How is kbmemused calculated in sar?

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux 10
  • Red Hat Enterprise Linux 9
  • Red Hat Enterprise Linux 8
  • Red Hat Enterprise Linux 7
  • sysstat-12.5.4-5.el9

Issue

  • Explanation of kbmemused in man sar has changed in RHEL9.
  • How is the value calculated? What is the difference compared with the older version?
RHEL8 and older version
# man sar
...
              kbmemused
                     Amount of used memory in kilobytes. This does not take into account memory used by  the  kernel
                     itself.

RHEL9 and RHEL10
# man sar
...
              kbmemused
                     Amount of used memory in kilobytes (calculated as total installed  memory  -  kbmemfree  -  kb‐
                     buffers - kbcached - kbslab).

Resolution

kbmemused is calculated in the following method:

RHEL9 and RHEL10 (11.7.4 and newer version)

kbmemused = total installed memory - kbmemfree - kbbuffer - kbcached - kbslab

RHEL8 and RHEL7 (older version than 11.7.4)

kbmemused = total installed memory - kbmemfree

Root Cause

  • As per the change log, the calculation method of kbmemused has changed since the upstream version 11.7.4.
2018/06/01: Version 11.7.4 - Sebastien Godard (sysstat <at> orange.fr)
...
        * sar: Change how used memory (kbmemused) is calculated to be
          consistent with free(1) and top(1)'s output [11.6.4] [11.4.10].
...
  • Here is a source code of sysstat-12.5.4-5.el9.
rndr_stats.c
...
 767                 nousedmem = smc->frmkb + smc->bufkb + smc->camkb + smc->slabkb;
 768                 if (nousedmem > smc->tlmkb) {
 769                         nousedmem = smc->tlmkb;
 770                 }
...
 780                 render(isdb, pre, PT_USEINT,
 781                        "-\tkbmemused", NULL, NULL,
 782                        smc->tlmkb - nousedmem, DNOVAL, NULL);
  • As the following source code shows, kbmemused is calculated in total installed memory - kbmemfree.
sysstat-11.7.3-9.el8
rndr_stats.c
...
 753                 render(isdb, pre, PT_USEINT,
 754                        "-\tkbmemfree", NULL, NULL,
 755                        smc->frmkb, DNOVAL, NULL);
...
 761                 render(isdb, pre, PT_USEINT,
 762                        "-\tkbmemused", NULL, NULL,
 763                        smc->tlmkb - smc->frmkb, DNOVAL, NULL);   <<<---

pidstat.c
  69 unsigned long tlmkb;            /* Total memory in kB */

Diagnostic Steps

  • Check the sysstat version and man sar.

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