Add comments on swap space (#154)

vllm/core/scheduler.py

@@ -409,7 +409,12 @@ class Scheduler:
         seq_group: SequenceGroup,
         blocks_to_swap_out: Dict[int, int],
     ) -> None:
-        assert self.block_manager.can_swap_out(seq_group)
+        if not self.block_manager.can_swap_out(seq_group):
+            # FIXME(woosuk): Abort the sequence group instead of aborting the
+            # entire engine.
+            raise RuntimeError(
+                "Aborted due to the lack of CPU swap space. Please increase "
+                "the swap space to avoid this error.")
         mapping = self.block_manager.swap_out(seq_group)
         blocks_to_swap_out.update(mapping)
         for seq in seq_group.get_seqs(status=SequenceStatus.RUNNING):