwen3.5 on CUDA only. Other families refuse the flag.
  • Release packaging from prebuilt fatbins needs qwen35_kv8.fatbin too.

- zig/src/platform: `sys`, imported by every module (zig/build/sys.zig wires
  it). On POSIX it is std.c, std.posix and std.DynLib unchanged; on Windows
  win_c.zig gives libc's surface over the UCRT and Win32 (integer
  descriptors, directory descriptors, readdir, pread/pwrite, high-resolution
  sleep, monotonic clock, /tmp and /proc/self/exe mapped), win_posix.zig the
  server's sockets over Winsock, console events as SIGINT/SIGTERM, and
  LoadLibraryW for DynLib.
- std.c and std.DynLib calls go through `platform` (mechanical).
- build.zig routes Windows targets to the CUDA build.
- Server: socket closes and writes are explicit, the key file's mode check
  is skipped on NTFS (ACLs), descriptor limits are a no-op.
- shard_edit: the shard's map is released before the rename, since Windows
  refuses to replace a mapped file.
- Tests that need symlink privilege or mode bits skip on Windows.

zig build test-cuda-host passes on x86_64-windows.
- Kernel fatbins build with nvcc and cl.exe: MSVC's conforming
  preprocessor is passed on Windows hosts (CUDA 13's headers need it),
  and explicit kernel instantiations carry the __restrict__ qualifiers
  their templates have (MSVC counts them in the function type; the
  generated code is unchanged).
- Nemotron's checkpoint source keeps io_uring to Linux; elsewhere its
  existing thread reads run.
- pull_exlr8 raises no descriptor limit on Windows.
- modelopt test: the lane's slice partials are allocated. GPUs without
  clusters (before sm_90) reduce split K through them; the test passed
  none, so it wrote through a null pointer on Ampere and Ada.

zig build native and install build with -Dnvcc=... -Dsm=89, and
tf-cuda-test's synthetic suites pass on an RTX 4090.
- pull: where Windows refuses the snapshot's symlink, the snapshot path
  becomes a hard link to its blob (CreateHardLinkW through `sys.c.link`):
  no extra space, and learning already refuses linked shards. Link
  failures are logged with their reason.
- HOME falls back to USERPROFILE (Windows sets no HOME) for the hub cache
  and every libc getenv("HOME").
- Tests read optional fixture variables through `platform.getenv`;
  std.process.Environ.getPosix does not build on Windows.
- The learned-state symlink tests skip on Windows (symlinks need
  privilege); `sys.c.symlink` asks for an unprivileged one.

zig build test passes on x86_64-windows.
- docs/WINDOWS.md: requirements, build, run, how the port works and how it
  differs from Linux; the README points to it.
- -Dstrip reaches tensorfold-native before its install step is made, so
  the step no longer expects a .pdb the stripped build does not write.
- zig fmt on the platform files.
8080 and 8000 are taken by too many other servers; serve listens on 8888
unless --port says otherwise, and the docs and bench scripts follow.
The MLX affine 4-bit checkpoint on the native CUDA server, beside Nemotron:
qwen35.cu holds the Metal engine's glue at its bf16 rounding points (gated
delta rule, causal convolution, gated attention), cuda.qlinear's affine-4
kernels do the projections, sample.cu draws on the device.

Windows of up to 16 rows record each row's recurrence and convolution tail,
so the lane core shares rounds across streams and keeps partial copies; a
window's rows equal one-row steps bit for bit. draw.zig is the host draw
both backends now share.

On an RTX 4090: 210 tok/s for one stream, 1250 tok/s across eight; logits
agree with an fp32 reference at every position of a 36-token check.
…mory

The 2B's CUDA engine now reads its widths from config.json (cuda_geometry.zig):
linear value heads over fewer key heads, more query heads a key/value head,
an untied head, an A_log stored as bf16. The 27B's logits agree with an fp32
reference at every position of a 36-token check, and its windows equal
one-row steps bit for bit; the 2B's output is unchanged.

Two Windows driver costs shaped the rest. Each kernel launch grew with the
process's allocation count (1.2 ms a launch over 1,900 weight buffers), so a
model's weights and scratch now live in one arena (cuda_weights.zig packs
each projection through a reused staging buffer: the 27B loads in 25 s, not
200). And a process's first forwards through a buffer stall for seconds, so
the engine warms every request path at load and the backend keeps a pool of
stream caches that `reserve` allocates and runs a row through once the
server knows its stream count.

On an RTX 4090: the 27B decodes at 37 tok/s in the runner, 31 to 35 tok/s
served, 210 tok/s across eight streams; the 2B stays at 210 and 1,250.
A forward's launches (the embedding to the kept rows' logits) are captured
the first time a shape (rows, windows, recording) is seen and replayed after;
the attention chunk count moves into the per-forward table so a graph stays
valid as caches grow. Prompt chunks stay uncaptured: their GEMMs dwarf the
launches, and each length would be a graph of its own.

A new graph's first run pays the Windows driver's stall (about a second on a
wide model), so `reserve` also captures the shared rounds' shapes, a row of
each of 1 to n streams, at load.

Bit for bit the same logits; on an RTX 4090 the 2B steps in 2.8 ms instead of
5.3 (served: 324 tok/s, 1,560 across eight streams) and the 27B in 23.8 ms
instead of 26.9 (served: 40 tok/s, 243 across eight).
--drafter loads z-lab's DFlash2 draft model behind the lane core's Drafted
wrapper: the target writes each row's residual after the drafter's five
tapped layers into a ring a stream, the drafter folds those rows into a
context of keys and values, and after every verify it drafts a block of
seven tokens past the pending one (the target's own embedding and head at
its ends, a codebook selector tracing one path through each row's best
sixteen). Its projections are quantized to affine-4 on the host at load and
run through the target's kernels; its glue is qwen35_draft.cu.

The Windows driver stalls a buffer's first kernel and a new graph's first
run for seconds, so the drafter pools its contexts and a captured forward
now serves every verify shape: its launches cover a bucket of rows and
every window, and the kernels read the true counts from the device.

On an RTX 4090 the 27B serves 52 to 58 tok/s greedy with drafts against 40
plain, about 100 tok/s on long replies, the same tokens either way; plain
decoding keeps its 24 ms step. The 2B is unchanged.
A verify window's rows may form a tree (each row's parent given): the
convolution taps, the delta rule and attention follow each row's own
branch, and the DFlash2 drafter fills the lane core's planned lanes
(each lane its parent's rank-r choice by the codebook selector), so a
lone stream verifies up to 12 lanes a round and the planner learns
which shapes land.

A verify no longer snapshots every row's state (1.3 GB shared, which
capped the 27B at 8 rows): each stream logs its rows' decay, write and
key, and keep replays the kept path onto the recurrence in one pass
(and moves a tree's kept keys down), so any window width fits. One-row
steps and prompts update in place as before. The lane host times the
target's windows at load, which the depth rule had lacked (it had been
capping chains at 3 drafts).

The runner's new tree mode verifies tree windows with junk branches
against step logits (bit-exact on both models).
What the port serves beyond Nemotron: Qwen3.5-2B and Qwen3.8-27B with
its DFlash2 drafter, the speeds measured on an RTX 4090, the load time
and why, the timing and chain-only switches, and what is not there.
The lean rule (tools/zig/lean_check.py) counts a comment that runs on
to a second line; the Windows port's new files had a hundred such runs.
Each is one line of at most 120 columns now, and the checker reports
the same 539 problems as main.
direct_io opened files with O_DIRECT on Linux alone. On Windows the
same open now asks for FILE_FLAG_NO_BUFFERING (which win_c maps O_DIRECT
to), and the 4 KiB-aligned spans it already reads satisfy the sector
alignment that needs; a file system that refuses it still falls back to
buffered reads. The direct-read test runs on Windows too, checking the
executable's bytes against buffered reads of it.
SIG.IGN is a function pointer made from 1, as std.posix has it. aarch64
requires function pointers to be aligned unless the type says align(1),
so the cross-build for aarch64-windows refused it. With align(1), as
std's own handler type has, tensorfold-native cross-compiles for
aarch64-windows with the GB10 (sm_121) fatbins; no hardware has run it.
The Windows page opens with what the build is, what it is tested on and
that it cross-compiles for aarch64-windows; the README's model table
and the CUDA paragraph say Windows where they said unofficial port.
The no-GPU workflow runs zig build test on windows-latest beside
ubuntu-latest, so the platform layer is built and tested on every pull
request.
Every other module calls libc through `sys`; the drafter's four getenv
calls were the last direct std.c calls in a file the Windows build
compiles.
After a prompt the lane core hands the drafter every prompt row's
features; the CUDA backend refused any request wider than its tap ring
(2,176 rows), so a 27B request with --drafter and a prompt past that
ended with RowsNotHeld, where the same prompt without the drafter ran.
The ring keeps the newest tap_rows rows, which hold the drafter's whole
context window, and absorb already reads only that window of what it is
handed; the backend now refuses only rows not yet written or all gone.

On an RTX 4090 the 27B with its drafter takes a 6,041-token prompt in
7.4 s, and its reply equals the one with "draft": false.
… attention cuts chunks at logical positions

- rowanc held a window's rows as bits of a u32, but prompt chunks run up to
  128 rows: rows from 32 on rotated at the wrong positions and attended the
  wrong keys. A chain row (no parents) now carries its index in its window
  under the top bit; tree windows (at most 16 rows) keep their bits.
- tf_q35_attn_partial cut its 128-key chunks at a key's cache row, so a kept
  tree path whose window crossed a chunk boundary summed in other chunks than
  one-row steps and changed bits. Chunks are now cut at the key's logical
  position (the stream's length plus the ancestors before it).

Checked on an RTX 5090 (Linux, sm_120), tf-qwen35-cuda over 300 ids on
Qwen3.5-2B and Qwen3.8-27B: step bits unchanged, window and tree equal step
byte for byte (tree differed from row 126 on the 2B and 254 on the 27B
before). Prompt mode's last row against step's at 16 to 300 ids: KL at most
0.003 and the same argmax in 20 of 20 (before: 27B at 40 ids KL 2.3 with a
different argmax, at 128 ids KL 1.35).
zig build test-golden and zig build fuzz-cortheon exist on every non-macOS
host since the golden step moved into zig/build/golden.zig, so the golden
check compiles against the platform layer on Windows:

- the platform's posix gains sockaddr.in, timeval, connect and kill for a
  child's handle (TerminateProcess; HUP has no equivalent), setsockopt turns
  a timeval for SO_RCVTIMEO or SO_SNDTIMEO into Winsock's milliseconds, and a
  socket read that times out is WouldBlock, as std.posix reports it.
- the check carries SystemRoot into the spawned server's environment, which
  Winsock needs to start, and drains the server's stdout after the port line
  on every host: the fake server logs each request there, and a full pipe
  (4 KiB on Windows) blocked it after sixteen cases.
- on Windows the check cannot send SIGTERM or SIGHUP, so the sigterm-exit-0
  outcome and the cases after a key rewrite are skipped and printed as such.

On Windows the goldens give 306 of 315 equal with the same nine known
differences as Linux, and the fuzz step passes its eight tests; the no-GPU
workflow's Windows job runs both.
The loader tried only libcublasLt.so.13 and libcublasLt.so, so the
cublaslt suite of tf-cuda-test reported LibraryUnavailable on Windows;
there it opens cublasLt64_13.dll from the toolkit's bin folder.
The platform handed O_APPEND to the C runtime as _O_APPEND, whose write
seeks to the end and then writes, so two threads appending at once could
overwrite each other's bytes: the request log lost lines under concurrent
requests, which the server goldens' request-log case caught on a slow
runner. A descriptor opened with O_APPEND now writes through WriteFile at
the end-of-file offset, which the kernel places whole, as POSIX does; the
file keeps its write access, so ftruncate on such a descriptor still works
(prompt_imprint repairs a torn index that way).

A request_log test appends 200 lines from each of 8 threads and expects
every line whole; it lost lines on Windows before this change.
The lifecycle/request-log outcome said only that the lines differ; it now
gives both logs' line counts and the first line that differs, cut to 100
bytes each, so a mismatch seen on a CI runner can be read from its log.
A checkout with core.autocrlf, as GitHub's Windows runners make, gives the
frozen request log CRLF line endings while the server writes LF, so the
lifecycle/request-log case differed at its first line's end on windows-latest
and nowhere else. Both logs compare with CRLF read as LF; the other goldens
are parsed as JSON and never saw the endings.
…'s widest stream

The chunk width came from the longest stream or window in the forward, so a 201-key row cut its keys into 128-key
chunks alone and 256-key chunks beside a stream past 8192, and a window crossing 8192 recut its earlier rows; either
changed the fp32 sums. Each row now takes chunk_keys(its logical keys), the width its one-row step uses, in both the
partial and the merge kernels, and the partials sit at a fixed max_chunks stride.

tf-qwen35-cuda takes TF_Q35_PREFILL=N (start after N cycled ids) and TF_Q35_BESIDE=N (step an N-token stream in each
forward), which cover both cases: before this change, windows from 8151 keys kept 36 of 120 rows and trees 35, and a
151-key stream beside one from 8189 kept 4 to 11 of 120; after it all of them equal one-row steps on the 2B and 27B.
…fits its selector rank

The integer reader accepted zero, so a conv_group_size or num_key_value_heads of 0 reached a remainder at
admission; sizes and divisors are now positive (mask_token_id alone may be 0). The host staging area held 16 rows at
rank 256, so a larger selector_rank overran it; it is now sized for the admitted rank. Config.parse takes the bytes,
so a host test covers both.
Each module loads at the width the config's quantization table names it. Codes
of 2 to 4 bits pack onto the affine-4 tiles and codes of 5 to 8 bits onto new
8-bit tiles, every code keeping its value, scale and bias, so nothing is
rounded. affine8.cu's lane GEMV is lane_gemv's arithmetic a byte wide (each
byte exact in bf16), with its split-K form; an odd-width embedding is widened
at load and 8-bit rows have their own embed kernel. A uniform 4-bit checkpoint
takes the same packs and kernels as before: the 2B's step, window, tree and
prompt logits are byte-equal to the previous build's.

The rope config's older "type" key is read as "rope_type" (mlx-community's
OptiQ conversions write it).

Checked on an RTX 4080 SUPER (sm_89):
- tf-cuda-test affine8: 24 cases over 2, 3, 4, 5, 6 and 8 bits; tiles equal
  the host-widened codes' tiles, 8-bit products within 0.32 of the float64
  bound, 1/3/9/16-row calls and split-K byte-equal.
- A 2B requantized at 2, 3, 5, 6 and 8 bits (and a 6-bit embedding) gives the
  same logits as the same codes stored widened, in step, window, tree and
  prompt modes.
- Qwen3.5-4B-OptiQ-4bit and Qwen3.5-9B-OptiQ-4bit: window and tree equal step
  over 300 ids; bench_concurrent --alone --serial at 1 and 4 streams all equal;
  about 130 and 78 tokens/s for one request.

8-bit modules have no prompt GEMM yet; prompt chunks run them on the decode
tiles, 16 rows at a time.
The mixed-width loader widened 2 and 3-bit codes onto the 4-bit tiles and 5 and
6-bit codes onto the 8-bit ones, so a mostly 2-bit checkpoint took the 4-bit
checkpoint's memory. affine8.cu's lane GEMV is now templated on the width: a
lane's 16 code pairs a group sit in `bits` words, first codes in the low halves
and second in the high, and bf16 (0x4300 | c) - 128 rebuilds each code exactly.
The MMAs see the operands the 8-bit tiles give them, so a code's products do not
depend on the width it is stored at.

- AffineN (was Affine8) carries its width; AffineNKernels resolves a gemv and a
  split-K kernel for 2, 3, 5, 6 and 8 bits, and tf_affinen_pack packs them.
- 4-bit modules keep the affine-4 tiles and prompt GEMM; uniform 4-bit
  checkpoints are unchanged. Embeddings still widen to 4 or 8 bits (rowBits).
- TensorFold/Qwen3.8-27B-oQ2 takes 10.3 GiB and fits a 16 GB card.

Checks on an RTX 4080 SUPER:
- tf-cuda-test affine8: 24 cases; 2, 3, 5 and 6-bit products byte-equal to
  the codes widened onto the 8-bit tiles at 1, 3, 9 and 16 rows, within 0.32 of
  the float64 bound, split-K equal to the slice loop.
- Qwen3.5-2B 4-bit: step, window, tree and prompt logits byte-equal to the
  build before.
- The 2B requantized at 2, 3, 5, 6 and 8 bits: byte-equal to the same codes
  widened to 8 bits in all four modes, and step/window/tree byte-equal to the
  previous build, where 2 and 3-bit codes ran on the 4-bit tiles.
- Qwen3.8-27B-oQ2: window, tree and prompt equal step over 300 ids;
  bench_concurrent --alone --serial at 1 and 2 streams all equal; about 45 to
  64 tokens/s for one request, about 750 tokens/s prompt.
Prompt chunks ran modules of every width but 4 on the decode tiles, 16 rows at
a time. affine8.cu now has qmm_prefill's GEMM over AffineN's tiles for 2, 3, 5,
6 and 8 bits: a 128 by 128 block on 2 by 4 warps, three stages, each weight
rounded once to bf16 (s * q + b) and one fp32 chain over K. Its operands are the
affine-4 prompt GEMM's, so a code of up to 4 bits gives the same bits as there.

Checks on an RTX 4080 SUPER:
- tf-cuda-test affine8: the prompt GEMM over 144 rows within 0.07 of a bound
  that allows for the bf16 weights; 2 and 3-bit byte-equal to the affine-4
  prompt GEMM of the same codes, 5 and 6-bit to the 8-bit tiles'.
- Qwen3.5-2B 4-bit: all four modes byte-equal to the build before.
- The 2B requantized at 2, 3, 5, 6 and 8 bits: byte-equal to the same codes at
  8 bits in all four modes, prompt included; decode modes unchanged.
- Qwen3.8-27B-oQ2: window and tree equal step, step unchanged; bench 0
  unequal. Prompts about 770 tokens/s (from 750): already bound by the bf16
  tensor rate with fp32 accumulation.
- Qwen3.5-9B MLX 8-bit: prompts about 1,940 tokens/s, from 1,520; bench at 1
  and 4 streams 0 unequal.
…ffine_codes.cuh, one-line comments

affine8.cu passed 600 lines with the prompt GEMM. The GEMM moves to its own
kernel module and the code readers both use to a header; lean_check reports
528 problems, as on the base.
The weight loader's staging buffer is now Engine.init's and freed once
the weights are on the card, so the server's stream budget counts that
memory; the drafter's loader stages its own for its load.
--kv-cache fp8 stores attention keys and values as e4m3 rows with an fp32
scale each (the tf_kv_quant format): qwen35_kv8.cu holds rope, the
attention partials and compaction for it, and the cache sizes, the
startup line and the refusals follow. Windows and trees stay equal to
steps bit for bit; the 9B 4-bit README perplexity is 9.267 against 9.289
with bf16 caches. On a 16 GiB card the 9B 8-bit serves a 262144-token
window and the 27B oQ2 a 106496-token one.
@ashhart

ashhart commented Oct 15, 2026

Copy link
Copy Markdown
Owner

Thank you, @BobClawblaw, for making fp8 an explicit cache option: zig/src/server/cli.zig:159 defaults to bf16, and line 355 enables fp8 only when requested.

At 0eebbd3, the standalone write, read and compaction layouts agree in source. Combining the image and reuse changes exposes two gaps: zig/kernels/cuda/qwen35_kv8.cu:40 uses scalar positions without the image delta and M-RoPE planes, while #888's snapshot offsets assume bf16 rows and omit fp8 scales.

Please make the combined vision/reuse path position- and format-aware, including step, window, tree and resume comparisons in its integration checks.

Posted by Ash's AI Assistant Synthena

…otate at the bf16 path's positions

rope_rows in qwen35.cuh computes each row's position and rotates its queries
and key once; tf_q35_rope and tf_q35_rope8 differ only in how they store the
key and value (bf16 rows, or e4m3 rows and scales). A later change to the
positions, such as an image prompt's M-RoPE planes and rope_delta, then
reaches both caches. Logits are byte-equal to before in step, window, tree
and prompt modes, with bf16 and fp8 caches, on the 2B and the 27B.
@BobClawblaw

Copy link
Copy Markdown
Contributor Author

Follow-up at c9e78c2: the bf16 and fp8 rope kernels now share one body (rope_rows in qwen35.cuh). They differ only in how they store keys and values (StoreBf16, StoreFp8), so fp8 rows rotate at exactly the bf16 path's positions. With #885, its M-RoPE planes and rope_delta reach both. Whichever of #885 and #886 lands second takes the M table argument and row_positions in the merge. The fp8 attention read uses the same slot indices as bf16, so no positions are involved there.

Checks, RTX 5090:

@BobClawblaw

Copy link
Copy Markdown
Contributor Author

Second-platform check of the follow-ups: #886 at c9e78c23 merged with #888 at f8a99737 (which carries #885 at 5296eb19). Run on an RTX 4080 SUPER (sm_89) under Windows 11, CUDA 13.3, built with -Dsm=89. This is a local merge, not a branch.

The merge. The conflicts are the ones the follow-up predicts. rope_rows in qwen35.cuh takes #885's M table. row_position becomes row_positions(sg, anc, m, M, pos): the [3][MAXR] rows for an image chunk, otherwise text plus sg.delta. rotate takes the M-RoPE pos[3], and tf_q35_rope and tf_q35_rope8 both pass M through. cuda_reuse.zig picks up the fp8 layout from #886's kv8 field as designed, with no changes.

tf-qwen35-cuda, 94 of 94 checks pass over the 2B and 27B oQ2, each with bf16 and fp8 caches (300 ids):

  • Text: byte-equal to qwen3_5 cuda: --kv-cache fp8, e4m3 attention caches with a scale a row #886 at 0eebbd32 in step, window and tree. The prompt mode was also checked with bf16 caches.
  • TF_Q35_DELTA=-37 after a 300-token prefill: window and tree equal step.
    • With TF_Q35_RESUME, each of step, window and tree equals fresh.
    • The control (delta 0) differs.
  • mrope: a table of text positions equals prompt, and TF_Q35_SHIFT=5 differs. The shifted table resumed at 128 equals fresh.
  • Image rows (a local harness addition, not pushed):
    • The prompt is 100 text rows, then a 12×16-patch image's 48 rows of seeded features at layout's positions, then 120 text rows, with the reply at delta -40. Window and tree equal step.
    • Restoring the state at row 128 (mid-image) or row 256 into a poisoned cache equals fresh in all three modes.
    • A 3,000-token stream beside it equals alone. The same rows at text positions differ.
    • These logits are byte-equal to a separate integration I built for the same review, with its own rope fix.
  • fp8 against bf16 on the image prompt: top-1 agrees on 296/300 (2B) and 297/300 (27B).

Served (--vision --parallel 2 --context 32768; 2B and 9B 4-bit, each with bf16 and fp8 caches; greedy, 48 tokens):

  • The test build: it adds a minimal PNG decoder, --prompt-cache-gib when /proc/meminfo is missing, and an env hook that keeps image prompts.
  • Each run: turn 1 is about 7,100 tokens of the README plus a 448×336 PNG.
    • Turn 2 resumes 7,168 tokens (turn 1's state), then 7,296 (its own), and equals a cold server's reply.
    • Drafts off, and beside another image prompt, it equals its cold reply too.
    • A different image with the same grid resumes nothing and equals its own cold reply.
  • 8 of 8 checks pass in each of the four runs.

Host: zig build test passes 552/559 (7 skipped), and zig build test-cuda-host passes 31/31 steps.

Windows gaps, already noted on #888 and outside this PR:

  • The server refuses image bytes until the shared decoder lands.
  • With no /proc/meminfo, a Windows server keeps no states.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment

Labels

None yet

Projects

None yet

Development

Successfully merging this pull request may close these issues.

2 participants