Connection Memory Usage

Provides a technical breakdown of how the server allocates and manages memory for each client connection and thread.

This page contains background information for MariaDB developers.

When creating a connection, a THD object is created for that connection. This contains all connection information and also caches to speed up queries and avoid frequent malloc() calls.

When creating a new connection, the following malloc() calls are done for the THD:

The following information is the state in MariaDB 10.6.1 when compiled without debugging.

Local Thread Memory

This is part of select memory_used from information_schema.processlist.

Amount allocated	Where allocated	Description
26646	THD::THD	Allocation of THD object
256	Statement_map::Statement_map(), my_hash_init(key_memory_prepared_statement_map, &st_hash	Prepared statements
256	my_hash_init(key_memory_prepared_statement_map, &names_hash	Names of used prepared statements
128	wsrep_wfc(), Opt_trace_context(), dynamic_array()
1024	Diagnostics_area::init(),init_sql_alloc(PSI_INSTRUMENT_ME, &m_warn_root
120	Session_sysvars_tracker, global_system_variables.session_track_system_variables	Tracking of changed session variables
280	THD::THD,my_hash_init(key_memory_user_var_entry,&user_vars
280	THD::THD,my_hash_init(PSI_INSTRUMENT_ME, &sequences	Cache of used sequences
1048	THD::THD, m_token_array= my_malloc(PSI_INSTRUMENT_ME, max_digest_length
16416	CONNECT::create_thd(), my_net_init(), net_allocate_new_packet()	This is for reading data from the connected user
16416	check_connection(), thd->packet.alloc()	This is for sending data to connected user

Objects Stored in THD->memroot During Connect

Amount allocated	Where allocated	Description
72	send_server_handshake_packet, mpvio->cached_server_packet.pkt=
64	parse_client_handshake_packet, thd->copy_with_error(...db,db_len)
32	parse_client_handshake_packet, sctx->user=
368	ACL_USER::copy(), root=	Allocation of ACL_USER object
56	ACL_USER::copy(), dst->user= safe_lexcstrdup_root(root, user)
56	ACL_USER::copy()	Allocation of other connect attributes
56	ACL_USER::copy()
64	ACL_USER::copy()
64	ACL_USER::copy()
32	mysql_change_db()	Store current db in THD
48	dbname_cache->insert(db_name)	Store db name in db name cache
40	mysql_change_db(), my_register_filename(db.opt)	Store filename db.opt
8216	load_db_opt(), init_io_cache()	Disk cache for reading db.opt
1112	load_db_opts(), put_dbopts()	Cache default database parameters

State at First Call to mysql_execute_command

(gdb) p thd->status_var.local_memory_used
$24 = 75496
(gdb) p thd->status_var.global_memory_used
$25 = 17544
(gdb) p thd->variables.query_prealloc_size
$30 = 24576
(gdb) p thd->variables.trans_prealloc_size
$37 = 4096

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