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kern/56309: swapctl -U is very inefficient (tages ages to eternities)

>Number:         56309
>Category:       kern
>Synopsis:       swapctl -U is very inefficient (tages ages to eternities)
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Jul 14 08:15:00 +0000 2021
>Originator:     Frank Kardel
>Release:        NetBSD 9.99.80 (also - .85 and likely beyond)
System: NetBSD pip 9.99.80 NetBSD 9.99.80 (PIPGEN) #0: Thu Feb 11 20:11:26 CET 2021 kardel@pip:/src/NetBSD/cur/src/obj.amd64/sys/arch/amd64/compile/PIPGEN amd64
Architecture: x86_64
Machine: amd64
	On a system with swap configured and swap having been used shutdown can
	take a very long time when shutting down in the "Removing block-type swap devices"
	phase. The read rate from the swap devices is very low for rust media and slow
	for SSD media. swapctl -U picks up significant CPU time. The whole process takes
	eternities for rust media and ages for SSDs.

Example (about 7 minutes into the shutdown):
   0 12876 23709 25309 117 -20    19376     1656 -       O<   ?       6:18.30 swapctl -U -t blk
   0   188 22490    84 117   0    17820     1400 tstile  D+   pts/2   0:00.00 swapctl -l

	On rust media the shutdown can take several hours. Even SSDs(NVME) can take several
	tens of minutes. 

	Use a system with swap enabled. Force the system to use swap space. Watch
	a very long shutdown.
	Workaround: set swapoff=NO in /etc/rc.conf.


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