Re: [RFC] mm: why cat /proc/pid/smaps | grep Rss is different from cat /proc/pid/statm?

[Xishi Qiu]

On 2016/3/22 22:47, Shakeel Butt wrote:

> 
> On Tue, Mar 22, 2016 at 6:55 AM, Xishi Qiu <qiuxishi@xxxxxxxxxx <mailto:qiuxishi@xxxxxxxxxx>> wrote:
> 
>     [root@localhost c_test]# cat /proc/3948/smaps | grep Rss
> 
> The /proc/[pid]/smaps read triggers the traversal of all of process's vmas and then page tables and accumulate RSS on each present page table entry.
> 
>     [root@localhost c_test]# cat /proc/3948/statm
>     1042 173 154 1 0 48 0
> 
> The files /proc/[pid]/statm and /proc/[pid]/status uses the counters (MM_ANONPAGES & MM_FILEPAGES) in mm_struct to report RSS of a process. These counters are modified on page table modifications. However the kernel implements an optimization where each thread keeps a local copy of these counters in its task_struct. These local counter are accumulated in the shared counter of mm_struct after some number of page faults (I think 32) faced by the thread and thus there will be mismatch with smaps file.
> 
> Shakeel

Hi Shakeel,

I malloc and memset 10M, then sleep. It seems that the problem is still exist,
the kernel version is v4.1

[root@localhost c_test]# cat /proc/13746/statm
3603 2767 250 1 0 2609 0
[root@localhost c_test]# cat /proc/13746/smaps | grep Rss
Rss:                   4 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:               10244 kB
Rss:                 924 kB
Rss:                   0 kB
Rss:                  16 kB
Rss:                   8 kB
Rss:                  12 kB
Rss:                 132 kB
Rss:                  12 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:                   4 kB
Rss:                   8 kB
Rss:                   0 kB
Rss:                   4 kB
Rss:                   0 kB