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I now understand a bit more RAM usage in linux OS and I knwo that I can use free -h to have correct knowledge about RAM utilization in my server.

but I also have SAR that is working on my server and I don't see the link between SAR outputs and free output.

this website write some relationship between these command outputs but it is not correct for my server. why ? https://www.catalyst2.com/knowledgebase/server-management/investigating-memory-usage-with-sar/

for instance here what I have in same time :

sar -f /var/log/sa/sa04 -r
Linux 3.10.0-1062.4.1.el7.x86_64 (XXXXX)      03/04/2024      _x86_64_        (32 CPU)

Average:    kbmemfree kbmemused  %memused kbbuffers  kbcached  kbcommit   %commit  kbactive   kbinact   kbdirty
229307171       394
10:15:01 AM  20288408 507419344     96.16     23956 425241152 139210760     25.57 259695284 230272148       148
10:20:01 AM  20105208 507602544     96.19     23956 425232140 139499180     25.62 259847160 230279820       348
10:25:01 AM  21172892 506534860     95.99     23956 425321436 138078624     25.36 258889072 230283168       200
10:30:01 AM  21162780 506544972     95.99     23956 425347776 138069932     25.36 258893704 230280008       216
10:35:01 AM  20702812 507004940     96.08     23956 425471328 139023364     25.53 259269580 230360248      7032
10:40:02 AM  20318412 507389340     96.15     23956 425443164 141154772     25.92 259582372 230358724      1192
10:45:02 AM  19954804 507752948     96.22     23956 425795580 141170648     25.93 259644240 230674360       196
10:50:01 AM  20096560 507611192     96.19     23956 425794692 140834740     25.87 259517620 230666828       204


free -h
              total        used        free      shared  buff/cache   available
Mem:           503G         75G         19G        1.6G        408G        425G
Swap:           15G        666M         15G

it seems that buff/cache value 408G is not equal to kbchached+kbbuffers... kbcached value seems to be equal to "available" output of free.

it is strange. is there a problem in SAR configuration ?

please note that these command are launched in same time and sar refresh is small enough so that RAM usage don't change a lot during the interval.

iostrym
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1 Answers1

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Both sar and free get their information from /proc/meminfo.

man free describes exactly which values from /proc/meminfo correspond to its output.

  • buffers value is Buffers in /proc/meminfo
  • cache value is Cached and SReclaimable in /proc/meminfo
  • buff/cache Sum of buffers and cache

man sar doesn't have such a description but from the source code of rd_stats.c it seems that:

  • kbcached equals Cached
  • kbbuffers equals Buffers.

The difference between sar and free output is in SReclaimable which free counts but sar does not.

AlexD
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