Unexplained MYSQL memory allocation

Question

I’m running MySQL server on Linux.

I have a long-standing issue on my database server — it keeps consuming an excessive amount of memory for no clear reason. As a result, I was forced to periodically restart it to bring the system back to a stable state. The attached screenshot shows the memory usage trend over the past 90 days. Here are some details:

OS: CentOS / RHEL 7

MySQL version: 5.7.x (can specify exact version) Configuration file: /etc/my.cnf.d/myserver.cnf

[mysqld@myserver]
net_read_timeout = 4800
tmp_table_size = 512M
innodb_lock_wait_timeout = 120
myisam_sort_buffer_size = 3G
read_rnd_buffer_size = 16M
max_heap_table_size = 512M
max_connect_errors = 10000
binlog_format = row
innodb_buffer_pool_size = 500M
myisam_max_sort_file_size = 1MB
thread_cache_size = 16
max_connections = 6000
max_allowed_packet = 256M
log_error = error.log
binlog_cache_size = 16m
innodb_log_buffer_size = 8M
slow_query_log_file = slow.log
innodb_max_dirty_pages_pct = 10
default_storage_engine = innodb
back_log = 50
explicit_defaults_for_timestamp = 1
innodb_log_file_size = 50331648
ft_min_word_len = 4
sql_mode = NO_ENGINE_SUBSTITUTION
thread_stack = 192K
wait_timeout = 3000
bind_address = 0.0.0.0
log_bin_trust_function_creators = 1
net_write_timeout = 4800
innodb_flush_log_at_trx_commit = 0
bulk_insert_buffer_size = 64M
transaction_isolation = REPEATABLE-READ
table_open_cache = 10000
sort_buffer_size = 4M
read_buffer_size = 4M
long_query_time = 2
default_authentication_plugin = mysql_native_password
default_password_lifetime = 0
key_buffer_size = 32M
join_buffer_size = 8M
log_slave_updates = 0
character_set_client_handshake = False
character_set_server = utf8mb4
collation_server = utf8mb4_unicode_ci
mysqlx = 0
skip_name_resolve
innodb_file_per_table
replicate_ignore_table = mysql.user
replicate_ignore_table = mysql.db
datadir = /var/lib/mysql_multi_instance/myserver
log_bin = myserver-bin
relay_log = myserver-relay
port = 32729
socket = /var/lib/mysql_multi_instance/myserver/mysql.sock
server_id = 3112729

Monitoring shows steady growth in RES and VIRT in top No large queries or import jobs are running Restarting the mysql servers temporarily fixes the issue, but memory slowly grows again

Things I’ve already checked:

No obvious long-running queries (SHOW PROCESSLIST clean) innodb_buffer_pool_size is set reasonably No memory leaks from client side (connections normal) Error log doesn’t show anything critical

My questions: What could cause MySQL to allocate memory unexpectedly even under low load? What tools or metrics can I use to identify which component (e.g., InnoDB, query cache, memory fragmentation, etc.) is consuming it? Are there recommended debugging steps for MySQL memory growth in multi-instance setups? Any advice or diagnostic steps would be appreciated.

UPDATE

To clarify — we are using InnoDB as the storage engine.

We have a single large physical server that runs several Java applications and four separate MySQL instances. Two of those database instances behave normally and maintain stable memory usage, but the other two continuously allocate additional memory over time.

What’s confusing is that all four MySQL servers share the same configuration, yet only two of them exhibit this memory growth. The databases themselves are different, but their configuration parameters (including InnoDB buffer pool, query cache, etc.) are identical.

Here’s a snapshot from the server showing the memory footprint of each instance:

$ ps -aux | grep mysqld
mysql     79974  0.5  4.8 9767484 6330264 ?     Ssl  Oct08  39:06 /usr/sbin/mysqld --defaults-group-suffix=@11129
mysql     80517  0.5  6.7 12127612 8852076 ?    Ssl  Oct08  42:00 /usr/sbin/mysqld --defaults-group-suffix=@11126
mysql    116894  0.4  1.2 3841544 1670672 ?     Ssl  Sep25 116:47 /usr/sbin/mysqld --defaults-group-suffix=@11119
mysql    118122  0.4  1.1 3826188 1535424 ?     Ssl  Sep25 119:09 /usr/sbin/mysqld --defaults-group-suffix=@11118

As you can see, two instances (@11126 and @11129) are consuming a significantly higher amount of memory compared to the others, even though they are configured the same way.

We’re trying to determine what could cause these specific instances to allocate more memory — whether it’s related to workload patterns, internal InnoDB behavior, or a memory leak in MySQL itself.

If you need any more additional information please write a comment