set default block_size and pa

vllm/attention/layer.py

@@ -75,7 +75,7 @@ class Attention(nn.Module):
             calculate_kv_scales = cache_config.calculate_kv_scales
         else:
             kv_cache_dtype = "auto"
-            block_size = 16
+            block_size = 64
             is_attention_free = False
             calculate_kv_scales = False
         if num_kv_heads is None:
@@ -298,7 +298,7 @@ class MultiHeadAttention(nn.Module):
         attn_backend = get_attn_backend(head_size,
                                         dtype,
                                         kv_cache_dtype=None,
-                                        block_size=16,
+                                        block_size=64,
                                         is_attention_free=False)
         backend = backend_name_to_enum(attn_backend.get_name())
         if backend in {_Backend.FLASH_ATTN, _Backend.FLASH_ATTN_VLLM_V1}:

vllm/envs.py

@@ -995,7 +995,7 @@ environment_variables: dict[str, Callable[[], Any]] = {
     
     # vLLM will use FlashAttention Backend for page attention computation on rocm
     "VLLM_USE_FLASH_ATTN_PA":
-    lambda: (os.environ.get("VLLM_USE_FLASH_ATTN_PA", "False").lower() in
+    lambda: (os.environ.get("VLLM_USE_FLASH_ATTN_PA", "True").lower() in
              ("true", "1")),
     
     # vLLM will use apex for rmsnorm