Re: computing drop-able caches

Daniel Walker <danielwa@xxxxxxxxx> · Thu, 28 Jan 2016 17:29:41 -0800



    On 01/28/2016 05:03 PM, Daniel Walker
      wrote:

    
    On
      01/28/2016 03:58 PM, Johannes Weiner wrote:
      

      On Thu, Jan 28, 2016 at 03:42:53PM -0800,
        Daniel Walker wrote:
        

        "Currently there is no way to figure out
          the droppable pagecache size
          

          from the meminfo output. The MemFree size can shrink during
          normal
          

          system operation, when some of the memory pages get cached and
          is
          

          reflected in "Cached" field. Similarly for file operations
          some of
          

          the buffer memory gets cached and it is reflected in "Buffers"
          field.
          

          The kernel automatically reclaims all this cached &
          buffered memory,
          

          when it is needed elsewhere on the system. The only way to
          manually
          

          reclaim this memory is by writing 1 to
          /proc/sys/vm/drop_caches. "
          

        [...]
        

        The point of the whole exercise is to
          get a better idea of free memory for
          

          our employer. Does it make sense to do this for computing free
          memory?
          

        /proc/meminfo::MemAvailable was added for this purpose. See the
        doc
        

        text in Documentation/filesystem/proc.txt.
        

        It's an approximation, however, because this question is not
        easy to
        

        answer. Pages might be in various states and uses that can make
        them
        

        unreclaimable.
        

      Khalid was telling me that our internal sources rejected
      MemAvailable because it was not accurate enough. It says in the
      description,
      

      "The estimate takes into account that the system needs some page
      cache to function well". I suspect that's part of the inaccuracy.
      I asked Khalid to respond with more details on this.
      

    Some quotes,

    
    "
          [regarding
            MemAvaiable]
          

          This
            new metric purportedly helps usrespace assess available
            memory. But,
          its
            again based on heuristic, it takes 1/2 of page cache as
            reclaimable..
          

          Somewhat
            arbitrary choice. Maybe appropriate for desktops, where page
          cache
            is mainly used as page cache, not as a first class store
            which is
          the
            case on embedded systems. Our systems are swap less, they
            have little
          secondary
            storage, they use in-memory databases/filesystems/shared
            memories/
          etc.
            which are all setup on page caches).. This metric as it is
            implemented
          in 3.14
            leads to a totally mis-leading picture of available memory"
        

    Daniel