Change tidb_memory_usage_alarm_ratio scope to instance (#5988) (#5992)

system-variables.md

@@ -548,6 +548,13 @@ Constraint checking is always performed in place for pessimistic transactions (d
 - This variable is used to set the threshold value of memory quota for a query.
 - If the memory quota of a query during execution exceeds the threshold value, TiDB performs the operation designated by the OOMAction option in the configuration file. The initial value of this variable is configured by [`mem-quota-query`](/tidb-configuration-file.md#mem-quota-query).
 
+### tidb_memory_usage_alarm_ratio
+
+- Scope: INSTANCE
+- Default value: `0.8`
+- TiDB triggers an alarm when the percentage of the memory it takes exceeds a certain threshold. For the detailed usage description of this feature, see [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio-new-in-v409).
+- You can set the initial value of this variable by configuring [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio-new-in-v409).
+
 ### tidb_metric_query_range_duration <span class="version-mark">New in v4.0</span>
 
 - Scope: SESSION
@@ -901,10 +908,3 @@ This variable is an alias for _transaction_isolation_.
 - Default value: ON
 - This variable controls whether to enable the dynamic memory control feature for the operator that reads data. By default, this operator enables the maximum number of threads that [`tidb_disql_scan_concurrency`](/system-variables.md#tidb_distsql_scan_concurrency) allows to read data. When the memory usage of a single SQL statement exceeds [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query) each time, the operator that reads data stops one thread.
 - When the operator that reads data has only one thread left and the memory usage of a single SQL statement continues to exceed [`tidb_mem_quota_query`](/system-variables.md#tidb_mem_quota_query), this SQL statement triggers other memory control behaviors.
-
-### `tidb_memory_usage_alarm_ratio`
-
-- Scope: SESSION
-- Default value: 0.8
-- TiDB triggers an alarm when the percentage of the memory it takes exceeds a certain threshold. For the detailed usage description of this feature, see [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio-new-in-v409).
-- You can set the initial value of this variable by configuring [`memory-usage-alarm-ratio`](/tidb-configuration-file.md#memory-usage-alarm-ratio-new-in-v409).