linux-kernel - Re: [GIT PULL] x86/misc for 6.5

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Date:   Tue, 27 Jun 2023 16:05:31 -0700
From:   Arjan van de Ven <arjan@...ux.intel.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Borislav Petkov <bp@...en8.de>,
        Noah Goldstein <goldstein.w.n@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] x86/misc for 6.5

On 6/27/2023 4:02 PM, Linus Torvalds wrote:
> On Tue, 27 Jun 2023 at 15:51, Arjan van de Ven <arjan@...ux.intel.com> wrote:
>>
>> I'm not surprised though; running 2 parallel streams (where one stream has a fixed zero as input,
>> so can run OOO any time) .. can really have a performance change like this
> 
> How much do people care?
> 
> One of the advantages of just having that single "update_csum_40b()"
> function is that it's trivial to then manually unroll.
> 
> With a 4-way unrolling, I get
> 
>    02: 184.0 / 184.5 cycles (8b414316) Upcoming linux kernel version
>    04: 184.0 / 184.2 cycles (8b414316) Specialized to size 40
>    06: 89.4 / 102.5 cycles (512daed6) New version
>    22: 184.6 / 184.4 cycles (8b414316) Odd-alignment handling removed
> 
> but doesn't most network hardware do the csum on its own anyway? How
> critical is csum_partial(), really?

the hardware does most cases..
in
https://lore.kernel.org/netdev/20211111181025.2139131-1-eric.dumazet@gmail.com/
Eric kind of implies it's for IPv6 headers in practice