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POHow to get accurate shared memory size of a process?
 

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  1. POHow to get accurate shared memory size of a process?
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    <p>I tried to get the shared memory size of a process on Linux. Here's the result of using 2 different commands:</p> <ol> <li><p>top and check with the SHR field:</p> <pre><code> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1734 root 20 0 201m 4072 1012 S 0.0 0.1 22:00.65 php-fpm </code></pre></li> <li><p>pmap -d :</p> <pre><code>mapped: 206672K writeable/private: 4352K shared: 128K </code></pre></li> </ol> <p>You can see that the shared memory size in pmap is much smaller than top.</p> <p>I read some source code to find the reason. It seems that top is reading the value from /proc//statm and the values are calculated by:</p> <pre><code>unsigned long task_statm(struct mm_struct *mm, unsigned long *shared, unsigned long *text, unsigned long *data, unsigned long *resident) { *shared = get_mm_counter(mm, MM_FILEPAGES); *text = (PAGE_ALIGN(mm-&gt;end_code) - (mm-&gt;start_code &amp; PAGE_MASK)) &gt;&gt; PAGE_SHIFT; *data = mm-&gt;total_vm - mm-&gt;shared_vm; *resident = *shared + get_mm_counter(mm, MM_ANONPAGES); return mm-&gt;total_vm; } </code></pre> <p>It seems that all the file pages are counted as shared memory?</p> <p>And the pmap command is reading the info from /proc//maps and then calculate the shared memory through some flags:</p> <pre><code>3dc822a000-3dc822d000 rw-p 0002a000 08:13 5134288 /usr/lib64/libmcrypt.so.4.4.8 start-end flags file_offset dev_major:dev_minor inode </code></pre> <p>If the flags[3] == 's' then this map will be counted as shared one.</p> <p>So my question is which one is more accurate? And why they have different methods to calculate the shared memory size?</p> <p>Thanks in advance!</p>
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