What Exactly Is “Compressed Memory”?
In Mavericks, the OS has one more option before it has to resort to swapping: compressed memory. Mavericks will find the least-recently-used data in memory and compress it, usually to about half its original size. Et voilà, more free memory.
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Memory compression is a triple play for Mavericks. It’s a performance win; compressing and decompressing data in RAM is much faster than reading from and writing to disk, even an SSD. It’s an energy win; the less time spent moving data between RAM and disk, the more time the system can spend in its idle state. And finally, it’s a capability win; Mavericks can handle much more demanding workloads than previous versions of OS X before crying uncle.
This seems like a great feature, but I’ve never fully understood how it’s reported in Activity Monitor. What do the “Compressed Memory” numbers for each process and the “Compressed” total actually mean? This AskDifferent post lists the columns in Activity Monitor, but commenter James K Polk has the same questions as me:
If the activity monitor says a given process uses 621.4 MB of memory and 615.4 MB of compressed memory, does that mean that the process is really only using up 6 MB of memory? Or that 615.4 out of 621.4 MB was compressed down to some unspecified size? Or something else?
In other words, does a high number for Compressed mean that the compression has been effective or that the system is nearing capacity because most of what can be compressed already is?
It’s also not clear how compressed memory interacts with the other reported numbers. For example, why did Siracusa’s App Memory go down when the Compressed memory went up? I would have expected that App Memory would include the part that’s compressed.
And, presumably the memory that is paged out to disk is compressed. Does Swap Used take that into account?
Update (2016-10-03): Activity Monitor in macOS 10.12 has a tooltip for the “Compressed Memory” column, which says “Memory sent to the VM compression.”
