Re: [Discuss-gnuradio] Flarging big FFTs

[Eric Blossom]

On Fri, Jan 23, 2009 at 01:01:24PM -0500, Marcus D. Leech wrote:
> I can't use FFTs larger than about 180000 bins without getting "shared
> memory allocation" errors in Gnu Radio.
> 
> Is there an easy fix for this?
> 
> For SETI work, being able to (for example) compute an 8 million point
> FFT for an 8Msps signal gives you 1Hz
>   resolution.  But I can't seem to get anywhere near this without the
> allocator griping at me.
> 

Hi Marcus,

You need to bump up the shared memory maximum segment size.
You can do this with the sysctl command.  On some systems there's also
an /etc/sysctl.conf file.  The defaults appear to depend on your
distribution.   The parameter you want to increase is kernel.shmmax.

  address@hidden trunk]$ sudo sysctl -a | grep shm
  kernel.shmmax = 268435456
  kernel.shmall = 2097152
  kernel.shmmni = 4096
  vm.hugetlb_shm_group = 0

The output above is from an x86 Fedora 9 system.  It's pretty big by default.

See also

 $ man sysctl
 $ man sysctl.conf

 http://bugzilla.kernel.org/show_bug.cgi?id=11381

Eric