Ick, that's some small chunks :(
I saw another thread which seems to describe another possibility (and
suggesting that visor has fixed it):
http://www.mail-archive.com/[email protected]/msg38382.html
The suggestion being that maybe I'm writing faster than my USB device
can handle, and that there's a backlog of URBs. It's entirely likely
that my device can't suck in more than about 15-20KB/s anyway. (Are
there any cpu usage implications in having a large number of outstanding
URBs, i.e., like an O(n) "look for the URB we just finished"?)
To test this, I added a periodic command to my device which causes a
response; I then block until I receive this response. (so: <stream a
bit><request><wait for response><stream a bit><request><wait for
response>...) The CPU usage is much lower, around 20% (still rather
absurdly high for ~15KB/s) and the computer doesn't seem to lock up.
Throughput was about the same as I'd gotten with much higher CPU usage.
This is evidence that I'm DoS'ing the computer, as suggested by the
other thread and the visor code.
It looks like visor_write() method is just returning 0 if too many URBs
are outstanding. Does this get translated into a userland write()
blocking, or would userland write() return 0? I might be rework ftdi_sio
to do the visor's URB limiting, too.
The problem is you only sending 32 bytes at a time. (SNIP)
I would suggest testing this out wih a small test applicate that sends
larger ammounts of data through the device.
I will test this, just to be sure. Isn't the maximum packet size on USB
64B anyway? If so, doesn't that mean that I can (at best) reduce the
number of URBs by half?
Thanks,
-Ed
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