It is configurable but I’ve seen enough variations to take the position ‘check the specifics for each setup’
/dev/shm is often half of RAM when it exist, but not /run/usr/$id (at least where I’ve been)
For example on a local workstation with a default install of CentOS 7 /run/usr/$id with one user is only 10% of RAM (3.2Gb of 32Gb) and on on some systems doesn’t exist at at all.
More importantly even assuming half RAM by default it’s a potential issue when allocating individual cores you end up with a system reporting a total of 0.5 * core-count of available RAM, leading to users having/causing issues with thrashing. – though user education seems the solution here.
Much less of an issue if you’re scheduling at the node level.
<But what do I know I’m out of my league here…>
Systemd - IDK, it might be calling systemd-tmpfiles