[TPU] [Perf] Improve Memory Usage Estimation (#15671)

Signed-off-by: Robert Shaw <robshaw@redhat.com>
Co-authored-by: Robert Shaw <robshaw@redhat.com>

Author: robertgshaw2-redhat

vllm/v1/worker/tpu_worker.py

@@ -161,7 +161,13 @@ def determine_available_memory(self) -> int:
         # intermediate activations.
         m = xm.get_memory_info(self.device)
         total_memory_size = m["bytes_limit"]
-        profiled = m["peak_bytes_used"]  # Weights + intermediate activations.
+        current_mem = m["bytes_used"]
+        # Ideally we would use profiled = m["peak_bytes_used"] to
+        # get weights + activations. But there is memory used during
+        # compilation / weight loading that impacts the peak and
+        # there is no way to reset peak memory in XLA, So we
+        # use the heuristic of 2% of weights.
+        profiled = current_mem * 1.02
 
         # Calculate the TPU KV cache size based on profiling.
         usable_memory_size = int(total_memory_size *