Date: Tue, 13 Jan 2026 14:03:25 +0000 From: Konstantin Belousov <kib@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: 457b940bfb6a - main - tuning.7: add more explanation about swap (over-)accounting Message-ID: <696650ad.a11e.704a7e09@gitrepo.freebsd.org>
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The branch main has been updated by kib: URL: https://cgit.FreeBSD.org/src/commit/?id=457b940bfb6a228af1281f357b627ecf1e26fea5 commit 457b940bfb6a228af1281f357b627ecf1e26fea5 Author: Konstantin Belousov <kib@FreeBSD.org> AuthorDate: 2026-01-12 03:54:58 +0000 Commit: Konstantin Belousov <kib@FreeBSD.org> CommitDate: 2026-01-13 14:03:14 +0000 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 --- share/man/man7/tuning.7 | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/share/man/man7/tuning.7 b/share/man/man7/tuning.7 index 44c427c4559d..a3709ce82b8c 100644 --- a/share/man/man7/tuning.7 +++ b/share/man/man7/tuning.7 @@ -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 thehome | help
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