Question

I was confused about the Pss column in /proc/pid/smaps, so I wrote a program to test it:

void sa();
int main(int argc,char *argv[])
{
    int fd;
    sa();
    sleep(1000);
}

void sa()
{
   char *pi=new char[1024*1024*10];
   for(int i=0;i<4;++i) {
        for(int j=0;j<1024*1024;++j){
                *pi='o';
                pi++;
        }
   }
   int cnt;
   for(int i=0;i<6;++i) {
        for(int j=0;j<1024*1024;++j){
                cnt+=*pi;
                pi++;
        }
   }
   printf("%d",cnt);
}

$cat /proc/`pidof testprogram`/smaps

08838000-0885b000 rw-p 00000000 00:00 0          [heap]
Size:                140 kB
Rss:                  12 kB
Pss:                  12 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:        12 kB
Referenced:           12 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
b6dcd000-b77d0000 rw-p 00000000 00:00 0 
Size:              10252 kB
Rss:               10252 kB
Pss:                4108 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:      4108 kB
Referenced:         4108 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB

Here I found Pss equal to Private_Dirty, but I wonder why.

BTW: Is there any detailed documentation for smaps?

Answer 1

Quoting from lwn.net

The “proportional set size” (PSS) of a process is the count of pages
it has in memory, where each page is divided by the number of
processes sharing it. So if a process has 1000 pages all to itself,
and 1000 shared with one other process, its PSS will be 1500

From Linux Kernel Documentation,

The /proc/PID/smaps is an extension based on maps, showing the memory
consumption for each of the process’s mappings. For each of mappings there
is a series of lines such as the following:

08048000-080bc000 r-xp 00000000 03:02 13130      /bin/bash
Size:               1084 kB
Rss:                 892 kB
Pss:                 374 kB
Shared_Clean:        892 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:          892 kB
Anonymous:             0 kB
Swap:                  0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Locked:              374 kB

The first of these lines shows the same information as is displayed
for the mapping in /proc/PID/maps. The remaining lines show the size
of the mapping (size), the amount of the mapping that is currently
resident in RAM (RSS), the process’ proportional share of this mapping
(PSS), the number of clean and dirty private pages in the mapping.
Note that even a page which is part of a MAP_SHARED mapping, but has
only a single pte mapped, i.e. is currently used by only one process,
is accounted as private and not as shared. “Referenced” indicates the
amount of memory currently marked as referenced or accessed.
“Anonymous” shows the amount of memory that does not belong to any
file. Even a mapping associated with a file may contain anonymous
pages: when MAP_PRIVATE and a page is modified, the file page is
replaced by a private anonymous copy. “Swap” shows how much
would-be-anonymous memory is also used, but out on swap.

This Question on Unix and Linux Stackexchange covers almost the topic. See Mat’s excellent answer which will surely clear all your doubts.