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32 lines
1.6 KiB
Markdown
32 lines
1.6 KiB
Markdown
# Memory overcommit
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Memory overcommit is an experimental technique intended to allow to set more flexible memory limits for queries.
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The idea of this technique is to introduce settings which can represent guaranteed amount of memory a query can use.
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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.
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When memory limit is reached any query will wait some time during atempt to allocate new memory.
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If timeout is passed and memory is freed, the query continues execution. Otherwise an exception will be thrown and the query is killed.
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Selection of query to stop or kill is performed by either global or user overcommit trackers depending on what memory limit is reached.
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## User overcommit tracker
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User overcommit tracker finds a query with the biggest overcommit ratio in the user's query list.
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Overcommit ratio is computed as number of allocated bytes divided by value of `max_guaranteed_memory_usage` setting.
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Waiting timeout is set by `memory_usage_overcommit_max_wait_microseconds` setting.
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**Example**
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```sql
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SELECT number FROM numbers(1000) GROUP BY number SETTINGS max_guaranteed_memory_usage=4000, memory_usage_overcommit_max_wait_microseconds=500
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```
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## Global overcommit tracker
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Global overcommit tracker finds a query with the biggest overcommit ratio in the list of all queries.
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In this case overcommit ratio is computed as number of allocated bytes divided by value of `max_guaranteed_memory_usage_for_user` setting.
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Waiting timeout is set by `global_memory_usage_overcommit_max_wait_microseconds` parameter in the configuration file.
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