Date: Sat, 6 Jul 2002 22:09:54 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Darren Pilgrim <dmp@pantherdragon.org> Cc: "Matthew D. Fuller" <fullermd@over-yonder.net>, ticso@cicely.de, freebsd-hackers@FreeBSD.ORG Subject: Re: How does swap work address spacewise? Message-ID: <200207070509.g6759snY065170@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> <200207062334.g66NYIhS063812@apollo.backplane.com> <3D27CB56.D5BEE938@pantherdragon.org>
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:Is NSWAP tied to the NSWAPDEV kernel option, or is it the actual number
:of active swap devices? If the prior, is setting NSWAPDEV to the
:actual number of swap devices a useful for improving memory usage? Is
:NSWAPDEV just a compile-time tunable, or is there a sysctl to do the
:same thing?
Sorry, I meant NSWAPDEV there.
:> 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).
:
:Can VM_SWZONE_SIZE_MAX be tuned down as well, or does the kernel
:already handle this efficiently enough to keep it at a minimum useful
:value sized relative to PHYS?
The kernel handles this fairly well now. It didn't used to. The limit
exists to prevent the kernel from reserving absurd amounts of KVM for
swblock's on systems with lots of memory (like 2G or 4G of ram). The
amount of KVM reserved for systems with less physical ram is fairly
small and you don't have to worry about it. Also, this is just KVM.
Real ram is only allocated for swblock structures when you actually
start paging to swap (the bitmaps are preallocated but the space for
the swblock structures is not).
The only time you would need to tune these parameters would be if you
really needed to configure more then around 14GB of swap and have it
all be useable by the system.
-Matt
Matthew Dillon
<dillon@backplane.com>
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