Date: Sat, 6 Jul 2002 16:34:18 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: "Matthew D. Fuller" <fullermd@over-yonder.net> Cc: Darren Pilgrim <dmp@pantherdragon.org>, ticso@cicely.de, freebsd-hackers@FreeBSD.ORG Subject: Re: How does swap work address spacewise? Message-ID: <200207062334.g66NYIhS063812@apollo.backplane.com> References: <20020705113532.GA11273@atrbg11.informatik.tu-muenchen.de> <20020705133515.GA295@HAL9000.wox.org> <20020705133837.GA513@HAL9000.wox.org> <20020705234126.GA12183@atrbg11.informatik.tu-muenchen.de> <3D2640A7.3EA2236B@pantherdragon.org> <20020706020656.GL48977@cicely5.cicely.de> <3D2762FE.9D9E0378@pantherdragon.org> <20020706215220.GG5512@over-yonder.net>
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:...
:>
:> Up to four, so then the theoretical limit for swap is 8TB?
:
:I hope not, since I have 6 of 'em. 4's just the default.
:
:
:> Do these management structures grow as swap grows, or do they only
:> change as the utilization increases?
:
:I believe they're pre-allocated, so it's the size of swap configured (not
:used) that eats it.
:
:--
:Matthew Fuller (MF4839) | fullermd@over-yonder.net
The nominal limit for swap space is around 14 GB due to limitations
in available KVM. There are three major limiting factors in the kernel:
* The swap bitmap eats 2 bits per page of swap. The bitmap is sized
to handle NSWAP (default 4) x size_of_largest_swap_partition.
* The system has to keep track of pages that are swapped out.
The system reserves 8 x <physical_pages_in_system> worth of
KVM to keep track of swapped out pages. A machine with 4G
of ram reserves enough KVA to hold 32GB worth of swapped out pages.
* The system limits the size of the above zone to VM_SWZONE_SIZE_MAX,
which is typically around 70 MB of KVM (enough to hold 14 GB worth
of swap mappings).
So the nominal limit is around 14 GB on a 32 bit architecture. With
tuning this limit can be bumped up, but the practical limit is
going to be around 60GB unless you give the kernel more KVA (reducing
the amount of VM a user process can access).
On 64 bit architectures the only real limitation is how much physical
memory one is willing to burn to hold active swap mappings.
And, no, there are no plans to swap out the swap mappings themselves...
that would be a complete waste of effort.
-Matt
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