[dpdk-dev] Missing prefetch in non-vector rx function

[Arnon Warshavsky <arnon@qwilt.com>]

Hi All

Moving from dpdk 1.5 to 2.0 we observed a PPS performance degradation of
~30%.
After chasing this one for a while we found the problem:

A) Between the 2 versions rte_mbuf was increased in size from 1 to 2 cache
lines.
B) The standard (non-vector)  rx function does not perform a prefetch for
the 2nd cache line of the mbuf (I see this bug exists in 2.1 as well) and
it touches it setting the next pointer to NULL.
I tested it in ixgbe, but it looks like it exists in all drivers in the
*_rx_recv_pkts() and *_rx_recv_scattered_pkts() functions.
Once added the prefetch for the 2nd line, we were back in our previous
numbers.

I believe this one slipped under the radar as the vector mode is now the
default.
We stumbled into it because we work in non-vector mode due to a different
mempool bug in 2.0 which sometimes crashes the application upon port stop.

I have 2 questions
1)
Could anyone tell if the regression tests are comparing performance while
building DPDK with the default set of flags alone, or are multiple options
examined?

2)
How are issues like that being tracked and later associated to a patch?


Thanks
/Arnon





*Arnon Warshavsky*
*Qwilt | work: +972-72-2221634 | mobile: +972-50-8583058 | arnon@qwilt.com
<arnon@qwilt.com>*