Say we have something like

Process Name   Memory    Compressed Mem
Firefox        2.60 GB   1 GB

What do the figures mean?

Is it:

  1. 1GB is compressed and we don't know how much it's compressed by so the app is using somewhere between 1.6 and 2.6 GB but we don't know how much
  2. 1GB is the amount of memory saved by compression - ie the app is using 2.6 - 1 = 1.6 GB of actual memory
  3. 1GB is the actual memory being used - ie after taking into account the compression of parts of the 2.6GB, we've been able to save 1.6GB and the process looks like it's only using 1GB
  4. Something else?

I intuitively thought it was (2) but a friend thought it might be (3). Now I'm thinking it's actually (1)!


Through various tests on my machine, I have discovered that: The app is using 2.6 GB in addition to the 1GB of compressed memory.

On the Safari Webpage Content processes, if you max out the memory used on your machine, you will see the memory go down as compressed go up. This is how I tested it, and I assume it will work for other apps, but safari usually uses the most memory for me.

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  • Hmmm, interesting but this can't be right - Adding up just my memory column alone is around 10GB. Adding up my compressed mem column is about 6GB. Adding those together would be 16GB. My physical memory is only 8GB and it's showing 7GB as used. (It's also showing 5GB of swap used...) – nevster Aug 25 '16 at 5:15
  • @nevster it adds up. Swap is hard drive space used for memory when you don't have enough RAM. – At0mic Aug 25 '16 at 21:22
  • How are you calculating that? If we add 7GB used + 5GB swap = 12 GB, not 16GB. What am I doing wrong? – nevster Aug 26 '16 at 0:21
  • @nevster I don't know what to tell you. Activity Monitor may not be completely accurate, and I doubt you actually took the time to add up 500 processes. – At0mic Aug 26 '16 at 13:54
  • It's not that hard to add up a page worth of one column - the rest is negligible - and if anything, it would cause me to under-estimate. – nevster Aug 27 '16 at 1:37

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