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

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Date:	Wed, 30 Mar 2016 17:39:47 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	Shakeel Butt <shakeelb@...gle.com>
CC:	Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] mm: why cat /proc/pid/smaps | grep Rss is different from
 cat /proc/pid/statm?

On 2016/3/30 15:16, Xishi Qiu wrote:

> On 2016/3/22 22:47, Shakeel Butt wrote:
> 
>>
>> On Tue, Mar 22, 2016 at 6:55 AM, Xishi Qiu <qiuxishi@...wei.com <mailto:qiuxishi@...wei.com>> wrote:
>>
>>     [root@...alhost 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@...alhost 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 misunderstand your meaning before. I know the reason now.

Thanks,
Xishi Qiu 

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