# Gr\_buffer::allocate\_buffer: warning and USRP tx underflow **URL:** \ **Category:** GNU Radio\ **Created:** [March 6, 2014, 10:03am UTC](https://www.ruby-forum.com/t/gr-buffer-warning-and-usrp-tx-underflow/239598 "2014-03-06T10:03:25Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![musicdenotation](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/ea666f/32.png) [@musicdenotation](https://www.ruby-forum.com/u/musicdenotation)\ **Post date:** [March 6, 2014, 10:03am UTC](https://www.ruby-forum.com/t/gr-buffer-warning-and-usrp-tx-underflow/239598/1 "2014-03-06T10:03:25Z")
Hi, My gnuradio flow graph has a custom block taking input from multiple USRP’s and the output of the block is connected to a USRP transmitter. My applications demands a sample rate of at least 5Msps and up to 10Msps. Inside the block around 500 (depends on the sample rate) complex samples from each of the USRP is stored in a buffer and processed.Without the transmitter connected, the flow graph runs smoothly and there is no overflow message in the grc. But with the transmitter USRP, I see a lot of underruns in the grc and the spectrum in signal analyzer shows discontinuity.Is there a way to overcome the underrun issue within the GRC platform? 1. Also, GRC produces the following warning, gr\_buffer::allocate\_buffer: warning: tried to allocate 16 items of size 4000. Due to alignment requirements 128 were allocated. If this isn’t OK, consider padding your structure to a power-of-two bytes. On this platform, our allocation granularity is 4096 bytes. Is this warning something of a concern? Regards, Saran National University of Singapore ---
**Author:** ![sarankumar](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/e36b37/32.png) [@sarankumar](https://www.ruby-forum.com/u/sarankumar)\ **Post date:** [March 6, 2014, 4:53pm UTC](https://www.ruby-forum.com/t/gr-buffer-warning-and-usrp-tx-underflow/239598/2 "2014-03-06T16:53:01Z")
On Thu, Mar 6, 2014 at 4:01 AM, sarankumar [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Saran > National University of Singapore Two things to keep in mind as you are connecting multiple USRPs together: 1. Make sure that the sample rates from your sources to sinks are exact. 2. If the sample rate matching is good, then the problem is probably due to processing speed. You’re computer is just not able to keep up with the flowgraph. You’ll have to work on optimizing your code and any blocks in the path for this problem. The warning itself is not necessarily a problem; it’s just telling you that more memory is allocated. That could be indicative of inefficiencies and/or errors in your block’s behavior dealing with the input/output streams. Tom