Glossary
VM compressor
macOS's in-RAM 'soft swap'. Compresses cold pages with WKdm/LZ4 to free physical frames before writing actual swap to disk.
The VM compressor is XNU's preferred response to memory pressure: instead of writing cold pages to a swap file on the SSD, it compresses them in RAM with WKdm (and LZ4 as a fallback), then frees the physical frame.
apple-oss-distributions/xnuosfmk/vm/vm_compressor.cThe compressor — algorithm choice and reclaim policy.View on GitHub(line —) apple-oss-distributions/xnuosfmk/vm/vm_compressor_swap_default.cThe fallback to actual swap files when the compressor is full.View on GitHub(line —)
Typical compression ratios are 2–4×, so the kernel reclaims half to three-quarters of the memory occupied by compressible pages without touching the disk. This is why SSD endurance on Macs is much better than the same workload would imply on a system that swaps eagerly.
Activity Monitor's Memory → Compressed column is the live size of the compressor pool. When that column climbs and Swap Used is still zero, the compressor is doing its job. When both climb, the compressor is full and the kernel has fallen back to disk swap.