[Fix] Don't deep-copy LogitsProcessors when copying SamplingParams (#3099)

vllm/engine/llm_engine.py

@@ -484,8 +484,9 @@ class LLMEngine:
             prompt_token_ids[:prefix_pos], lora_request.lora_int_id
             if lora_request else 0) if prefix_pos is not None else None
 
-        # Defensive copy of SamplingParams, which are used by the sampler
-        sampling_params = copy.deepcopy(sampling_params)
+        # Defensive copy of SamplingParams, which are used by the sampler,
+        # this doesn't deep-copy LogitsProcessor objects
+        sampling_params = sampling_params.clone()
 
         # Create the sequence group.
         seq_group = SequenceGroup(request_id, [seq], sampling_params,

vllm/sampling_params.py

 """Sampling parameters for text generation."""
+import copy
 from enum import IntEnum
 from functools import cached_property
 from typing import Callable, List, Optional, Union
@@ -237,6 +238,20 @@ class SamplingParams:
             return SamplingType.RANDOM_SEED
         return SamplingType.RANDOM
 
+    def clone(self) -> "SamplingParams":
+        """Deep copy excluding LogitsProcessor objects.
+
+        LogitsProcessor objects are excluded because they may contain an
+        arbitrary, nontrivial amount of data.
+        See https://github.com/vllm-project/vllm/issues/3087
+        """
+
+        logit_processor_refs = None if self.logits_processors is None else {
+            id(lp): lp
+            for lp in self.logits_processors
+        }
+        return copy.deepcopy(self, memo=logit_processor_refs)
+
     def __repr__(self) -> str:
         return (
             f"SamplingParams(n={self.n}, "