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Subject: Re: [PATCH net-next 03/21] ethtool, stats: introduce standard XDP
 statistics
To:     Jakub Kicinski <kuba@kernel.org>, David Ahern <dsahern@gmail.com>,
        "Karlsson, Magnus" <magnus.karlsson@intel.com>
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Message-ID: <5653e19f-75fa-9a94-4b45-0cf110fd6e36@redhat.com>
Date:   Thu, 12 Aug 2021 14:19:30 +0200
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On 04/08/2021 18.44, Jakub Kicinski wrote:
> On Wed, 4 Aug 2021 10:17:56 -0600 David Ahern wrote:
>> On 8/4/21 6:36 AM, Jakub Kicinski wrote:
> 
>> Does anyone have data that shows bumping a properly implemented counter
>> causes a noticeable performance degradation and if so by how much? You
>> mention 'yet another cacheline' but collecting stats on stack and
>> incrementing the driver structs at the end of the napi loop should not
>> have a huge impact versus the value the stats provide.
> 
> Not sure, maybe Jesper has some numbers. Maybe Intel folks do?

(sorry, behind on emails after vacation ... just partly answering inside 
this thread, not checking if you did a smart counter impl.).

I don't have exact numbers, but I hope Magnus (Intel) would be motivated 
to validate performance degradation from this patchset.  As I know Intel 
is hunting the DPDK numbers with AF_XDP-zc, where every last cycle *do* 
count.

My experience is that counters can easily hurt performance, without the 
developers noticing the small degradation's.  As Ahern sketch out above 
(stats on stack + end of napi loop update), I do believe that a smart 
counter implementation is possible to hide this overhead (hopefully 
completely in the CPUs pipeline slots).

I do highly appreciate the effort to standardize the XDP stats!
So, I do hope this can somehow move forward.

--Jesper