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from free to meminfo.md

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tags: linux, memory

memory: from free to meminfo

It's very common to use free to get memory usage of system.

❯ free -m
              total        used        free      shared  buff/cache   available
Mem:          64429       12329       18055         793       34043       50630
Swap:             0           0           0

So what does it mean and where does free get it from? After reading the source code of free, I found that free get the result from /proc/meminfo.

memory fields

The total memory field comes from MemTotal of /proc/meminfo. And the free field comes from MemFree.

The used field is calculated as following:

mem_used = kb_main_total - kb_main_free - kb_main_cached - kb_main_buffers;
  if (mem_used < 0)
    mem_used = kb_main_total - kb_main_free;
  kb_main_used = (unsigned long)mem_used;

So we can find that buff/cache is not included in used memory if calculated result is positive.

The shared field comes from Shmem.

The buff/cache is sum of buffers and cache. The buffers field comes from Buffers.

The cache is also calculated. It is sum of Cached field and SReclaimable.

  kb_main_cached = kb_page_cache + kb_slab_reclaimable;

Available memory is calculated as following and it will fallback to kb_main_free if the result is negative or large than total memory.

      watermark_low = kb_min_free * 5 / 4; /* should be equal to sum of all 'low' fields in /proc/zoneinfo */
      mem_available = (signed long)kb_main_free - watermark_low
      + kb_inactive_file + kb_active_file - MIN((kb_inactive_file + kb_active_file) / 2, watermark_low)
      + kb_slab_reclaimable - MIN(kb_slab_reclaimable / 2, watermark_low);

The kb_min_free is value of /proc/sys/vm/min_free_kbytes. So what does it mean?

You'll get trouble if the kernel use out all memory. Kernel needs to reserve a small number of memory to keep the kernel running without problems. The /proc/sys/vm/min_free_kbytes is used to tune this value. You'd better not set this lower than 1024.

The kb_inactive_file comes from field Inactive(file) while the kb_active_file comes from Active(file).

swap fields

The swap total comes from SwapTotal while the swap free comes from SwapFree. Swap used is minus of the two.

From above we know free just extracts information from /proc/meminfo and gives us a simple and friendly output. You can go to /proc/meminfo to get detailed information about memory.

references