[V1] Revert the default `max_num_seqs` to V0 values for most hardware (#16158)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

docs/source/getting_started/v1_user_guide.md

@@ -156,10 +156,3 @@ vLLM V1 is currently optimized for decoder-only transformers. Models requiring
   cross-attention between separate encoder and decoder are not yet supported (e.g., `BartForConditionalGeneration`, `MllamaForConditionalGeneration`).
 
 For a complete list of supported models, see the [list of supported models](https://docs.vllm.ai/en/latest/models/supported_models.html).
-
-## Frequently Asked Questions
-
-**I'm using vLLM V1 and I'm getting CUDA OOM errors. What should I do?**
-The default `max_num_seqs` has been raised from `256` in V0 to `1024` in V1. If you encounter CUDA OOM only when using V1 engine, try setting a lower value of `max_num_seqs` or `gpu_memory_utilization`.
-
-On the other hand, if you get an error about insufficient memory for the cache blocks, you should increase `gpu_memory_utilization` as this indicates that your GPU has sufficient memory but you're not allocating enough to vLLM for KV cache blocks.

tests/v1/engine/test_engine_args.py

@@ -64,15 +64,17 @@ def test_defaults_with_usage_context():
         # For H100 and H200, we use larger default values.
         default_llm_tokens = 16384
         default_server_tokens = 8192
+        default_max_num_seqs = 1024
     else:
         default_llm_tokens = 8192
         default_server_tokens = 2048
+        default_max_num_seqs = 256
 
-    assert vllm_config.scheduler_config.max_num_seqs == 1024
+    assert vllm_config.scheduler_config.max_num_seqs == default_max_num_seqs
     assert vllm_config.scheduler_config.max_num_batched_tokens == default_llm_tokens  # noqa: E501
 
     engine_args = EngineArgs(model="facebook/opt-125m")
     vllm_config = engine_args.create_engine_config(
         UsageContext.OPENAI_API_SERVER)
-    assert vllm_config.scheduler_config.max_num_seqs == 1024
+    assert vllm_config.scheduler_config.max_num_seqs == default_max_num_seqs
     assert vllm_config.scheduler_config.max_num_batched_tokens == default_server_tokens  # noqa: E501

vllm/engine/arg_utils.py

@@ -1666,12 +1666,14 @@ class EngineArgs:
                 UsageContext.LLM_CLASS: 16384,
                 UsageContext.OPENAI_API_SERVER: 8192,
             }
+            default_max_num_seqs = 1024
         else:
             # TODO(woosuk): Tune the default values for other hardware.
             default_max_num_batched_tokens = {
                 UsageContext.LLM_CLASS: 8192,
                 UsageContext.OPENAI_API_SERVER: 2048,
             }
+            default_max_num_seqs = 256
 
         use_context_value = usage_context.value if usage_context else None
         if (self.max_num_batched_tokens is None
@@ -1682,7 +1684,6 @@ class EngineArgs:
                 "Setting max_num_batched_tokens to %d for %s usage context.",
                 self.max_num_batched_tokens, use_context_value)
 
-        default_max_num_seqs = 1024
         if self.max_num_seqs is None:
             self.max_num_seqs = default_max_num_seqs