Ron is correct about the defaults. USB 3.0 has a MTU of 1024 B on the bus and USB 2.0 has a MTU of 512 B, so you might get something by reducing the send_frame_size to those values. I recommend also increasing the num_send_frames to something like 256 if you do. One thing to keep in mind is that smaller packets have lower latency on the wire, but the overhead per transfer may negate that.
My guess is that the latency on the transport is probably <1ms even with the default settings so I'm sure other things will result in more of an improvement. I recommend profiling your application to see where the bottlenecks may be. My understanding is that each block in GnuRadio runs in a separate thread and has buffering on the input and output. Minimizing the number of blocks and the buffering in the blocks is where I would focus, but definitely profile first to see where you should be looking. Regards, Michael On Sat, Aug 5, 2017 at 3:34 AM, Ron Economos via USRP-users < usrp-users@lists.ettus.com> wrote: > I'm pretty sure the B200 defaults are: > > send_frame_size = 8192 > > num_send_frames = 16 > > https://github.com/EttusResearch/uhd/blob/maint/ > host/lib/usrp/b200/b200_impl.cpp#L532 > > Ron > On 08/05/2017 12:00 AM, Dan CaJacob via USRP-users wrote: > > Hi, > > I have a GnuRadio modem I've developed and I'd like to reduce latency. It > does IP encapsulation and for testing, I use two USRPs to implement an OTA > connection between the modems. > > I've reduced latency so far by 1) adding fill frames/bytes only at the > very last block before the USRP sink and 2) increasing the sample rate to > the device. This has gotten my slightly under 100 ms latency, which is good > compared to what I started with, but I think we can do better. > > My assumption is that there is still some latency in the last buffer > between the last block and the USRP sink and perhaps some in the USRP USB > transport itself. I am using B200s in this case, BTW. > > I suspect that what I really must do is now tweak the usrp buffer params > from here: https://files.ettus.com/manual/page_transport.html But, I have > no idea what they are set to by default (any way to tell)? And some naive > manipulation of them hasn't improved anything. > > I also tried tweaking the mx output buffer size on the last block before > the USRP sink without any improvement. > > Any advice from Ettus folks would be appreciated :) I am giving a talk at > GRCon that addresses the latency reduction work so far, so if we can get > any more, I'll throw that in as well. > > - Dan > -- > Very Respectfully, > > Dan CaJacob > > > _______________________________________________ > USRP-users mailing > listUSRP-users@lists.ettus.comhttp://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > > > > _______________________________________________ > USRP-users mailing list > USRP-users@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com > >
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