[dpdk-dev] TX write threshold vs Report Status threshold

[Patrick Mahan <mahan@mahan.org>]

I'm not sure this is a bug or feature.

First, I've been away for a few weeks being on vacation and other stuff.  My client liked my findings on DPDK well enough that I have been loaned a dual-cpu system (2 Xeon E5-2690 @ 2.9 GHz) and two 82599 cards.  This is so I can eliminate the PCIe bus contention and to test the Numa capabilities. 

So I am now seeing (almost) line rate (~14.32 Mpps) between the two cards.  But it is not quite line rate (14.88 Mpps), so I have started playing around with the TX/RX threshold parameters.

Reading the datasheet for the 82599, section 7.2.3.5.1 seems to indicate that the write threshold (TXDCTL[n].WTHRESH) should (could?) set this value to obtain more efficient use of the PCIe bus and memory.

However, testpmd fails because the TX rs threshold (set to 0 by testpmd) is defaulting to 32 inside the ixgbe pmd code, which then fails because I have set the TX write threshold to 16.
And I cannot force the TX rs threshold < 0 via the command line.

Personally I think this is a bug, but maybe it's a feature :-) but is there a reason to force use of the rs threshold over the write threshold?

Thanks,

Patrick