Understanding Compressed Memory on Your Mac

Summary: This post explains compressed memory, a type of memory on your Mac, including what it is, how it works, how to check memory usage on Mac, etc.

You may have heard of different types of memory on macOS such as RAM, virtual memory, wired memory, purgeable memory, and compressed memory. In this post, we will dive into the compressed memory on Mac to help you understand it.

What is compressed memory?

Compressed memory is a technology used by macOS to optimize the use of your system's RAM (Random Access Memory). When your Mac's RAM starts to fill up, instead of offloading inactive data to the slower disk (paging/swap memory), macOS compresses this data to make it smaller and keeps it in RAM. This approach helps maintain the system's performance and responsiveness.

How does compressed memory work?

macOS continuously monitors RAM usage. When the system detects that RAM is getting full, it looks for data that is inactive or used less frequently. The system then compresses this data using efficient algorithms, significantly reducing its size. The compressed data is stored back in RAM, freeing up space for active processes and applications.

Even if some inactive data is compressed and stored on RAM, the OS will also move inactive data to the hard drive when the available RAM is insufficient for the newly launched apps or memory-intensive apps. That is to say, what the compressed memory can achieve is limited.

When an app requests access to data that is stored in a compressed region of RAM, the operating system automatically decompresses the data and makes it accessible. There shouldn't be any performance degradation during the compression or decompression process because the processes operate simultaneously on one of the CPU cores.

How to check compressed memory?

Apple provides a tool to monitor how the Mac uses RAM, that is Activity Monitor, you can check the compressed memory via the Memory tab. Here's how:

Open Activity Monitor from the Finder > Applications > Utilities.
Select the Memory tab from the toolbar.
From the chart at the bottom, find Memory Used. It shows the amount of RAM being used.
Then to the right, you can see where the used memory is allocated,
From there, you can find Compressed Memory and the amount.

Lots of compressed memory on Mac? Fixed!

Some Mac users find the compressed memory taking a large amount of the RAM. It indicates that there is a lot of data that is inactive or used less frequently or you are working with some memory-intensive apps/programs causing the OS to compress unused data to release RAM.

However, there is no way to check which apps' inactive data is compressed, to fix the high compressed memory on Mac, you can try to close some unused apps running in the background, quit memory-intensive apps/programs, or simply restart the Mac to release the RAM.

Help more people understand the compressed memory on Mac!