[dpdk-dev] [PATCH 0/6] app/testpmd: add runtime config

[Dharmik Thakkar]

> On Aug 26, 2020, at 11:41 AM, Bruce Richardson <bruce.richardson at intel.com> wrote:
> 
> On Wed, Aug 26, 2020 at 05:33:20PM +0100, Ferruh Yigit wrote:
>> On 7/14/2020 10:51 PM, Dharmik Thakkar wrote:
>>> Meson build system lacks support for
>>> CONFIG_RTE_TEST_PMD_RECORD_CORE_CYCLES and
>>> CONFIG_RTE_TEST_PMD_RECORD_BURST_STATS configuration options.
>>> 
>>> One solution is to add these options within meson_options.txt
>>> 
>>> Since adding these as runtime config causes no performance impact,
>> 
>> Hi Dharmik,
>> 
>> These are on the datapath, and even disable there will be additional
>> checks, isn't it expected to have some impact?  Did you do any
>> measurements for it?

Hi Ferruh,

In my measurements, I saw a maximum performance degradation of 0.3% for rx and tx throughput (pps)
with runtime option (both disabled and enabled cases) when compared to compile time option..
There was no difference in cycles per packet measurement.
I did these measurements with Mellanox-ConnectX-5 card on N1SDP server with ’set fwd mac retry’ option.

>> 
> Branches that always predict the same way can be very cheap, and unless
> proven to be a problem, I'd see no issue with having a few on the datapath
> - especially if it just one or two per burst. If we start seeing a
> significant number, or ones that occur for every packet, then we perhaps
> need to be more cautious.
> 
> I also think that using lots of different CFLAGS for turning things on and
> off is as bad - or even worse - than using the old build-time config
> options, just this time we don't have a single list of them somewhere!
> Therefore, I think we really need to start converting these to runtime
> options where we can.
> 
> Regards,
> /Bruce