Add docs

docs/en/operations/server-configuration-parameters/settings.md

@@ -1616,3 +1616,14 @@ Possible values:
 
 Default value: `10000`.
 
+## global_memory_usage_overcommit_max_wait_microseconds {#global_memory_usage_overcommit_max_wait_microseconds}
+
+Sets maximum waiting time for global overcommit tracker.
+
+Possible values:
+
+-   Positive integer.
+
+Default value: `0`.
+
+

docs/en/operations/settings/memory-overcommit.md

@@ -0,0 +1,31 @@
+# Memory overcommit
+
+Memory overcommit is an experimental technique intended to allow to set more flexible memory limits for queries.
+
+The idea of this technique is to introduce settings which can represent guaranteed memory amount of memory a query can use.
+When memory overcommit is enabled and memory limit is reached ClickHouse will select the most overcommit query and try to free memory by killing this query.
+
+When memory limit is reached any query will wait some time during atempt to allocate new memory.
+If selected query is killed and memory is freed within waiting timeout, query will continue execution after waiting, otherwise it'll be killed too.
+
+Selection of query to stop is performed by either global or user overcommit trackers depending on what memory limit is reached.
+
+## User overcommit tracker
+
+User overcommit tracker finds a query with the biggest overcommit ratio in the user's query list.
+Overcommit ratio is computed as number of allocated bytes divided by value of `max_guaranteed_memory_usage` setting.
+
+Waiting timeout is set by `memory_usage_overcommit_max_wait_microseconds` setting.
+
+**Example**
+
+```sql
+SELECT number FROM numbers(1000) GROUP BY number SETTINGS max_guaranteed_memory_usage=4000, memory_usage_overcommit_max_wait_microseconds=500
+```
+
+## Global overcommit tracker
+
+Global overcommit tracker finds a query with the biggest overcommit ratio in the list of all queries.
+In this case overcommit ratio is computed as number of allocated bytes divided by value of `max_guaranteed_memory_usage_for_user` setting.
+
+Waiting timeout is set by `global_memory_usage_overcommit_max_wait_microseconds` parameter in the configuration file.