tuning.7: add more explanation about swap (over-)accounting

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D54572

This commit is contained in:

Konstantin Belousov

2026-01-12 05:54:58 +02:00

parent d160447129

commit 457b940bfb

1 changed files with 15 additions and 0 deletions

+15

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@@ -222,6 +222,21 @@ and
 .Va vm.stats.vm.v_wire_count
 sysctls, respectively).
 .Pp
 Due to the architecture of the
 .Fx
 virtual memory subsystem, the use of copy on write (CoW) anonymous
 memory, e.g. on
 .Xr fork 2 ,
 causes swap reservation for all three regions (VM objects),
 as in the original pre-fork mapping, and its copies in
 the parent and child, instead of only two.
 Eventually the subsystem tries to optimize the internal layout
 of the tracking for CoW and often removes (collapses) no longer
 needed backing objects, re-assigning its pages and swap
 reservations to the copies.
 Coolapsing frees the swap reserve, but it is not guaranteed
 to happen.
 .Pp
 The
 .Va kern.ipc.maxpipekva
 loader tunable is used to set a hard limit on the