Time to bump default VM_SWZONE_SIZE_MAX?

[Dag-Erling Smørgrav]

Colin Percival <cperciva at freebsd.org> writes:
> If I'm understanding things correctly, the "maxswzone" value -- set by the
> kern.maxswzone loader tunable or to VM_SWZONE_SIZE_MAX by default -- should
> be approximately 9 MiB per GiB of swap space.

Far less, in fact.  As mentioned in loader(8), the default 32 MB limit
is enough for slightly more than 7 GB of swap space - assuming 100%
efficient use of swblocks.  The man page recommends not using more than
half the theoretical limit, or, with 16 pages per swblock,

                          1     maxswzone x s
                         --- x ---------------
                          2          16

where s = 276 on 32-bit systems and 288 on 64-bit systems.

> The current default for VM_SWZONE_SIZE_MAX was set in August 2002 to 32 MiB;
> meaning that anyone who wants to use more than ~ 3.5 GB of swap space ought
> to set kern.maxswzone in /boot/loader.conf.
>
> Is it time to increase this default on amd64?  (I understand that keeping the
> value low on i386 is important due to KVA limitations, but amd64 has far more
> address space available...)

There is no reason to keep this limit at all.  The algorithm used to
size the zone is "physpages / 2 * sizeof(struct swblock) or maxswzone,
whichever is smaller" where maxswzone == VM_SWZONE_SIZE_MAX unless
kern.maxswzone was set in loader.conf.  On amd64, the cutoff point is
slightly below 1 GB of physical memory (233,016 4,096-byte pages), so
the limit doesn't help machines that actually need to conserve memory,
and it hurts machines that have plenty of it and therefore also plenty
of swap (assuming the user followed the old "twice the amount of RAM"
rule of thumb).

DES
-- 
Dag-Erling Smørgrav - des at des.no

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