Re: [Discuss-gnuradio] ofdm reception affected by python threading?

[Veljko Pejovic]

Hi Eric,

Thanks for the tip. I tried it out on a faster machine, and I observed
no such problems. The first machine that produces this problem has a
single core 2.8GHz CPU and the second one, which works fine, has a
dual core Intel Atom D510, both machines have 2GB of RAM each. I
should note that the faster machine also has a better GIgEthernet
card. Interestingly, I do not observe any other signs of CPU
throttling, such as dropped packets when I see the problem.

Cheers,

Veljko


2010/5/24 Eric Blossom <address@hidden>:
> On Fri, May 21, 2010 at 02:15:15PM -0700, Veljko Pejovic wrote:
>> Hi,
>>
>> I'm using two USRP2 with XCVR2450s, a week old gnuradio git master and
>> Ubuntu 9.10.
>>
>> A few months ago while using the OFDM code I observed that I tend to
>> receive (or send?) packets in groups, although I was sending them
>> individually.
>>
>> Now, I finally isolated the problem. I added a few simple print lines:
>>
>> "Message inserted in HAVE_HEADER" - whenever a message is enqueued in
>> STATE_HAVE_SYNC of ofdm_frame_sink.cc
>>
>> and
>>
>> "Message picked up from the queue" - in queue_watcher_thread in
>> ofdm.py, every time the message is fetched from the queue.
>>
>> The output that I expected was:
>>
>> Message inserted in HAVE_HEADER
>> Message picked up from the queue
>> Message inserted in HAVE_HEADER
>> Message picked up from the queue
>> ...
>>
>> However, I'm getting rather inconsistent output (this is a sample,
>> check the attachment for the first few seconds of the output):
>>
>> Message inserted in HAVE_HEADER
>> Message inserted in HAVE_HEADER
>> Message inserted in HAVE_HEADER
>> Message inserted in HAVE_HEADER
>> Message picked up from the queue
>> Message picked up from the queue
>> Message picked up from the queue
>> Message picked up from the queue
>> Message inserted in HAVE_HEADER
>> Message inserted in HAVE_HEADER
>> Message inserted in HAVE_HEADER
>> Message inserted in HAVE_HEADER
>> Message inserted in HAVE_HEADER
>> Message picked up from the queue
>> Message picked up from the queue
>> Message picked up from the queue
>> Message picked up from the queue
>> Message picked up from the queue
>>
>> So, it is definitely the receiver that has problems. It looks as if
>> "msg = self.rcvd_pktq.delete_head()" in queue_watcher_thread in
>> ofdm.py is not checked frequently enough. Could it be that Python does
>> not switch threads fast enough? Note, I do have real time scheduling
>> enabled.
>> Any ideas on how to get around this?
>
> Is your machine out of CPU cycles?
> If it's not fast enough, it won't keep up.
>
> Eric
>