memory configuration settings

memory configuration settings

max_memory_usage. Single query memory usage

max_memory_usage - the maximum amount of memory allowed for a single query to take. By default, it’s 10Gb. The default value is good, don’t adjust it in advance.

There are scenarios when you need to relax the limit for particular queries (if you hit ‘Memory limit (for query) exceeded’), or use a lower limit if you need to discipline the users or increase the number of simultaneous queries.

Server memory usage

Server memory usage = constant memory footprint (used by different caches, dictionaries, etc) + sum of memory temporary used by running queries (a theoretical limit is a number of simultaneous queries multiplied by max_memory_usage).

Since 20.4 you can set up a global limit using the max_server_memory_usage setting. If something will hit that limit you will see ‘Memory limit (total) exceeded’ in random places.

By default it 90% of the physical RAM of the server. https://clickhouse.tech/docs/en/operations/server-configuration-parameters/settings/#max_server_memory_usage https://github.com/ClickHouse/ClickHouse/blob/e5b96bd93b53d2c1130a249769be1049141ef386/programs/server/config.xml#L239-L250

You can decrease that in some scenarios (like you need to leave more free RAM for page cache or to some other software).

How to check what is using my RAM?

altinity-kb-who-ate-my-memory.md"

Mark cache

https://github.com/ClickHouse/clickhouse-presentations/blob/master/meetup39/mark-cache.pdf

Last modified 2021.08.12 : General corrections and updates. (5969b688)