Memory Pressure - explain like I'm five?

Discussion in 'OS X Mavericks (10.9)' started by Lily7, Jan 20, 2014.

  1. Lily7 macrumors newbie

    Dec 29, 2013
    Pretty much what the title says. All I understand is: green = good, yellow = pushing it, red = bad.

    Here's what I'm unclear on: when it's measuring memory pressure, does it measure the actual "pressure" vertically? And the horizontal is representative of time? So the higher it gets to the top, the worse the memory pressure is or was in that moment of time as it ticks by horizontally?

    I'm not having memory pressure issues as it's always green, but I don't really know what it would look like if there were problems, so I'm not exactly sure how to "read it". I'd like to know how to read it in case I have issues in the future.

    So, if anyone could explain this like I'm five years old, I'd appreciate it. ;)
  2. simsaladimbamba

    Nov 28, 2010
  3. Lily7 thread starter macrumors newbie

    Dec 29, 2013
    That does help, thank you. Apparently I had the right idea.

    Is there a way to know what apps were causing the problem when it goes yellow or red? For example if I opened my activity monitor and saw what is shown in their screenshot - it would mean that most recently, memory pressure is fine, but it wasn't before then. How would I know which apps caused it to go yellow or red after the fact?

    I'd also still like to understand "Swap used" and everything on the right side.
  4. w0lf macrumors 65816


    Feb 16, 2013
    Just like normal memory you can run out of space for compressed memory. Pressure gets too high and the system wont be able to compress any more and is forced to use the disk. That's pretty much it.

    Yellow and red don't actually mean bad. It's more like a measure of being full.

    Green empty, Yellow filling up, Red nearing capacity/at capacity.

    That being said if you're frequently hitting red/swapping you probably don't have enough RAM.
  5. ElectricSheep macrumors 6502


    Feb 18, 2004
    Wilmington, DE
    Starting form the left side:

    Physical Memory - Self explanatory: This is how much physical memory you have installed.
    Memory Used - This is the portion of physical memory that mapped to a virtual memory object. A virtual memory object could be memory allocated by applications or memory-mapped files.
    Virtual Memory - This is the sum of Physical Memory and Swap. Activity Monitor used to present this as the size of the Virtual Address Space, but that statistic really isn't useful for typical users.
    Swap Used - This is the amount of space occupied in the swap file on disk. Note that the size of the swap file and this number need not agree; swap files are typically allocated in growing powers of two with portions of the file being 'filled' with swapped out data

    Right side:

    App Memory - This is memory which was directly allocated by an application; buffers, structures, etc. This is contrasted with the File Cache.
    File Cache - This is memory which has been mapped to files on disk. Contrasted from App Memory. Note that even if an application un-maps a file, the file remains resident in memory. This takes advantage of temporal locality: The same data is typically accessed at times which are closer together rather than far apart. Keeping the file mapped in memory makes re-accessing it much faster.
    Wired Memory - This is memory which has been 'wired-down' by the kernel and cannot be compressed or swapped out to disk. Applications themselves are discouraged from directly wiring memory, but their activities and calls to the kernel can cause additional memory to be wired-down as a side effect.
    Compressed - This is the amount of memory residing in the new Page Compressor. Mavericks treats App Memory preferentially. As demand for memory increases, it will discard memory-mapped files from the File Cache before filling the compressor with App Memory. Since some mapped-files may currently be in use, the File Cache will not be completely drained before compression takes place. As demand further increases, compressed memory is written to swap in its compressed form.

    You can watch all of this happen in real-time by running the command:
    sudo memory_pressure -l critical
    in a Terminal. This will allocate memory to simulate a critical memory pressure situation. Watch the numbers and the memory pressure graph to see how the system responds to high memory demands. Hit ctrl-C in the terminal to stop the simulation and relieve the pressure.
  6. Lily7 thread starter macrumors newbie

    Dec 29, 2013
    Thank you both! Both of those were helpful, I am still having trouble grasping it completely but I definitely understand a lot more than I did before and that helps. I guess I'll just have to hope I can figure it out completely at the time if I ever have any issues.

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