[riku_iki]

> There are various knobs in ClickHouse that allow you to trade memory usage for performance.

but what knobs to use and what values to use in each specific case? Query just usually fails with some generic OOM message without much information.

[alright2565]

It's not actually so esoteric. The two main knobs are

- max_concurrent_queries, since each query uses a certain amount of memory

- max_memory_usage, which is the max per-query memory usage

Here's my full config for running clickhouse on a 2GiB server without OOMs. Some stuff in here is likely irrelevant, but it's a starting point.

    diff --git a/clickhouse-config.xml b/clickhouse-config.xml
    index f8213b65..7d7459cb 100644
    --- a/clickhouse-config.xml
    +++ b/clickhouse-config.xml
    @@ -197,7 +197,7 @@
     
         <!-- <listen_backlog>4096</listen_backlog> -->
     
    -    <max_connections>4096</max_connections>
    +    <max_connections>2000</max_connections>
     
         <!-- For 'Connection: keep-alive' in HTTP 1.1 -->
         <keep_alive_timeout>3</keep_alive_timeout>
    @@ -270,7 +270,7 @@
         -->
     
         <!-- Maximum number of concurrent queries. -->
    -    <max_concurrent_queries>100</max_concurrent_queries>
    +    <max_concurrent_queries>4</max_concurrent_queries>
     
         <!-- Maximum memory usage (resident set size) for server process.
              Zero value or unset means default. Default is "max_server_memory_usage_to_ram_ratio" of available physical RAM.
    @@ -335,7 +335,7 @@
              In bytes. Cache is single for server. Memory is allocated only on demand.
              You should not lower this value.
           -->
    -    <mark_cache_size>5368709120</mark_cache_size>
    +    <mark_cache_size>805306368</mark_cache_size>
     
     
         <!-- If you enable the `min_bytes_to_use_mmap_io` setting,
    @@ -981,11 +980,11 @@
         </distributed_ddl>
     
         <!-- Settings to fine tune MergeTree tables. See documentation in source code, in MergeTreeSettings.h -->
    -    <!--
         <merge_tree>
    -        <max_suspicious_broken_parts>5</max_suspicious_broken_parts>
    +        <merge_max_block_size>2048</merge_max_block_size>
    +        <max_bytes_to_merge_at_max_space_in_pool>1073741824</max_bytes_to_merge_at_max_space_in_pool>
    +        <number_of_free_entries_in_pool_to_lower_max_size_of_merge>0</number_of_free_entries_in_pool_to_lower_max_size_of_merge>
         </merge_tree>
    -    -->
     
         <!-- Protection from accidental DROP.
              If size of a MergeTree table is greater than max_table_size_to_drop (in bytes) than table could not be dropped with any DROP query.
    diff --git a/clickhouse-users.xml b/clickhouse-users.xml
    index f1856207..bbd4ced6 100644
    --- a/clickhouse-users.xml
    +++ b/clickhouse-users.xml
    @@ -7,7 +7,12 @@
             <!-- Default settings. -->
             <default>
                 <!-- Maximum memory usage for processing single query, in bytes. -->
    -            <max_memory_usage>10000000000</max_memory_usage>
    +            <max_memory_usage>536870912</max_memory_usage>
    +
    +            <queue_max_wait_ms>1000</queue_max_wait_ms>
    +            <max_execution_time>30</max_execution_time>
    +            <background_pool_size>4</background_pool_size>
    +
     
                 <!-- How to choose between replicas during distributed query processing.
                      random - choose random replica from set of replicas with minimum number of errors

[riku_iki]

> The two main knobs are

my experience is that those are not enough, multiple algorithms will just fail saying you hit max memory limit. There are many other knobs, for example: when to start external aggregation or sorting. For some cases I couldn't figure out setup and query just hits OOM without any ideas how to fix it.

[FridgeSeal]

How is your table setup? It’s plausible the on-disk/index layout is not amenable to the kinds of queries you’re trying to do.

What kind of queries are you trying to do? Also, what kind of machine are you running on?

[riku_iki]

Trivial example would be to run select count(distinct) from large table with high cardinality values: https://github.com/ClickHouse/ClickHouse/issues/47520