mmap max size (qemu VM memsize limit)

Gergely CZUCZY phoemix at
Mon Nov 26 07:03:50 PST 2007

On Mon, Nov 26, 2007 at 08:59:12AM -0600, Guy Helmer wrote:
> Gergely CZUCZY wrote:
> >Hello,
> >
> >I'm trying to set up a qemu with 4G memory to the host system
> >on a 7-BETA2, and I've hit some kind of limit in freebsd.
> >
> >When i give more then ~2000MB of memory to qemu, it returns
> >an after trying to mmap() it, saying "Could not map physical memory".
> >
> >At the end of mmap(2) there's a note saying:
> >"     The len argument is limited to the maximum file size or available user-
> >     land address space.  Files may not be able to be made more than 1TB large
> >     on 32 bit systems due to file systems restrictions and bugs, but address
> >     space is far more restrictive.  Larger files may be possible on 64 bit
> >     systems."
> >
> >I guess I've found this limit. I've tried to check a few sysctls but I wasn't
> >able to find the one effecting this limit.
> >
> >Could someone point me into the right direction in increasing the maximum
> >mmap-able size?
> >
> >  
> If you are using the 32-bit i386 system, then you could be limited by a couple of things, the kern.maxdsiz loader tunable or the datasize limit are 
> likely.  On i386, I believe 1073741824 (1G) is the default maxdsiz.  With 4GB of RAM, I think you ought to be able to double kern.maxdsiz in 
> /boot/loader.conf.
It's an amd64 with 14Gs of RAM in it. Sorry for accidentally omitting this.


Gergely Czuczy
mailto: gergely.czuczy at

Weenies test. Geniuses solve problems that arise.
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