system-variables: add doc for tidb_max_dist_task_nodes

system-variables.md

@@ -3778,6 +3778,22 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
 - Range: `[100, 16384]`
 - This variable is used to set the maximum number of schema versions (the table IDs modified for corresponding versions) allowed to be cached. The value range is 100 ~ 16384.
 
+### tidb_max_dist_task_nodes <span class="version-mark">New in v9.0.0</span>
+
+- Scope: SESSION | GLOBAL
+- Persists to cluster: Yes
+- Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
+- Type: Integer
+- Default value: `-1`
+- Range: `-1` or `[1, 128]`
+- This variable defines the maximum number of TiDB nodes that the Distributed eXecution Framework (DXF) tasks can use. The default value is `-1`, which indicates that automatic mode is enabled. In automatic mode, TiDB dynamically calculates the value as `min(3, tikv_nodes / 3)`, where `tikv_nodes` represents the number of TiKV nodes in the cluster.
+
+> **Note:**
+> 
+> If you explicitly set the [`tidb_service_scope`](#tidb_service_scope-new-in-v740) system variable for some TiDB nodes, the Distributed eXecution Framework schedules tasks only to these nodes. In this case, even if you set `tidb_max_dist_task_nodes` to a larger value, the framework uses no more than the number of nodes explicitly configured with `tidb_service_scope`.
+>
+> For example, if the cluster has 10 TiDB nodes, and 4 of them are configured with `tidb_service_scope = group1`, then even if you set `tidb_max_dist_task_nodes = 5`, only 4 nodes participate in task execution.
+
 ### tidb_max_paging_size <span class="version-mark">New in v6.3.0</span>
 
 - Scope: SESSION | GLOBAL