How is the global process area implemented in ClickHouse?

Shiv Iyer
Posted on March 2, 2023

In ClickHouse, the global process area is implemented as a shared-memory segment that is shared between all instances of the ClickHouse server running on the same machine. This shared-memory segment is used to store various types of data, including metadata about the database, tables, and columns, as well as information about running queries and other server-level data.

The shared-memory segment used for the global process area is created when the ClickHouse server starts up and is destroyed when the server shuts down. The size of the shared-memory segment is configurable and can be set using the –global-config-file command-line option or the max_shared_memory_size server configuration parameter.

One of the key benefits of using a shared-memory segment for the global process area is that it allows multiple instances of the ClickHouse server to share data and communicate with each other more efficiently. For example, if one instance of the server updates metadata about a table, that update is immediately visible to all other instances of the server that are sharing the same global process area.

However, shared-memory segments can also pose security risks if not properly managed. To mitigate these risks, ClickHouse implements various security mechanisms, such as using file permissions and ownership to restrict access to the shared-memory segment and limiting the size of the shared-memory segment to prevent it from consuming too much system resources.

Configuring the global process area in ClickHouse

In ClickHouse, you can configure the global process area by setting the max_shared_memory_size server configuration parameter to the desired size of the shared-memory segment. Here’s how to do it:

  1. Open the ClickHouse server configuration file, which is usually located at /etc/clickhouse-server/config.xml.
  2. Add or modify the max_shared_memory_size parameter in the <tcp_port> section of the configuration file to set the maximum size of the shared-memory segment. For example, to set the size to 1 gigabyte (GB), you would add the following line:


  1. Note that the max_shared_memory_size parameter is specified in bytes.
  2. Save the configuration file and restart the ClickHouse server for the changes to take effect.

The max_shared_memory_size parameter is set to 268435456 (256 MB) by default. Suppose you are running multiple instances of the ClickHouse server on the same machine. In that case, you may need to adjust this parameter to prevent the shared-memory segment from consuming too much memory.

It’s important to note that setting the max_shared_memory_size parameter too high can cause the ClickHouse server to consume too much memory and potentially cause the system to become unresponsive. Therefore, you should set the parameter to a reasonable value based on the available system resources and the expected workload of the server.