Controlling memory usage

In Memgraph, you can control memory usage by limiting, inspecting and deallocating memory.

You can control the memory usage of:

• a whole instance by setting the --memory-limit within the configuration file
• a query by including the QUERY MEMORY clause at the end of a query
• a procedure by including a PROCEDURE MEMORY clause

Controlling the memory usage of an instance

By setting the --memory-limit flag in the configuration file, you can set the maximum amount of memory (in MiB) that a Memgraph instance can allocate during its runtime. If the memory limit is exceeded, only the queries that don't require additional memory are allowed. If the memory limit is exceeded while a query is running, the query is aborted and its transaction becomes invalid.

If the flag is set to 0, it will use the default values. Default values are:

• 90% of the total memory if the system doesn't have swap memory.
• 100% of the total memory if the system has swap memory.

Controlling the memory usage of a query

Each Cypher query can include the following clause at the end:

QUERY MEMORY ( UNLIMITED | LIMIT num (KB | MB) )

If you use the LIMIT option, you have to specify the amount of memory a query can allocate for its execution. You can use this clause in a query only once at the end of the query. The limit is applied to the entire query.

Examples:

MATCH (n) RETURN (n) QUERY MEMORY LIMIT 10 KB;

MATCH (n) RETURN (n) QUERY MEMORY UNLIMITED;

Controlling the memory usage of a procedure

Each procedure call can contain the following clause:

PROCEDURE MEMORY ( UNLIMITED | LIMIT num ( KB | MB) )

If you use the LIMIT option, you can specify the amount of memory that the called procedure can allocate for its execution. If you use the UNLIMITED option, no memory restrictions will be imposed when the procedure is called. If you don't specify the clause, the memory limit is set to a default value of 100 MB.

One procedure call can have only one PROCEDURE MEMORY clause at the end of the call. If a query contains multiple procedure calls, each call can have its own limit specification.

Examples:

CALL example.procedure(arg1, arg2, ...) PROCEDURE MEMORY LIMIT 100 KB YIELD result;

CALL example.procedure(arg1, arg2, ...) PROCEDURE MEMORY LIMIT 100 MB YIELD result;

CALL example.procedure(arg1, arg2, ...) PROCEDURE MEMORY UNLIMITED YIELD result;

Inspecting memory usage

Run the following query to inspect memory usage:

SHOW STORAGE INFO;

Find out more about SHOW STORAGE INFO query on Server stats.

Deallocating memory

Memgraph has a garbage collector that deallocates unused objects, thus freeing the memory. The rate of the garbage collection in seconds can be specified in the configuration file by setting the --storage-gc-cycle-sec.

You can free up memory by using the following query:

FREE MEMORY;

This query tries to clean up as much unused memory as possible without affecting currently running transactions.

Storage modes

Memgraph can work either in an IN_MEMORY_ANALYTICAL or IN_MEMORY_TRANSACTIONAL storage mode. The IN_MEMORY_TRANSACTIONAL is the default storage mode that ensures strongly-consistent ACID transactions using WAL files and periodic snapshots.

You can reduce memory usage by switching to the IN_MEMORY_ANALYTICAL mode in which Memgraph no longer creates additional memory objects during data modification, and doesn't provide any ACID guarantees. You can use it for the initial data import as it also speeds up import up to 6x.

You can switch between modes within a single session using the STORAGE MODE IN_MEMORY_TRANSACTIONAL; and STORAGE MODE IN_MEMORY_ANALYTICAL; Cypher queries, but you must have the STORAGE_MODE privilege.