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MemAvailable metric for Linux kernels before 3.14 in /proc/meminfo

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A great new metric has been introduced in “/proc/meminfo” in the Linux 3.14 kernel — MemAvailable:

An estimate of how much memory is available for starting new applications, without swapping. Calculated from MemFree, SReclaimable, the size of the file LRU lists, and the low watermarks in each zone.

The estimate takes into account that the system needs some page cache to function well, and that not all reclaimable slab will be reclaimable, due to items being in use. The impact of those factors will vary from system to system.

I recommend that you read the kernel commit description for further details.

Since many people are still using Linux kernels before 3.14, I’ve backported this kernel patch to Perl. You can download the sources from GitHub: https://github.com/famzah/linux-memavailable-procfs

Many system administrators rely on the “free” tool to get a quick overview of the system’s memory usage. Unfortunately, the latest “procps” package still doesn’t interpret the “MemAvailable” metric, and even if it did, we don’t have it in Linux kernels before 3.14. Actually, the developers of the “procps-ng” package (which is the Debian, Fedora and openSUSE fork of “procps“) have reacted and did the same thing as me. For kernels before 3.14 they emulate the metric in the same way, and for kernels after 3.14, they display the native metric from “/proc/meminfo”. This makes my Perl port more or less redundant.

This is the reason I wrote a quick replacement of the “free” tool in Perl. A few examples of it follow.

Typical memory usage overview, in MBytes:

famzah@vbox:~$ ./free.pl -m
             total       used       free  anonymous     kernel     caches     others
Mem:          2488       1228       1259        608         24        580         15
  -/+ caches              648       1840
Swap:         1565          0       1565

Typical memory usage overview, in percentage:

famzah@vbox:~$ ./free.pl -mp
             total       used       free  anonymous     kernel     caches     others
Mem:          2488        49%        51%        24%         1%        23%         1%
  -/+ caches              26%        74%
Swap:         1565         0%       100%

Extended memory usage overview, in MBytes:

famzah@vbox:~$ ./free.pl -me
             total       used       free  anonymous     kernel     caches     others
Mem:          2488       1228       1260        608         24        580         14
  -/+ caches              647       1840
Swap:         1565          0       1565

Extended memory usage info:
  Buffers                  83
  Cached                  785
  SwapCached                0
  Shmem                   308
  AnonPages               300
  Mapped                  122
  Unevict+Mlocked           5
  Dirty+Writeback           0
  NFS+Bounce                0

Extended memory usage overview, in percentage:

famzah@vbox:~$ ./free.pl -mep
             total       used       free  anonymous     kernel     caches     others
Mem:          2488        49%        51%        24%         1%        23%         1%
  -/+ caches              26%        74%
Swap:         1565         0%       100%

Extended memory usage info:
  Buffers                  3%
  Cached                  32%
  SwapCached               0%
  Shmem                   12%
  AnonPages               12%
  Mapped                   5%
  Unevict+Mlocked          0%
  Dirty+Writeback          0%
  NFS+Bounce               0%

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Author: Ivan Zahariev

An experienced Linux & IT enthusiast, Engineer by heart, Systems architect & developer.

2 thoughts on “MemAvailable metric for Linux kernels before 3.14 in /proc/meminfo

  1. Hi
    i have a query regarding “Available Memory” i.e MemAvailable value in /proc/meminfo which you compute in a perl backport of the same in linux kernels > 3.14.
    In Kernel C source as per the git link you mention when caclulating it they do finally for Slab reclaimable portion as follows :
    /*
    + * Part of the reclaimable swap consists of items that are in use,
    + * and cannot be freed. Cap this estimate at the low watermark.
    + */
    + available += global_page_state(NR_SLAB_RECLAIMABLE) –
    + min(global_page_state(NR_SLAB_RECLAIMABLE) / 2, wmark_low);

    But you seem to be missing the “/2” in your perl backport module for same “Available Memory” calculation as below :

    $available += $slab_reclaimable – min($slab_reclaimable, $wmark_low);

    Am i missing anything here ?

    thanks in adv
    Kishore

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