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non-steady wlan tcp data transfer

Question asked by Rob. on Oct 4, 2013
Latest reply on Oct 11, 2013 by Aaronwu

Doing iperf tests with an ADSP-BF527 target, using a WLAN-USB stick of ath9k family, configured as hostap in a bridge configuration, I encounter non-steady data transfer. Transfer is established using an iperf client on computer #1 (connected with the wlan port of the BF527-target), while computer #2 provides the eth0 connection to the same target board. A test using a "short" link between an iperf server and client over et0 yields approx. 45 Mbps rate without dips. The iperf interval chunks are specified as 1 sec. (-i1).

The plot below shows the throughput over time, with rate dips every two minutes. (each dip lasts at least 4 seconds). Any cronjob or so aren't responsible for this.


Next plot proves, that the dips are not related to other CPU usage at that time, since idle is increasing to approx. 45% during this time. Note: the 90% sirq load is related to the small (1Kbyte) FIFO buffer in musb subsystem (interrupt rate approx.  6 kHz). Further, the musb's DMA capabilities are not utilized here, since DMA with musb for Blackfin is still not working with kernels 3.10. The actual test is done with 2011R1 (3.0.8).

My question: what cause may be associated to this curious behaviour?