Time to bump default VM_SWZONE_SIZE_MAX?
Locked
 
|
|
|
In reply to this post by Colin Percival-4
On 2012-Aug-12 15:44:07 -0700, Colin Percival <[hidden email]> wrote:
>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. I'm not sure how you got that value. By default, struct swblock is 288 bytes (280 bytes on 32-bit archs) and can store up to 32 pages of swap (the comment in vm/swap_pager.c:swap_pager_swap_init() is wrong). For x86, this is 2.25 MiB per GiB (best case). >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. In practice, you can't fully populate each swblock. I did a test on my amd64 box by running multiple copies of a program that allocates and dirties a big chunk of RAM and then pause()s. That gave me a 90% swblock utilisation - which I suspect is higher than a typical scenario where memory pressure pushes more randomly unused pages out. Realistically, I'd say that the default VM_SWZONE_SIZE_MAX can handle about 9GB swap (at least, that was my experience). BTW, if you plan on allocating lots of swap, be aware that each swap device is limited to 32GiB - see vm/swap_pager.c:swaponsomething(). -- Peter Jeremy |
attachment0 (203 bytes)
|
In reply to this post by Colin Percival-4
Colin Percival <[hidden email]> 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 - [hidden email] _______________________________________________ [hidden email] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "[hidden email]" |
|
|
Slightly better patch (improved documentation)
DES -- Dag-Erling Smørgrav - [hidden email] _______________________________________________ [hidden email] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "[hidden email]" |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Loading...
| Powered by Nabble | Edit this page |