limit the max buffer size of PDML write

src/sql/engine/pdml/static/ob_pdml_op_batch_row_cache.cpp

@@ -177,7 +177,14 @@ int ObPDMLOpBatchRowCache::add_row(const ObExprPtrIArray &row, ObTabletID tablet
   int ret = OB_SUCCESS;
   ObChunkDatumStore::StoredRow *stored_row = NULL;
   ObChunkDatumStore *row_store = NULL;
-  if (OB_FAIL(process_dump())) {
+  // to address storage write throttle, we limit the max buffer size of PDML write.
+  // the 2MB config is tested optimal under PDML concurrency=4 and concurrency=8 cases
+  // TODO: maybe we can introduce a dynamic control policy
+  //       concidering the tenant overall access behavior to storage
+  constexpr int64_t max_pdml_cache_size_per_thread = 2 * 1024 * 1024;
+  if (!with_barrier_ && cached_rows_size_ > max_pdml_cache_size_per_thread) {
+    ret = OB_EXCEED_MEM_LIMIT;
+  } else if (OB_FAIL(process_dump())) {
     if (OB_EXCEED_MEM_LIMIT != ret) {
       LOG_WARN("fail process dump for PDML row cache", K(ret));
     }