MacPro 2008 up to 10.9.3 - USB performance drop

Question

I have a vanilla early 2008 8-core Mac Pro with 8GB of RAM and about 500 GB of HD.
I use it exclusively for software defined radio, which is an application that in my instance, transfers a lot of data to the computer via USB from a device called an SDR-IQ (by RFSPACE.) Very little outgoing data is involved, it's almost all in the device to computer direction. Data rates involved range from 200 kB/sec to 800 kB/sec, depending on the bandwidth selected in my application.
I was previously using OSX 10.6.8, and everything worked fine at all data rates.
I upgraded to 10.9.3, and now I get a few seconds of data transfer and correct operation, and then there is either a data stall or a significantly diminished transfer rate,  -- and that's the end of that. This happens at all data rates including the slowest.
Running the same application, but with a unit that transfers the same amount of data, and also at all the higher rates, over TCP/ethernet, there is no problem. This establishes that 10.9 is not slowing down my actual application; instead it seems to be a problem specifically related to USB. I do have access to the source code which is handling the USB transfer, but there's nothing in it that was previously broken, and nothing stands out to me as something that is being done wrong, funny or in any way that would otherwise would raise suspicion.
I know of one change Apple made between 10.6 and 10.9 that breaks USB compatibility; it was the addition of a serial kernal module for the FTDI chip which effectively blocks the direct access library and completely breaks anything that tries to talk to the FTDI chip in the (previously) standard manner. This must be unloaded, and I have done so:
     sudo kextunload -b com.apple.driver.AppleUSBFTDI
At this point, software like mine that uses the FTDI chip works again, but as I said, it's very slow and/or stalls during data transfer.
Anyone know of any hints, tips, or hopefully outright fixes?

Answer 1

Ok, found it. Mavericks has this "app nap" thing. The software uses a server that talks to the SDR-IQ via USB, then re-encapsulates that data as a standard TCP stream at fairly high rates. The SDR receive software receives and interprets that stream.
"App nap" takes windowed applications that are not at the "front" of OSX's GUI and interferes with their processing such that they don't get the cycles they need to run normally; that's exactly what was happening to the USB-to-TCP server. It was getting "put to sleep" in the middle of doing it's time-critical work, the TCP output stream would stall, and SdrDx in turn would suffer a data famine.
The solution is to right click on the application icon and select "show info", then checkmark the "ignore app nap" or similar nomenclature. I did this for both the server and SdrDx, because there are *many* times when SdrDx is receiving but is not the front application, and it may not be making any sound, either, which is apparently one of the (few) things the app nap "feature" looks at to see if it is ruining your day.
So if you get weird USB (or other) speed issues under Mavericks, first thing to try is turn off App Nap. That's a by-default-on OSX feature that is not nearly smart enough to do what it does to applications. Thank goodness Apple provided a way to turn it off!