Compressed memory

A large Compressed figure means macOS has had to make room. This is often why memory pressure turns yellow. It is the step before swapping to disk, so it is a sign that apps are asking for more RAM than is free.

In the Memory tab, choose View, Columns, and add the compressed memory column. Sort by it to see which processes have the most compressed data. Those are often apps that have been open a long time without being used.

Activity Monitor describes Compressed as physical memory used to store a compressed version of data that has not been used recently. So the figure is the space the compressed data takes up in RAM, not the size the data would be once decompressed.

The data inside is larger than the Compressed figure. That difference is the memory macOS has gained by compressing it.

Apple explains that as the Mac approaches its maximum memory capacity, inactive apps in memory are compressed, which makes more memory available to the apps you are using. That is why apps left in the background for a long time tend to hold the most compressed memory, and why switching back to one of them can take a moment while its data is decompressed.

macOS manages a memory shortage in stages. Each stage shows up in a different place in Activity Monitor.

1. Data that has not been used recently is compressed and kept in RAM. The Compressed figure grows.
2. When compressing is not enough, macOS moves compressed data out to the startup disk. Swap Used grows.
3. When an app needs that data again, macOS reads it back from disk and decompresses it. If this happens a lot, the Mac feels slow and memory pressure turns yellow or red.

Two read-only commands show the same information Activity Monitor uses. Comparing the stored and occupied figures tells you how well your data is compressing. The bigger the gap between them, the more memory compression is saving.

1. Run top -l 1 | grep PhysMem. The compressor figure in parentheses is the memory holding compressed data.
2. Run vm_stat. Pages occupied by compressor is the RAM the compressed data takes up. Pages stored in compressor is how large that data would be uncompressed. Multiply either by the page size on the first line to get bytes.
3. Run vm_stat 5 to print a new line every 5 seconds, and press Control-C to stop. After the first line, which shows totals since startup, the comprs and dcomprs columns show how many pages were compressed and decompressed in each interval.

Normal: Compressed stays at a steady level while memory pressure is green. macOS is simply keeping old data in a smaller form. It is common to see some compressed memory even on a Mac with plenty of RAM, especially after it has been on for a while.

Worrying: Compressed keeps rising through the day, Swap Used rises with it, and memory pressure is often yellow or red. The Mac is short on RAM for what you are running, so quit apps you are not using or run fewer heavy apps at the same time.