Sudden grow of memory in "Laundry" state

[Robert]

Hi, if I understood correctly, "written back to swap device" means they 
come from swap at some point, but it's not the case (see attached graph).

Swap was 16GB, and slightly reduced when pages rapidly started to move 
from free (or "Inactive"?) into "Laundry" queue.

Here is vmstat output:

vmstat -s
821885826 cpu context switches
3668809349 device interrupts
470487370 software interrupts
589970984 traps
3010410552 system calls
        25 kernel threads created
    378438  fork() calls
     21904 vfork() calls
         0 rfork() calls
      1762 swap pager pageins
      6367 swap pager pages paged in
     61678 swap pager pageouts
   1682860 swap pager pages paged out
      1782 vnode pager pageins
     16016 vnode pager pages paged in
         0 vnode pager pageouts
         0 vnode pager pages paged out
         3 page daemon wakeups
1535368624 pages examined by the page daemon
        12 clean page reclamation shortfalls
   2621520 pages reactivated by the page daemon
  18865126 copy-on-write faults
      5910 copy-on-write optimized faults
  36063024 zero fill pages zeroed
     21137 zero fill pages prezeroed
     12149 intransit blocking page faults
704496861 total VM faults taken
      3164 page faults requiring I/O
         0 pages affected by kernel thread creation
  15318548 pages affected by  fork()
    738228 pages affected by vfork()
         0 pages affected by rfork()
  61175662 pages freed
   1936826 pages freed by daemon
  24420300 pages freed by exiting processes
   3164850 pages active
   2203028 pages inactive
   6196772 pages in the laundry queue
    555637 pages wired down
    102762 pages free
      4096 bytes per page
2493686705 total name lookups
           cache hits (99% pos + 0% neg) system 0% per-directory
           deletions 0%, falsehits 0%, toolong 0%

What do you think? How pages could be moved into "Laundry" without being 
in Swap?

Thanks.


On 09/10/18 17:54, Mark Johnston wrote:
> On Mon, Sep 10, 2018 at 11:44:52AM -0700, Robert wrote:
>> Hi, I have a server with FreeBSD 11.2 and 48 Gigs of RAM where an app
>> with extensive usage of shared memory (24GB allocation) is running.
>>
>> After some random amount of time (usually few days of running), there
>> happens a sudden increase of "Laundry" memory grow (from zero to 24G in
>> a few minutes).
>>
>> Then slowly it reduces.
>>
>> Are described symptoms normal and expected? I've never noticed anything
>> like that on 11.1.
> The laundry queue contains dirty inactive pages, which need to be
> written back to the swap device or a filesystem before they can be
> reused.  When the system is short for free pages, it will scan the
> inactive queue looking for clean pages, which can be freed cheaply.
> Dirty pages are moved to the laundry queue.  My guess is that the
> system was running without a page shortage for a long time, and
> suddenly experienced some memory pressure.  This caused lots of
> pages to move from the inactive queue to the laundry queue.  Demand
> for free pages will then cause pages in the laundry queue to be
> written back and freed, or requeued if the page was referenced after
> being placed in the laundry queue.  "vmstat -s" and "sysctl vm.stats"
> output might make things more clear.
>
> All this is to say that there's nothing particularly abnormal about what
> you're observing, though it's not clear what effects this behaviour has
> on your workload, if any.  I can't think of any direct reason this would
> happen on 11.2 but not 11.1.