mmap max size (qemu VM memsize limit)

Guy Helmer ghelmer at
Mon Nov 26 07:16:30 PST 2007

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 

Hope this helps,

Guy Helmer, Ph.D.
Chief System Architect
Palisade Systems, Inc.

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