I'm a bit confused on how the memory figures add up in top.
If I've understood correctly, the amount of "really" used memory is total - free - buffers - cached. In the example below that would be 14370248k - 75736k - 178892k - 10459552k = 3656068k, which is 25% of the total memory (3656068/14370248).
Again, if I've understood correctly, the numbers shown in RES and %MEM columns show the amount of physical memory a process "really" uses. The sum of the RES values of just httpd processes is 7254m, or 7428096k (7254*1024), which is 52% of the total memory (7428096/14370248). The sum of %MEM values of httpd processes is 53%, close enough.
How come the summary part shows just 25% of memory being used, while httpd processes alone claim to use over 50% of memory? What am I missing here?
top - 15:37:00 up 117 days, 18:27, 6 users, load average: 4.82, 4.44, 3.60
Tasks: 433 total, 4 running, 429 sleeping, 0 stopped, 0 zombie
Cpu(s): 59.6%us, 6.5%sy, 0.0%ni, 33.0%id, 0.1%wa, 0.0%hi, 0.8%si, 0.0%st
Mem: 14370248k total, 14294512k used, 75736k free, 178892k buffers
Swap: 8388600k total, 116k used, 8388484k free, 10459552k cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
3566 mysql 21 0 554m 60m 4576 S 74.6 0.4 11422:52 mysqld
23305 apache 15 0 327m 27m 3792 S 3.2 0.2 0:01.30 httpd
23474 apache 15 0 327m 27m 3800 S 3.2 0.2 0:01.09 httpd
...