Possibly incorrect optimizations in 90-usrp.conf

[drws]

The 90-usrp.conf file for setting kernel parameters in /etc/sysctl.d/ contains 3 lines, only 2 of which are unique. Furthermore, it doesn't set rmem_default and wmem_default as per the USRP docs. I believe the contents of the file should be:

net.core.rmem_default=33554432
net.core.wmem_default=33554432
net.core.rmem_max=33554432
net.core.wmem_max=33554432

The value 33554432 means 32MiB whereas the current 50MB doesn't seem practical for a memory chunk size. Is there a reason a higher value than 32MiB is used, possibly because of some other SDR hardware than USRP?

[bastibl]

I guess, there is no right or wrong optimization. Just one that works better or worse. I don't know what is better and also Ettus is not clear on that: https://github.com/EttusResearch/uhd/search?q=rmem_max&type=code. IIRC, I didn't make up this value but got also got it from the recommendation of the driver. I'm happy to merge 32MB, if you prefer that. Can you open a PR?

[drws]

I'm not actually using instant-gnuradio, I was just checking out its code and left the comment since it's good to have working GR builds around. I also don't know what value would be best, so you can just leave it for now. But there's still the duplicated line and no setting of *_default values (which might be redundant).