memory utilization by a linux process - linux

Question

I have a lot of instances of a process running on my host, each seemingly consuming large amount of memory.
ps aux on the processes give me the following information
blah1 18634 0.0 0.4 131852 31188 pts/15 Ssl+ 00:27 0:00 myPgm
blah2 18859 0.0 0.3 131292 30656 pts/32 Sl+ 01:17 0:00 myPgm
blah3 19813 0.0 0.4 131960 31220 pts/44 Ssl+ 01:17 0:00 myPgm
blah4 20228 0.1 0.3 131728 31036 pts/54 Ssl+ 01:41 0:00 myPgm
blah5 20238 0.0 0.3 131688 30932 pts/20 Sl+ Nov15 0:00 myPgm
blah6 21181 0.0 0.3 131304 30632 pts/25 Sl+ Nov15 0:00 myPgm
blah7 21278 0.0 0.3 131824 31096 pts/61 Ssl+ Nov15 0:00 myPgm
blah8 21821 0.0 0.3 131444 30808 pts/7 Sl+ 00:54 0:00 myPgm
So VSZ is always around 130 MB and RSS around 30 MB. pmap for the processes have the following data:
For 18634:
mapped: 131852K writeable/private: 59692K shared: 28K
For 21181:
mapped: 131304K writeable/private: 59144K shared: 28K
and similar values for other processes as well. The host has 7GB of physical memory. At times I have around 700 to 800 instances of the same process running on the host. I am trying to understand how much memory each process consumes in reality. If i take "writeable/private" as the actual memory usage in each process then 58MB for each process would lead to 45 GB (for 800 process) which is crazy. Can anyone explain if im doing it wrong and how should be calculation be done?
Also free -k gives
total used free shared buffers cached
Mem: 7782580 4802104 2980476 0 380192 1931708
-/+ buffers/cache: 2490204 5292376
Swap: 1048568 32 1048536
Looks like not much swap is being used, now where does the memory for each process come from? Thanks.

Answer 1

You don't know what VSZ is. You think you know, but evidence suggests otherwise, therefore you need to find out what it is.
VZS is Virtual Memory Size and that's all memory required by the process, including shared memory. That's why you can't just sum(VSZ) and expect to get less than the physical amount of memory + swap.
The mapped memory in pmap probably corresponds to VSZ and writable/private, I guess, is memory that is shared by some processes, where each of them has write access to that memory (like allocated by their parent process or so).
To understand this, you need to understand how memory allocation and access works, which is difficult. http://emilics.com/blog/article/mconsumption.html this article seems to explain it in some detail (but I read that only in cursory way)