[Qwen4-Exp] Build the offloaded PLE table on the meta device so --ple-offload-embedding never materialises it on the accelerator (#39928)

Co-authored-by: Yangmin Li <yangminl@nvidia.com>

python/sglang/srt/models/qwen4_exp.py

@@ -508,7 +508,11 @@ class Qwen4ExpNGramEmbedding(nn.Module):
             and not self.use_attn_tp_ngram
         )
         ngram_prefix = f"{prefix}.ngram_embedding" if prefix else "ngram_embedding"
-        self.ngram_embedding = VocabParallelEmbedding(
+        offload_embedding = bool(config.ple_offload_embedding)
+        # Offload only needs this embedding's metadata: build it on meta so the
+        # shard is never allocated on the device.
+        with torch.device("meta") if offload_embedding else nullcontext():
+            ngram_embedding = VocabParallelEmbedding(
                 padded_vocab_size,
                 self.head_dim_per_ngram,
                 params_dtype=(
@@ -519,9 +523,17 @@ class Qwen4ExpNGramEmbedding(nn.Module):
                 output_dtype=torch.bfloat16,
                 use_attn_tp_group=self.use_attn_tp_ngram,
             )
-        self.ngram_embedding.register_buffer(
+        # weight_scale stays a real device tensor.
+        ngram_embedding.register_buffer(
             "weight_scale", torch.ones(1, dtype=torch.bfloat16), persistent=True
         )
+        if offload_embedding:
+            ngram_embedding = Qwen4ExpPinnedHostEmbedding(
+                ngram_embedding,
+                backend=getattr(config, "ple_offload_backend", "pinned"),
+                table_dir=getattr(config, "ple_offload_dir", None),
+            )
+        self.ngram_embedding = ngram_embedding
 
     @classmethod
     def _splitmix64(cls, x: int) -> int:
@@ -771,6 +783,8 @@ class Qwen4ExpPinnedHostEmbedding(VocabParallelEmbedding):
 
     The table stays in its checkpoint storage dtype (fp8 with a per-tensor
     weight_scale for fp8 checkpoints, bf16 otherwise); gathers emit bf16.
+
+    The source weight may be on the meta device; only its metadata is used.
     """
 
     _COPIED_ATTRIBUTES = (
@@ -931,12 +945,6 @@ class Qwen4ExpPLELayer(nn.Module):
             quant_config=quant_config,
             prefix=f"{prefix}.ple_embedding" if prefix else "ple_embedding",
         )
-        if config.ple_offload_embedding:
-            self.ple_embedding.ngram_embedding = Qwen4ExpPinnedHostEmbedding(
-                self.ple_embedding.ngram_embedding,
-                backend=getattr(config, "ple_offload_backend", "pinned"),
-                table_dir=getattr(config, "ple_offload_dir", None),
-            )
         self.short_conv_dilation = self.ple_embedding.ngram_size
         self.short_conv_state_len = (
             self.conv_kernel_size - 1

test/registered/kernels/ops/embeddings/test_qwen4_ple_offload.py

@@ -6,6 +6,7 @@ import pytest
 import torch
 from torch import nn
 
+from sglang.srt.configs.qwen4_exp import Qwen4ExpTextConfig
 from sglang.srt.layers.quantization.unquant import UnquantizedEmbeddingMethod
 from sglang.srt.layers.vocab_parallel_embedding import (
     VocabParallelEmbeddingShardIndices,
@@ -15,6 +16,7 @@ from sglang.srt.models.qwen4_exp import (
     Qwen4ExpPinnedHostEmbedding,
     Qwen4ExpPLELayer,
 )
+from sglang.srt.runtime_context import get_context, get_parallel
 from sglang.srt.utils import set_weight_attrs
 from sglang.test.ci.ci_register import register_cuda_ci
 
@@ -191,6 +193,52 @@ def test_qwen4_ple_prefetch_buffer_lifecycle(monkeypatch):
     assert set(layer._graph_prefetch_buffers) == {3, 5}
 
 
+@pytest.fixture
+def single_rank_runtime_context():
+    """``Qwen4ExpPLELayer.__init__`` reads the TP topology through
+    ``VocabParallelEmbedding``; pin it to one rank without a process group."""
+    override = get_context().override_server_args(tp_size=1)
+    override.install()
+    try:
+        with get_parallel().override(
+            tp_rank=0, tp_size=1, attn_tp_rank=0, attn_tp_size=1
+        ):
+            yield
+    finally:
+        override.restore()
+
+
+def test_qwen4_ple_offload_avoids_device_table(single_rank_runtime_context):
+    # sgl-project/sglang#39841: the table was built on the device before the
+    # host table existed, so the flag needed a full per-rank shard of free VRAM.
+    # Small everywhere except the n-gram table (16 heads x ~20k rows x 4 dims),
+    # which must dominate the layer's footprint for the peak check to bite.
+    config = Qwen4ExpTextConfig(
+        vocab_size=64,
+        hidden_size=16,
+        hc_count=2,
+        ple_embed_dim=64,
+        ngram_size=3,
+        heads_per_ngram=8,
+        ngram_vocab_size_base=20_000,
+        eos_token_id=1,
+        ple_offload_embedding=True,
+    )
+    torch.cuda.synchronize()
+    torch.cuda.reset_peak_memory_stats()
+    base = torch.cuda.memory_allocated()
+    with torch.device("cuda"):  # the model loader builds every layer this way
+        layer = Qwen4ExpPLELayer(config, prefix="ple", layer_id=0, ple_layer_index=0)
+    peak = torch.cuda.max_memory_allocated() - base
+
+    emb = layer.ple_embedding.ngram_embedding
+    table_bytes = emb.weight.numel() * emb.weight.element_size()
+    assert peak < table_bytes // 2, (peak, table_bytes)
+    assert emb.weight.device.type == "cpu" and emb.weight.is_pinned()
+    assert emb.weight_scale.is_cuda
+    assert not any(t.is_meta for t in (*layer.parameters(), *layer.buffers()))
+
+
 def _file_backend_supported() -> bool:
     from sglang.srt.models.qwen4_exp_ple_table import device_uses_host_page_tables