$ cat /proc/sys/vm/min_free_kbytes
67584

$ free -h
              total        used        free      shared  buff/cache   available
Mem:          7.7Gi       3.2Gi       615Mi       510Mi       3.9Gi       3.7Gi
Swap:         2.0Gi       707Mi       1.3Gi

$ grep -r min_free_kbytes /etc/sysctl*  # No manual configuration
$

$ uname -r  # My kernel version
5.0.17-200.fc29.x86_64

/*
 * Initialise min_free_kbytes.
 *
 * For small machines we want it small (128k min).  For large machines
 * we want it large (64MB max).  But it is not linear, because network
 * bandwidth does not increase linearly with machine size.  We use
 *
 *  min_free_kbytes = 4 * sqrt(lowmem_kbytes), for better accuracy:
 *  min_free_kbytes = sqrt(lowmem_kbytes * 16)
 *
 * which yields
 *
 * 16MB:    512k
 * 32MB:    724k
 * 64MB:    1024k
 * 128MB:   1448k
 * 256MB:   2048k
 * 512MB:   2896k
 * 1024MB:  4096k
 * 2048MB:  5792k
 * 4096MB:  8192k
 * 8192MB:  11584k
 * 16384MB: 16384k
 */