macOS vm pressure level thresholds kVMPressureWarning and kVMPressureCritical

Facts

• Definitions: `AVAILABLE_NON_COMPRESSED_MEMORY` = active + inactive + free + speculative pages (N); `VM_PAGE_COMPRESSOR_COUNT` = resident pages of the compressor object (C, "Pages occupied by compressor" in vm_stat); `AVAILABLE_MEMORY` = N + C. [source]
• Normal to Warning (compressor active): N < VM_PAGE_COMPRESSOR_COMPACT_THRESHOLD = AVAILABLE_MEMORY x 10 / minorcompact_divisor. [source]
• On macOS with more than 3 GB of RAM the divisors are minorcompact 20, majorcompact 25, unthrottle 35, catchup 50; with 3 GB or less they are 11, 13, 20, 35. [source]
• So for a Mac above 3 GB, Normal to Warning fires when N < (N + C) x 10/20, that is N < C. Derived. [source]
• Warning to Critical: `vm_compressor_low_on_space()` OR N < 1.2 x AVAILABLE_MEMORY x 10 / 35. That is about N < 0.52 C. Derived. [source]
• Hysteresis, down: Warning to Normal needs N > 1.2 x the warning threshold, which is N > 0.6 (N + C), about N > 1.5 C. Critical to Warning needs N > 1.4 x the unthrottle threshold, about N > 0.67 C. [source]
• If the compressor is not active the thresholds are page-count based: warning below `memorystatus_get_soft_memlimit_page_shortage_threshold`, critical below the critical page shortage threshold, and recovery needs 115 percent of the threshold. [source]
• The non-compressor percentages (iOS style) are listed in kern_memorystatus.c by DRAM size: 6 GB and up is critical 4 percent, idle 8, pressure 12, reaper 16, freeze 50; 0 to 3 GB is 5, 10, 15, 20, 50. On a compressor-active Mac these are not what drives the level. [source]
• `vm_pressure_response` also sets `memorystatus_level` = available x 100 / total pages (the sysctl `kern.memorystatus_level`), but the level transitions above do not use that percentage. [source]
• The level re-broadcasts after sitting in a non-normal level for 30 minutes (`vm_pressure_level_transition_threshold`). [source]
• Measured: N was 1,910,295 pages and C was 746,105 pages (N/C about 2.56) with `kern.memorystatus_vm_pressure_level` = 1 (Normal) and `kern.memorystatus_level` = 46 at the same moment, consistent with the N versus C rule. [asserted] (measured locally 2026-10-04, Apple M5 Max 64 GB, macOS 27.2; vm_stat, sysctl) [source]
• Not documented in sources read. [source]
• Wired memory (1,477,245 pages on the measured host) is excluded from N, so a large wired GPU allocation shrinks N without growing C, and the Warning line N < C is reached when active + inactive + free fall below the compressor's own footprint. [source]
• `kern.memorystatus_level` can read 46 percent while pressure is Normal and a "low" percentage does not imply Warning. [source]
• The Warning line moves as C grows: heavy compression raises C and so raises the N needed to stay Normal. [source]
• Common folklore ties the Activity Monitor memory-pressure colors to a free-percentage (for example 10 percent / 5 percent). The source read shows a compressor-relative rule on macOS instead. No Apple document confirming the folklore percentages was found. [source]
• The numeric enum values of kVMPressureUrgent and kVMPressureCritical were not found in the files read. [source]
• Whether VM_CONFIG_COMPRESSOR_IS_ACTIVE is true on every Apple silicon Mac build; macOS defaults to compressor-with-swap, so it should be. [source]
• The long-run behavior of Activity Monitor's graph versus this level. [source]
• On Macs above 3 GB the Warning threshold is N < C (non-compressed available pages below the compressor's resident pages). [source]
• Critical is low-on-space or N below about half of C. [source]
• Hysteresis uses 120 percent (warning to normal) and 140 percent (critical to warning) multipliers. [source]
• The compressor divisors differ for 3 GB or less versus larger Macs. [source]
• A stuck non-normal level re-notifies every 30 minutes. [source]