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Last weekend, I upgraded my machine to have 32 GB of RAM. I knew I didn't really need this much memory, but I figured macOS would use it for file caching or some such.

Memory Usage according to Activity Monitor

According to Activity Monitor, around 13 GB of memory are being used by applications, around 6 GB are being used for caching, and a whopping 13 GB are just being left idle. These numbers vary a bit depending on time and current workload, but in general, macOS consistently fails to use more than around 20 GB of memory.

Are there any steps I can take to make macOS utilize my additional memory? For example, it would be great if I could make macOS cache files more aggressively, to offset my computer's relatively slow SSD.

  • I wonder how long the system was up when the image was taken. The system starts with caching nothing and you need to let it run and read in files, they only cache once they're read off disk. The system doesn't pre-fill the cache and the fact that it's not used and slow shows your constraint wasn't RAM. – bmike Nov 1 '18 at 21:59
  • @bmike System had been up for two days at time of screenshot, but stats seem to stay relatively consistent regardless. Shame the cache isn't pre-filled, I wish I could change that... – Wowfunhappy Nov 2 '18 at 0:40
  • To make MacOS cache more aggressively you need to read in more from the disk than you have. – Thorbjørn Ravn Andersen Nov 2 '18 at 7:52
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macOS will by default aggressively cache files, etc. in memory. In your case, it seems your programs simply aren't loading in that much data - and then there's really nothing more to cache.

If you keep the system running longer and look at the values again, you'll most probably see that the numbers have changed so that less memory is left idle.

You can selectively use more memory by adding a RAM disk to the system. I.e. a virtual "disk" that is really in RAM. You could then copy files to that disk to ensure that they stay in RAM. It is hugely impractical for ordinary desktop usage.

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