===== Some VM subsystem important knobs =====

There are at least two hysteresis-like cycles in the VM code.

One regulates when the page daemon performs paging out.

The other regulates when page allocation succeeds.

Important levels of available pages (//v_free_count// + //v_cache_count//), from higher to lower.

Prior to [[http://svnweb.freebsd.org/changeset/base/254304]]:

| Threshold | Sample values from untuned amd64 system with 8GB RAM | Description and useful information | Page daemon active | Normal requests are satisfied | Normal waiters waken up |
| **v_cache_min + v_free_target** | 104572 = 52286 + 52286 | a level that a page scan pass tries to reach; at this level and above **vm_paging_target()** == 0, below it is 1. |  | X | X |
| **v_cache_min + v_free_reserved** | 54904 = 52286 + 2618 | a level below which //vm_page_alloc()// still succeeds for all request classes, but sets //vm_pages_needed// flag and triggers a page scan; **vm_paging_needed()** == 1 below this level. | X | X | X |
| **v_free_min** | 12417 | at this level threads waiting for pages in //VM_WAIT// are waken up after page shortage (and //vm_pages_needed// flag is reset) in //vm_page_free_wakeup()//; **vm_page_count_min()** == 1 below this level. //vm_page_free_wakeup()// is called by //vm_page_free_toq()// and //vm_page_cache()//, that is, upon increasing available pages count. | X | X | X |
| **v_free_severe** | 7517 | below this level some users of //vm_page_alloc()// decide to not call it and instead voluntary wait until more pages are available. Important cases of this check are: //vm_fault()// and //vm_forkproc()//.  **vm_page_count_severe()** == 1 below this level. | X | X |  |
| **v_free_reserved** | 2618 | at this level //vm_page_alloc()// would return //NULL// for normal requests; threads needing (//M_WAITOK//) memory would call //VM_WAIT// before retrying. | X |  |  |
| **v_pageout_free_min** | 34 | if the page daemon thread runs into a page shortage and ends up in //VM_WAIT//, then it would be waken up when this threshold is exceeded | X |  |  |
| **v_interrupt_free_min** | 2 | at this level //vm_page_alloc()// would fail even for //VM_ALLOC_SYSTEM// requests, only //VM_ALLOC_INTERRUPT// requests can succeed | X |  |  |
| zero | 0 | with zero available pages //vm_page_alloc()// would fail for all requests, all memory is exhausted | X |  |  |

After [[http://svnweb.freebsd.org/changeset/base/254304]]:

| Threshold | Sample values from untuned amd64 system with 8GB RAM | Description and useful information | Page daemon active | Normal requests are satisfied | Normal waiters waken up |
| **v_free_target** | 40536 | a level that a page scan pass tries to reach; at this level and above **vm_paging_target()** == 0, below it is 1. |  | X | X |
| **vm_pageout_wakeup_thresh** | 13233 | a level below which //vm_page_alloc()// still succeeds for all request classes, but sets //vm_pages_needed// flag and triggers a page scan; **vm_paging_needed()** == 1 below this level. | X | X | X |
| **v_free_min** | 12417 | at this level threads waiting for pages in //VM_WAIT// are waken up after page shortage (and //vm_pages_needed// flag is reset) in //vm_page_free_wakeup()//; **vm_page_count_min()** == 1 below this level. //vm_page_free_wakeup()// is called by //vm_page_free_toq()// and //vm_page_cache()//, that is, upon increasing available pages count. | X | X | X |
| **v_free_severe** | 7517 | below this level some users of //vm_page_alloc()// decide to not call it and instead voluntary wait until more pages are available. Important cases of this check are: //vm_fault()// and //vm_forkproc()//.  **vm_page_count_severe()** == 1 below this level. | X | X |  |
| **v_free_reserved** | 2618 | at this level //vm_page_alloc()// would return //NULL// for normal requests; threads needing (//M_WAITOK//) memory would call //VM_WAIT// before retrying. | X |  |  |
| **v_pageout_free_min** | 34 | if the page daemon thread runs into a page shortage and ends up in //VM_WAIT//, then it would be waken up when this threshold is exceeded | X |  |  |
| **v_interrupt_free_min** | 2 | at this level //vm_page_alloc()// would fail even for //VM_ALLOC_SYSTEM// requests, only //VM_ALLOC_INTERRUPT// requests can succeed | X |  |  |
| zero | 0 | with zero available pages //vm_page_alloc()// would fail for all requests, all memory is exhausted | X |  |  |