Sycl --split-mode tensor#24152
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* SYCL: tensor parallelism (--split-mode tensor) for dual-GPU
Adds the comm_init/comm_free/comm_allreduce_tensor trio that the
meta-backend queries via get_proc_address to enable backend-specific
all-reduce, mirroring the pattern used by ggml-cuda.cu.
For N=2 (the common dual-GPU case) implements a degenerate ring
all-reduce with two size-branched paths:
* Small (nelem < 32768): FP32 direct memcpy + per-device ADD kernel
chained via depends_on(memcpy_event). 4 SYCL submissions/call.
* Large (nelem >= 32768): BF16-compressed. Each device compresses
FP32 -> BF16 in a local outbox, cross-device memcpys to the peer's
inbox (HALF the PCIe bytes), then decompresses + adds into the
local FP32 partial. 6 SYCL submissions/call but PCIe bytes halved
-- wins for any tensor where PCIe dominates kernel time.
Threshold and BF16 path pattern mirror the CUDA NCCL allreduce.
Storage: ONE persistent uint8_t buffer per device, 4 * nelem bytes
(matches both path layouts: FP32 nelem floats; BF16 outbox+inbox =
2 * nelem uint16_t each). Single alloc+free per device keeps the
SYCL pool's strict-LIFO invariant trivial.
Initial impl handles N=2 FP32 contiguous tensors. Other cases return
false, causing the meta-backend to use its generic butterfly fallback.
Per-call sync is intentionally omitted. SYCL in-order queue semantics
ensure that the meta-backend's next compute on the same per-device
queue waits for our final ADD, and the next allreduce's first op on
the same persistent buffer waits via the same queue. Only comm_free
does an explicit final wait.
OneCCL is NOT used: OneCCL 2021.17 hardcodes single-device-per-process
in communicator_impl.hpp:47 (condition devices.size() == 1), which is
incompatible with llama.cpp's single-process multi-GPU model.
Measured on dual Intel Arc Pro B70 (NEO 26.05.x, oneAPI 2025.3 +
DPC++ nightly):
Llama-3.3-70B Q4_K_M, -sm tensor -fa 1 -ctk f16 -ctv f16:
pp512 = 377.08 t/s (vs 313.65 layer mode = +20.2%)
tg128 = 17.40 t/s (vs 9.74 layer mode = +78.6%)
Qwen3-Coder-Next-80B-A3B Q3_K_M (MoE):
pp512 = 216.56 t/s (vs 156.58 meta-backend butterfly = +38.3%)
tg128 = 17.60 t/s (vs 14.31 meta-backend butterfly = +23.0%)
Qwen3-4B Q4_K_M:
pp64 = 984.51 t/s, tg16 = 49.29 t/s
Llama-3.3-70B in SYCL TP now comfortably beats production layer mode
on both prefill and decode. Coder-Next-80B-A3B (MoE) also wins on
both — the BF16 path is what unlocks the many-medium-allreduces
prefill pattern.
Build/CMake: no changes. No new dependencies. ~210 lines added across
ggml-sycl.h and ggml-sycl.cpp.
* Fix comments
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Overview
Adds the comm_init/comm_free/comm_allreduce_tensor trio that the meta-backend wants via get_proc_address to enable backend-specific all-reduce, mirroring the pattern used by ggml-cuda.cu.
For N=2 (dual-GPUs) implements a degenerate ring allreduce with two size-branched paths to mirror CUDA NCCL allreduce:
Small (N< 32k): FP32 direct memcpy + per-device ADD kernel chained via depends_on(memcpy_event). 4 submissions/call.
Large (N>= 32k): BF16-compressed. Each device compresses FP32 -> BF16 locally, cross-device memcpys to the peer, then decompresses + adds into the local FP32 partial. 6 SYCL submissions/call but PCIe bytes halved. Wins overall for larger dense models.
Storage: A single persistent uint8_t buffer per device, 4 * N bytes (matches both path layouts: FP32 N floats; BF16 outbox+inbox = 2 * N uint16_t each). Single alloc+free per device keeps the SYCL pool's strict-LIFO invariant.
Initial impl handles N=2 FP32 contiguous tensors. Other cases return false, causing the meta-backend to use its generic butterfly fallback. This should be easy to extend to N>2, but I don't have the hardware.
Measured on dual Intel Arc Pro B70 (NEO 26.05.x, oneAPI 2025.3 + DPC++ nightly) with a Threadripper 2970WX, 128GB system running Ubuntu Server 26 LTS.
Llama-3.3-70B Q4_K_M, -sm tensor -fa 1 -ctk f16 -ctv f16:
pp512 = 377.08 t/s (vs 313.65 layer mode = +20.2%)
tg128 = 17.40 t/s (vs 9.74 layer mode = +78.6%)
Qwen3-Coder-Next-80B-A3B Q3_K_M (MoE):
pp512 = 216.56 t/s (vs 156.58 meta-backend butterfly = +38.3%)
tg128 = 17.60 t/s (vs 14.31 meta-backend butterfly = +23.0%)
Qwen3-4B Q4_K_M:
pp64 = 984.51 t/s, tg16 = 49.29 t/s
Requirements
Build/CMake: no changes. No new dependencies. ~200 lines added across ggml-sycl.h and ggml-sycl.cpp.