mirror of
https://github.com/ClickHouse/ClickHouse.git
synced 2024-11-27 18:12:02 +00:00
41 lines
2.0 KiB
Markdown
41 lines
2.0 KiB
Markdown
---
|
|
slug: /en/operations/settings/memory-overcommit
|
|
---
|
|
# 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 amount of memory a query can use.
|
|
When memory overcommit is enabled and the memory limit is reached ClickHouse will select the most overcommitted query and try to free memory by killing this query.
|
|
|
|
When memory limit is reached any query will wait some time during attempt to allocate new memory.
|
|
If timeout is passed and memory is freed, the query continues execution.
|
|
Otherwise an exception will be thrown and the query is killed.
|
|
|
|
Selection of query to stop or kill is performed by either global or user overcommit trackers depending on what memory limit is reached.
|
|
If overcommit tracker can't choose query to stop, MEMORY_LIMIT_EXCEEDED exception is thrown.
|
|
|
|
## User overcommit tracker
|
|
|
|
User overcommit tracker finds a query with the biggest overcommit ratio in the user's query list.
|
|
Overcommit ratio for a query is computed as number of allocated bytes divided by value of `memory_overcommit_ratio_denominator` setting.
|
|
|
|
If `memory_overcommit_ratio_denominator` for the query is equals to zero, overcommit tracker won't choose this query.
|
|
|
|
Waiting timeout is set by `memory_usage_overcommit_max_wait_microseconds` setting.
|
|
|
|
**Example**
|
|
|
|
```sql
|
|
SELECT number FROM numbers(1000) GROUP BY number SETTINGS memory_overcommit_ratio_denominator=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 `memory_overcommit_ratio_denominator_for_user` setting.
|
|
|
|
If `memory_overcommit_ratio_denominator_for_user` for the query is equals to zero, overcommit tracker won't choose this query.
|
|
|
|
Waiting timeout is set by `memory_usage_overcommit_max_wait_microseconds` parameter in the configuration file.
|