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Differential D53688

swap_pager: Improve swzone warning
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Authored by des on Tue, Nov 11, 3:31 PM.
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imp
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kib
Summary

Currently, we emit a warning if more swap is configured than half the
maximum amount we can manage (to account for fragmentation). Change the
code to emit a warning if we exceed the actual maximum, and a different,
softer warning it we exceed the recommended value.

MFC after: 1 week

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rG FreeBSD src repository
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Buildable 68540
Build 65423: arc lint + arc unit

Event Timeline

des created this revision.Tue, Nov 11, 3:31 PM
Herald added a subscriber: imp. · View Herald TranscriptTue, Nov 11, 3:31 PM
des requested review of this revision.Tue, Nov 11, 3:31 PM
Harbormaster completed remote builds in B68540: Diff 166238.Tue, Nov 11, 3:31 PM
des retitled this revision from swap_pager: Improve maxswzone ergonomy to swap_pager: Improve swzone warning.Tue, Nov 11, 5:22 PM
des edited the summary of this revision. (Show Details)
des updated this revision to Diff 166251.Tue, Nov 11, 5:22 PM

update

Harbormaster completed remote builds in B68548: Diff 166251.Tue, Nov 11, 5:22 PM
markmi_dsl-only.net added a subscriber: markmi_dsl-only.net.Tue, Nov 11, 5:23 PM

man 8 loader_simp reports:

kern.maxswzone
              . . .

              Note that swap metadata can be fragmented, which means that
              the system can run out of space before it reaches the
              theoretical limit.  Therefore, care should be taken to not
              configure more swap than approximately half of the
              theoretical maximum.

              . . .
des added a comment.Tue, Nov 11, 5:24 PM

I dropped the part of the patch that sets maxswzone if it is zero because we already have vm.swzone which shows the actual value.

What we really need is better documentation — maybe a paragraph in tuning(7) that explains kern.maxswzone and vm.swzone.

markmi_dsl-only.net added a comment.Tue, Nov 11, 5:38 PM

Yea. Another paragraph from that part of that man pages says:

Running out of space for swap metadata can leave the system
in an unrecoverable state.  Therefore, you should only
change this parameter if you need to greatly extend the KVM
reservation for other resources such as the buffer cache or
kern.ipc.nmbclusters.  Modifies kernel option
VM_SWZONE_SIZE_MAX.

So there are, apparently, other considerations. I wonder if the "only change" wording should explicitly indicate a specific direction (increase vs. decrease) instead.

kib accepted this revision.Wed, Nov 12, 12:30 AM

I generally dislike such teaching printfs in kernel, it should be explained in man pages, while kernel only provide a concise values print. The long explanation, including description of units of output, and examples how to correct it, would go into tuning(7).

sys/vm/swap_pager.c
2689–2690

Wouldn't it be useful to show the swap_maxpages (without /2) in this case as well?

This revision is now accepted and ready to land.Wed, Nov 12, 12:30 AM

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sys/
vm/
29 lines

Diff 166238

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sys/vm/swap_pager.c

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