add Instance-level execution plan cache

Signed-off-by: Aolin <aolin.zhang@pingcap.com>

releases/release-8.4.0.md

@@ -112,6 +112,17 @@ Quick access: [Quick start](https://docs.pingcap.com/tidb/v8.4/quick-start-with-
 
     In SaaS scenarios, you might need to batch-create a large number of users, rotate passwords periodically, and complete these tasks within a specific time window. Starting from v8.4.0, the performance of batch user creation and password rotation has been significantly improved. Additionally, you can further enhance performance by increasing concurrency through a higher number of session connections, which greatly reduces execution time for these operations.
 
+* Instance-level execution plan cache (experimental) [#54057](https://github.com/pingcap/tidb/issues/54057) @[qw4990](https://github.com/qw4990) **tw@Oreoxmt** <!--1569-->
+
+    TiDB v8.4.0 introduces instance-level execution plan cache as an experimental feature. This feature allows all sessions within the same TiDB instance to share the execution plan cache, significantly reducing TiDB latency, improving cluster throughput, decreasing the likelihood of execution plan fluctuations, and maintaining stable cluster performance. Compared with session-level execution plan cache, instance-level execution plan cache offers the following advantages:
+
+    - Eliminates redundancy, caching more execution plans with the same memory consumption.
+    - Allocates a fixed-size memory on the instance, limiting memory usage more effectively.
+
+    In v8.4.0, instance-level execution plan cache only supports caching query execution plans and is disabled by default. You can enable this feature using [`tidb_enable_instance_plan_cache`](/system-variables.md#tidb_enable_instance_plan_cache-new-in-v840) and set its maximum memory usage using [`tidb_instance_plan_cache_max_size`](/system-variables.md#tidb_instance_plan_cache_max_size-new-in-v840). Before enabling this feature, disable [Prepared execution plan cache](/sql-prepared-plan-cache.md) and [Non-prepared execution plan cache](/sql-non-prepared-plan-cache.md).
+
+    For more information, see [documentation](/system-variables.md#tidb_enable_instance_plan_cache-new-in-v840).
+
 * Partitioned tables support global indexes (GA) [#45133](https://github.com/pingcap/tidb/issues/45133) @[mjonss](https://github.com/mjonss) @[Defined2014](https://github.com/Defined2014) @[jiyfhust](https://github.com/jiyfhust) @[L-maple](https://github.com/L-maple)
 
     In previous versions of partitioned tables, some limitations exist because global indexes are not supported. For example, the unique key must use every column in the table's partitioning expression. If the query condition does not use the partitioning key, the query will scan all partitions, resulting in poor performance. Starting from v7.6.0, the system variable [`tidb_enable_global_index`](/system-variables.md#tidb_enable_global_index-new-in-v760) is introduced to enable the global index feature. But this feature was under development at that time and it is not recommended to enable it.