RES != CODE + DATA in the output information of the top command,why? - linux

Question

what 'man top' said is: RES = CODE + DATA
q: RES -- Resident size (kb)
The non-swapped physical memory a task has used.
RES = CODE + DATA.
r: CODE -- Code size (kb)
The amount of physical memory devoted to executable code, also known as the 'text resident set' size or TRS.
s: DATA -- Data+Stack size (kb)
The amount of physical memory devoted to other than executable code, also known as the 'data >resident set' size or DRS.
what when i run 'top -p 4258',i get the following:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ CODE DATA COMMAND
258 root 16 0 3160 1796 1328 S 0.0 0.3 0:00.10 476 416 bash
1796 != 476+416
why?
ps:
linux distribution:
linux-iguu:~ # lsb_release -a
LSB Version: core-2.0-noarch:core-3.0-noarch:core-2.0-ia32:core-3.0-ia32:desktop-3.1-ia32:desktop-3.1-noarch:graphics-2.0-ia32:graphics-2.0-noarch:graphics-3.1-ia32:graphics-3.1-noarch
Distributor ID: SUSE LINUX
Description: SUSE Linux Enterprise Server 9 (i586)
Release: 9
Codename: n/a
kernel version:
linux-iguu:~ # uname -a
Linux linux-iguu 2.6.16.60-0.21-default #1 Tue May 6 12:41:02 UTC 2008 i686 i686 i386 GNU/Linux

Answer 1

I'll explain this with the help of an example of what happens when a program allocates and uses memory. Specifically, this program:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
int main(){
int *data, size, count, i;
printf( "fyi: your ints are %d bytes large\n", sizeof(int) );
printf( "Enter number of ints to malloc: " );
scanf( "%d", &size );
data = malloc( sizeof(int) * size );
if( !data ){
perror( "failed to malloc" );
exit( EXIT_FAILURE );
}
printf( "Enter number of ints to initialize: " );
scanf( "%d", &count );
for( i = 0; i < count; i++ ){
data[i] = 1337;
}
printf( "I'm going to hang out here until you hit <enter>" );
while( getchar() != '\n' );
while( getchar() != '\n' );
exit( EXIT_SUCCESS );
}
This is a simple program that asks you how many integers to allocate, allocates them, asks how many of those integers to initialize, and then initializes them. For a run where I allocate 1250000 integers and initialize 500000 of them:
$ ./a.out
fyi: your ints are 4 bytes large
Enter number of ints to malloc: 1250000
Enter number of ints to initialize: 500000
Top reports the following information:
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ SWAP CODE DATA COMMAND
<program start>
11129 xxxxxxx 16 0 3628 408 336 S 0 0.0 0:00.00 3220 4 124 a.out
<allocate 1250000 ints>
11129 xxxxxxx 16 0 8512 476 392 S 0 0.0 0:00.00 8036 4 5008 a.out
<initialize 500000 ints>
11129 xxxxxxx 15 0 8512 2432 396 S 0 0.0 0:00.00 6080 4 5008 a.out
The relevant information is:
DATA CODE RES VIRT
before allocation: 124 4 408 3628
after 5MB allocation: 5008 4 476 8512
after 2MB initialization: 5008 4 2432 8512
After I malloc'd 5MB of data, both VIRT and DATA increased by ~5MB, but RES did not. RES did increase after I touched 2MB of the integers I allocated, but DATA and VIRT stayed the same.
VIRT is the total amount of virtual memory used by the process, including what is shared and what is over-committed. DATA is the amount of virtual memory used that isn't shared and that isn't code-text. I.e., it is the virtual stack and heap of the process. RES is not virtual: it is a measurment of how much memory the process is actually using at that specific time.
So in your case, the large inequality CODE+DATA < RES is likely the shared libraries included by the process. In my example (and yours), SHR+CODE+DATA is a closer approximation to RES.
Hope this helps.
There's a lot of hand-waving and voodoo associated with top and ps. There are many articles (rants?) online about the descrepancies. E.g., this and this.

Answer 2

This explanation is terrific to resolve my some queries. Thanks!
And meanwhile, trying to add something got during my understanding of linux memory management knowledge. If any misunderstand, please correct me!
Modern OS process concepts are based on virtual memory. Virtual memory system includes the RAM+SWAP;
So I think most of the memory concepts related with processes refer to the virtual memory, except that there are some supplement notes.
Any virtual address(page) allocated to a process is in below state:
a) allocated, but no mapping to any physical memory(something like COW)
b) allocated, already mapped to physical memory
c) allocated, already mapped to swapped memory.
The fields ouput of top command:
a) VIRT -- it refers to all virtual memory that the process have the right
to access, no matter it is already mapped to physical memory or swapped
memory, or even has no any mapping.
b) RES -- it refers to the virtual address already mapped to physical address and it still in RAM.
c) SWAP -- refers to the virtual address already mapped to physical address and it is swapped into SWAP space.
d) SHR -- it refers to the shared memory available to a process(VM?)
e) CODE + DATA -- CODE could be in a state of 2.b/2.c, and DATA could be in any of 3 state 2.a/2.b/3.c, and 3.b/3.c also have a fields name called "USED".
4) So the calculation maybe look like:
a) VIRT(VM) = RES(VM in memory) + SWAP(VM in swap) + VM unmapped(DATA, SHR?).
b) USED = RES + SWAP
c) SWAP = CODE(vm in memory) + DATA(vm in memory) + SHR(vm in memory?)
d) RES = CODE(vm in memory) + DATA(vm in memory) + SHR(vm in memory?)
At least DATA segment still have a "DATA(VM unmapped)", this could be observed from above malloc example. That's a little different from the manpage of top command which says "DATA: The amount of physical memory devoted to other than executable code, also known as the Data Resident Set size or DRS". Thanks again.
So amount of (CODE + DATA + SHR) usually larger than RES, because at least DATA(vm unmapped) actually calculated in "DATA", not like the manpge claiming.
Regards,