4x sqlalchemy pool size (#405)

* 4x sqlalchemy pool size

* don't update nullpool

Author: yunfeng-scale

model-engine/model_engine_server/db/base.py

@@ -79,25 +79,32 @@ def get_engine_url(env: Optional[str] = None, read_only: bool = True, sync: bool
     echo=False,
     future=True,
     pool_pre_ping=True,
+    pool_size=20,
+    max_overflow=30,
 )
 pg_engine_read_only = create_engine(
     get_engine_url(read_only=True, sync=True),
     echo=False,
     future=True,
     pool_pre_ping=True,
+    pool_size=20,
+    max_overflow=30,
 )
 pg_engine_async = create_async_engine(
     get_engine_url(read_only=False, sync=False),
     echo=False,
     future=True,
     pool_pre_ping=True,
+    pool_size=20,
+    max_overflow=30,
 )
 pg_engine_read_only_async = create_async_engine(
     get_engine_url(read_only=True, sync=False),
     echo=False,
     future=True,
     pool_pre_ping=True,
-    max_overflow=5,
+    pool_size=20,
+    max_overflow=30,
 )
 pg_engine_async_null_pool = create_async_engine(
     get_engine_url(read_only=False, sync=False),