MariaDB, Apache, etc, config tweaks to reduce swap usage?

Our Moodle server (Moodle 4.5) seems to be using more and more swap gradually over time until it starts to get to the point where this is making the server less responsive.We have been keeping a closer eye on this recently and are looking into tweaks that we can make to improve performance (one thing we are certainly planning to do is to schedule overnight reboots once a month, or perhaps more frequently if necessary, as a simple, but perhaps inelegant, workaround). Arguably we should have perhaps being doing this already, but we try to minimise reboots as much as possible (within reason).The server has 64 GB RAM, but (as far back as our monitoring logging goes) doesn't ever seem to use more than 50% of this. Despite there being lots of available RAM, swap usage gradually increases over time (around 2 - 4 GB a month), and, not unsurprisingly, the server starts to get sluggish once much more than 4 GB swap (of 16 GB available) is being used.I suppose it is not unusual for a small amount of swap to be used, but it seems a bit odd for the amount to be constantly increasing, especially given that there is available RAM (my guess is that perhaps something is not returning memory properly when it should?).As far as I can tell, ourApacheandMariaDBsettings (mostly) seem fairly reasonable for our server usage and capacity, although I will need to look more carefully the next time we approach a usage peak.For our current use level, for Apache (2.4), using mpm_prefork, we seem to have enough (and not excessive)SpareServersandMaxRequestWorkers(looking at usage via the Apache server-status page). I have set a limit onMaxConnectionsPerChild(currently 4096, rather than the default 0/no limit) in case Apache child processes are indeed gradually leaking memory, but I don't know whether that's too high a value (I didn't want to start far too low!) - so many of these things seem to be trial and error and gradual tweaking!For MariaDB,innodb_log_file_sizeandinnodb_buffer_pool_sizeare set in the recommended 1:4 ratio and to values (potentially) using ~70% of RAM.ApacheMaxRequestWorkersand MariaDBmax_connectionsare still at their respective defaults (150, 151) as these values seem more than enough for our current load (from Apache server-status and MariaDB's status variables, respectively), and I'm assuming that a 1:1 ratio for these is appropriate?I have run mysqltuner, which is mostly (but not entirely) happy. Oddly, the (older) distro packaged version saysTotal buffers: 40.3G global + 102.9M per thread (151 max threads)but the current, directly downloaded, version saysℹ  Total buffers: 40.3G global + 2.9M per thread (151 max threads)has a very very different description of how much memory each thread requires (not sure why or which is correct!)?The only MariaDB settings which it suggests I tweak are:join_buffer_size (> 256.0K, or always use indexes with JOINs)table_definition_cache (400) > 866 or -1 (autosizing if supported)These are not settings that I have ever changed in the past, or am familiar with. I obviously want to be extra cautious when changing settings on a live site: do these recommendations seem sensible to follow, and are they likely to have much impact on memory usage?The only other thing that I can think of is that more recently it seems from monitoring that the swap usage has been increasing in small 'chunks' around 01:15 every morning. This seems to coincide with the automated course backups which are underway at that time (starting earlier than this and finishing later than this), so I am wondering if it is maybe the backup of a large course that is perhaps consuming more (and not returning) extra memory?Thanks for any advice!