- 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