What does monit consider to be memory usage?

Solution 1:

This could be get from the source of monit https://github.com/arnaudsj/monit/tree/master/process.

For linux the value is computed in sysdep_LINUX.c and comes from /proc/meminfo :

si->total_mem_kbyte = systeminfo.mem_kbyte_max - mem_free - buffers - cached;

In other words monit use as memory usage MemTotal - MemFree - Buffers - Cached.

In your case 998 - 146 - 114 - 70 = 668