FAQ
Hi,


I have two 64 bit cent Os machine. I googled that in linux always show
entire memory as used. But below showing only 1 GB detected.




*Machine 1 :*
i) It has 31 GB of ram.


[root at machine1]# free -g
total used free shared buffers
cached
Mem: 31 1 30 0 0 0
-/+ buffers/cache: 0 30
Swap: 49 0 49
------------------------------------------


*Machine 2 :*
i)It has 11 GB of ram.


[root at machine2]# free -g
total used free shared buffers
cached
Mem: 11 11 0 0 0 9
-/+ buffers/cache: 1 10
Swap: 49 0 49




Could you please anyone can explain why ?




Thanks,
Ashik
-*
*

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  • Ashish Yadav at Dec 18, 2012 at 6:26 am
    free displays the total amount of free and used physical and swap memory in
    the system. In your case "free -g" it showing you how much memory you are
    using right now in gigabits.




    On Tue, Dec 18, 2012 at 11:52 AM, AshikAli.m wrote:

    Hi,

    I have two 64 bit cent Os machine. I googled that in linux always show
    entire memory as used. But below showing only 1 GB detected.


    *Machine 1 :*
    i) It has 31 GB of ram.

    [root at machine1]# free -g
    total used free shared buffers
    cached
    Mem: 31 1 30 0 0 0
    -/+ buffers/cache: 0 30
    Swap: 49 0 49
    ------------------------------------------

    *Machine 2 :*
    i)It has 11 GB of ram.

    [root at machine2]# free -g
    total used free shared buffers
    cached
    Mem: 11 11 0 0 0 9
    -/+ buffers/cache: 1 10
    Swap: 49 0 49


    Could you please anyone can explain why ?


    Thanks,
    Ashik
    -*
    *
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  • John R Pierce at Dec 18, 2012 at 6:35 am

    On 12/17/2012 10:22 PM, AshikAli.m wrote:
    I have two 64 bit cent Os machine. I googled that in linux always show
    entire memory as used. But below showing only 1 GB detected.

    no, that first one shows you have 31gb ram, about 1gb is being used by
    software, and the other 30GB are in use as buffers/cache


    the second shows 11gb ram, 1gb used, and 10gb as buffers/cache


    here's a machine fo mine thats been running for a couple months with
    large databases (both postgres and oracle)


    # free -g
    total used free shared buffers cached
    Mem: 47 43 3 0 0 38
    -/+ buffers/cache: 5 42
    Swap: 49 0 49


    out of 47gb total, 5gb is actually used, 38gb is in use as disk cache,
    and 3gb is completely unused currently, so 42gb is available for use by
    software.


    the main line to look at is the -/+ buffers/cache. THAT is the real
    usage with discardable buffers and cache subtracted and the 'free' value
    on that line is the buffers + cache + freespace
  • AshikAli.m at Dec 18, 2012 at 7:19 am
    Thank you very much for your great explanation.


    I little bit doubt that why +/- buffers/cache is not tallying for machine1.
    I understood after displaying it as mb.Here is output in mb which is not
    tallied.


    [root at machine1 ~]# free -m
    total used free shared buffers cached
    Mem: 32183 1309 30873 0 290 485
    -/+ buffers/cache: 533 31649
    Swap: 51199 0 51199


    Thanks again,
    Ashik






    On Tue, Dec 18, 2012 at 9:35 AM, John R Pierce wrote:

    On 12/17/2012 10:22 PM, AshikAli.m wrote:
    I have two 64 bit cent Os machine. I googled that in linux always show
    entire memory as used. But below showing only 1 GB detected.
    no, that first one shows you have 31gb ram, about 1gb is being used by
    software, and the other 30GB are in use as buffers/cache

    the second shows 11gb ram, 1gb used, and 10gb as buffers/cache

    here's a machine fo mine thats been running for a couple months with
    large databases (both postgres and oracle)

    # free -g
    total used free shared buffers cached
    Mem: 47 43 3 0 0 38
    -/+ buffers/cache: 5 42
    Swap: 49 0 49

    out of 47gb total, 5gb is actually used, 38gb is in use as disk cache,
    and 3gb is completely unused currently, so 42gb is available for use by
    software.

    the main line to look at is the -/+ buffers/cache. THAT is the real
    usage with discardable buffers and cache subtracted and the 'free' value
    on that line is the buffers + cache + freespace


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  • John R Pierce at Dec 18, 2012 at 7:44 am

    On 12/17/2012 11:19 PM, AshikAli.m wrote:
    [root at machine1 ~]# free -m
    total used free shared buffers cached
    Mem: 32183 1309 30873 0 290 485
    -/+ buffers/cache: 533 31649
    Swap: 51199 0 51199

    so you have about 32gb total physical memory. 1.3gb of that is 'used'
    but of that 290MB is buffers and 485MB is cache, so really only 533MB
    (the 2nd line) is in use by software, and 31649MB is available (which is
    the 30873 free on the 1st line, plus the 290 buffers plus the 485
    cached, as all that is considered 'discardable' [*].


    you also have 51199MB of swapfile, entirely unused.


    this looks to me like a system that was just booted, and hasn't done much.




    [*] under heavy loads, like a buay large scale database server, this sum
    isn't exactly true, some of the buffer memory is 'dirty' so not
    considered as free without first flushing it to disk...
  • AshikAli.m at Dec 18, 2012 at 8:57 am
    yes. yes exactly the calculation is correct. In that machine only DRBD and
    HA application running.


    I tell the story,


    I have a telephony server with HA and DRBD mirroring(2 machines) .
    Unfortunately I installed 32 bit OS on those. I come to know that after
    upgrading it to 64 system will detect all ram memory which are inserted.


    In this HA, important process runs on which is production node. For
    migration purpose; currently I made machine2 as production node which has
    12G ram.


    [root at machine2~]# free -m
    total used free shared buffers cached
    Mem: 12007 11961 46 0 579 9956
    -/+ buffers/cache: 1424 10582
    Swap: 51199 0 51199


    Here,


    Used memory ( 12007 - 46 ) = 11961


    In that used memory 11961 - ( 579 + 9956 ) = 1429( Actually in use )


    so,
    46(free)+579(buffer)+9956(cached) = 10581(fraction) is free.




    So, Always look into -/+ buffers/cache. That is the exactly correct.


    Thanks,
    Ashik.
















    On Tue, Dec 18, 2012 at 10:44 AM, John R Pierce wrote:

    On 12/17/2012 11:19 PM, AshikAli.m wrote:
    [root at machine1 ~]# free -m
    total used free shared buffers cached
    Mem: 32183 1309 30873 0 290 485
    -/+ buffers/cache: 533 31649
    Swap: 51199 0 51199
    so you have about 32gb total physical memory. 1.3gb of that is 'used'
    but of that 290MB is buffers and 485MB is cache, so really only 533MB
    (the 2nd line) is in use by software, and 31649MB is available (which is
    the 30873 free on the 1st line, plus the 290 buffers plus the 485
    cached, as all that is considered 'discardable' [*].

    you also have 51199MB of swapfile, entirely unused.

    this looks to me like a system that was just booted, and hasn't done much.


    [*] under heavy loads, like a buay large scale database server, this sum
    isn't exactly true, some of the buffer memory is 'dirty' so not
    considered as free without first flushing it to disk...

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    CentOS at centos.org
    http://lists.centos.org/mailman/listinfo/centos





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