Re: [Discuss-gnuradio] New implementation for fusb_linux withoutallocs/frees

[Eric Blossom]

On Tue, Feb 24, 2009 at 10:03:05AM -0500, Marcus D. Leech wrote:
> Stefan Bruens wrote:
> > Hi,
> >
> > attached is a new version for fusb_linux.cc.
> >
> > The current implementation uses three std::list lists for free, pending and 
> > completed urbs, so submitting a single urb causes three allocs and three 
> > frees 
> > (pushing and popping of the list).
> >
> > The new implementation uses a circular list for the urbs, where each urb is 
> > marked as free, pending or completed. As the total number of allocated urbs 
> > is 
> > constant, no allocs or frees are needed.
> >
> > Benchmark:
> > usrp/host/apps/test_usrp_standard_tx -B 512 -N 64 -M 128
> >
> > old code needs ~990e6 instructions, new code 690e6 instructions. The call 
> > to 
> > usrp_basic_tx::write goes down from 380e6 to 80e6 (so almost down to a 
> > fifth 
> > ...), the remaining instructions is the pattern fill for the sample buffer.
> >
> > Regards,
> >
> > Stefan
> >  
> >   
> Have you tested receive performance, and is it improved?
> 
> Bandwidth is my dearest friend in radio astronomy (in the absence of
> RFI), so getting the best USB performance
>   that's possible given CPU constraints is important to me.
> 

Marcus, in my experience, USB performance has not be limited by cpu
cycles.  It seems to be primarily a function of the design of the host
controller, the firmware in the device, and a reasonable way to get
the data into user mode.  In most apps I've benchmarked, the overhead
of all usrp related stuff is typically on the order of 5 to 10% of the
total cycles consumed.

Eric