Sum of cached memory and shared memory exceed total memory [duplicate]

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When I run free -h, I get:

             total       used       free     shared    buffers     cached
Mem:          251G       208G        43G       179G       144M       190G
-/+ buffers/cache:        17G       233G
Swap:         5.6G       5.6G       1.1M

How can shared memory(179G) + cached memory(190G) exceed total memory (251G) ? Can memory be count as both of cached memory and shared memory?

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marked as duplicate by Romeo Ninov, schily, Jesse_b, RalfFriedl, G-Man Nov 30 at 3:02

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    Welcome to U&L! Yes, apparently it can. See, for example, this explanation from Red Hat. See the line: “Cached: Memory in the pagecache (Diskcache and Shared Memory)”
    – fra-san
    Nov 29 at 12:47

up vote
2
down vote

favorite

This question already has an answer here:

  • `free`: output format

    2 answers

When I run free -h, I get:

             total       used       free     shared    buffers     cached
Mem:          251G       208G        43G       179G       144M       190G
-/+ buffers/cache:        17G       233G
Swap:         5.6G       5.6G       1.1M

How can shared memory(179G) + cached memory(190G) exceed total memory (251G) ? Can memory be count as both of cached memory and shared memory?

share|improve this question

marked as duplicate by Romeo Ninov, schily, Jesse_b, RalfFriedl, G-Man Nov 30 at 3:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • 1

    Welcome to U&L! Yes, apparently it can. See, for example, this explanation from Red Hat. See the line: “Cached: Memory in the pagecache (Diskcache and Shared Memory)”
    – fra-san
    Nov 29 at 12:47

up vote
2
down vote

favorite

up vote
2
down vote

favorite

This question already has an answer here:

  • `free`: output format

    2 answers

When I run free -h, I get:

             total       used       free     shared    buffers     cached
Mem:          251G       208G        43G       179G       144M       190G
-/+ buffers/cache:        17G       233G
Swap:         5.6G       5.6G       1.1M

How can shared memory(179G) + cached memory(190G) exceed total memory (251G) ? Can memory be count as both of cached memory and shared memory?

share|improve this question

This question already has an answer here:

  • `free`: output format

    2 answers

When I run free -h, I get:

             total       used       free     shared    buffers     cached
Mem:          251G       208G        43G       179G       144M       190G
-/+ buffers/cache:        17G       233G
Swap:         5.6G       5.6G       1.1M

How can shared memory(179G) + cached memory(190G) exceed total memory (251G) ? Can memory be count as both of cached memory and shared memory?

This question already has an answer here:

  • `free`: output format

    2 answers

linux memory

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asked Nov 29 at 12:28

Hansol Shin

162

162

marked as duplicate by Romeo Ninov, schily, Jesse_b, RalfFriedl, G-Man Nov 30 at 3:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

marked as duplicate by Romeo Ninov, schily, Jesse_b, RalfFriedl, G-Man Nov 30 at 3:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • 1

    Welcome to U&L! Yes, apparently it can. See, for example, this explanation from Red Hat. See the line: “Cached: Memory in the pagecache (Diskcache and Shared Memory)”
    – fra-san
    Nov 29 at 12:47

  • 1

    Welcome to U&L! Yes, apparently it can. See, for example, this explanation from Red Hat. See the line: “Cached: Memory in the pagecache (Diskcache and Shared Memory)”
    – fra-san
    Nov 29 at 12:47

1

1

Welcome to U&L! Yes, apparently it can. See, for example, this explanation from Red Hat. See the line: “Cached: Memory in the pagecache (Diskcache and Shared Memory)”
– fra-san
Nov 29 at 12:47

Welcome to U&L! Yes, apparently it can. See, for example, this explanation from Red Hat. See the line: “Cached: Memory in the pagecache (Diskcache and Shared Memory)”
– fra-san
Nov 29 at 12:47

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All shared memory is also counted as cached.

shared memory is implemented using tmpfs internally. tmpfs is implemented as a thin wrapper for the page cache, just without having any backing store (except that tmpfs is swappable).


man free does not explain this. At least on my system (provided by procps-ng, last updated 2016-06-03). Sorry. All man free tells you is that cache is taken from Cached in /proc/meminfo. If you read man proc (from man-pages, updated 2017-09-15), it has documentation for the fields in the meminfo file, but it still fails to inform you that Cached includes Shmem.

You can see this with an experiment. While you have free memory (not available), you can create a file in a tmpfs, for example:

dd bs=1M count=100 < /dev/zero > /dev/shm/test.tmp

The result of this is that both the shared and cached figures in free -m increase by 100.

If you only have available memory, and do not have enough free memory to test this properly, you can create free memory by dropping as much page cache as possible, by running echo 1 | sudo tee /proc/sys/vm/drop_caches. Of course dropping page cache can be very bad for performance. Do not do this on a real server :-).

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    down vote

    All shared memory is also counted as cached.

    shared memory is implemented using tmpfs internally. tmpfs is implemented as a thin wrapper for the page cache, just without having any backing store (except that tmpfs is swappable).


    man free does not explain this. At least on my system (provided by procps-ng, last updated 2016-06-03). Sorry. All man free tells you is that cache is taken from Cached in /proc/meminfo. If you read man proc (from man-pages, updated 2017-09-15), it has documentation for the fields in the meminfo file, but it still fails to inform you that Cached includes Shmem.

    You can see this with an experiment. While you have free memory (not available), you can create a file in a tmpfs, for example:

    dd bs=1M count=100 < /dev/zero > /dev/shm/test.tmp
    

    The result of this is that both the shared and cached figures in free -m increase by 100.

    If you only have available memory, and do not have enough free memory to test this properly, you can create free memory by dropping as much page cache as possible, by running echo 1 | sudo tee /proc/sys/vm/drop_caches. Of course dropping page cache can be very bad for performance. Do not do this on a real server :-).

    share|improve this answer

      up vote
      3
      down vote

      All shared memory is also counted as cached.

      shared memory is implemented using tmpfs internally. tmpfs is implemented as a thin wrapper for the page cache, just without having any backing store (except that tmpfs is swappable).


      man free does not explain this. At least on my system (provided by procps-ng, last updated 2016-06-03). Sorry. All man free tells you is that cache is taken from Cached in /proc/meminfo. If you read man proc (from man-pages, updated 2017-09-15), it has documentation for the fields in the meminfo file, but it still fails to inform you that Cached includes Shmem.

      You can see this with an experiment. While you have free memory (not available), you can create a file in a tmpfs, for example:

      dd bs=1M count=100 < /dev/zero > /dev/shm/test.tmp
      

      The result of this is that both the shared and cached figures in free -m increase by 100.

      If you only have available memory, and do not have enough free memory to test this properly, you can create free memory by dropping as much page cache as possible, by running echo 1 | sudo tee /proc/sys/vm/drop_caches. Of course dropping page cache can be very bad for performance. Do not do this on a real server :-).

      share|improve this answer

        up vote
        3
        down vote

        up vote
        3
        down vote

        All shared memory is also counted as cached.

        shared memory is implemented using tmpfs internally. tmpfs is implemented as a thin wrapper for the page cache, just without having any backing store (except that tmpfs is swappable).


        man free does not explain this. At least on my system (provided by procps-ng, last updated 2016-06-03). Sorry. All man free tells you is that cache is taken from Cached in /proc/meminfo. If you read man proc (from man-pages, updated 2017-09-15), it has documentation for the fields in the meminfo file, but it still fails to inform you that Cached includes Shmem.

        You can see this with an experiment. While you have free memory (not available), you can create a file in a tmpfs, for example:

        dd bs=1M count=100 < /dev/zero > /dev/shm/test.tmp
        

        The result of this is that both the shared and cached figures in free -m increase by 100.

        If you only have available memory, and do not have enough free memory to test this properly, you can create free memory by dropping as much page cache as possible, by running echo 1 | sudo tee /proc/sys/vm/drop_caches. Of course dropping page cache can be very bad for performance. Do not do this on a real server :-).

        share|improve this answer

        All shared memory is also counted as cached.

        shared memory is implemented using tmpfs internally. tmpfs is implemented as a thin wrapper for the page cache, just without having any backing store (except that tmpfs is swappable).


        man free does not explain this. At least on my system (provided by procps-ng, last updated 2016-06-03). Sorry. All man free tells you is that cache is taken from Cached in /proc/meminfo. If you read man proc (from man-pages, updated 2017-09-15), it has documentation for the fields in the meminfo file, but it still fails to inform you that Cached includes Shmem.

        You can see this with an experiment. While you have free memory (not available), you can create a file in a tmpfs, for example:

        dd bs=1M count=100 < /dev/zero > /dev/shm/test.tmp
        

        The result of this is that both the shared and cached figures in free -m increase by 100.

        If you only have available memory, and do not have enough free memory to test this properly, you can create free memory by dropping as much page cache as possible, by running echo 1 | sudo tee /proc/sys/vm/drop_caches. Of course dropping page cache can be very bad for performance. Do not do this on a real server :-).

        share|improve this answer

        share|improve this answer

        share|improve this answer

        edited Nov 29 at 16:11

        answered Nov 29 at 15:44

        sourcejedi

        22.3k43398

        22.3k43398

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