netdev - Re: [PATCH v3 net-next 16/16] tcp_bbr: add BBR congestion control

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Date:   Mon, 19 Sep 2016 16:33:53 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Neal Cardwell <ncardwell@...gle.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>, Van Jacobson <vanj@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Nandita Dukkipati <nanditad@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH v3 net-next 16/16] tcp_bbr: add BBR congestion control

>
> It generates some slightly smaller code.
>         if (bbr->lt_rtt_cnt < bbr_lt_intvl_min_rtts)
> - 3e7:  0f b6 c0                movzbl %al,%eax
> - 3ea:  83 f8 03                cmp    $0x3,%eax
> - 3ed:  0f 86 d4 00 00 00       jbe    4c7 <bbr_lt_bw_sampling.isra.6+0x157>
> + 3e7:  3c 03                   cmp    $0x3,%al
> + 3e9:  0f 86 d1 00 00 00       jbe    4c0 <bbr_lt_bw_sampling.isra.6+0x150>
>
> And different code for abs
>                 /* Is new bw close to the lt_bw from the previous interval? */
>                 diff = abs(bw - bbr->lt_bw);
> - 47a:  44 89 e2                mov    %r12d,%edx
> - 47d:  29 c2                   sub    %eax,%edx
> - 47f:  89 d1                   mov    %edx,%ecx
> - 481:  89 d3                   mov    %edx,%ebx
> + 475:  44 89 e3                mov    %r12d,%ebx
> + 478:  29 c3                   sub    %eax,%ebx
> + 47a:  89 da                   mov    %ebx,%edx
> + 47c:  c1 fa 1f                sar    $0x1f,%edx
> + 47f:  31 d3                   xor    %edx,%ebx
> + 481:  29 d3                   sub    %edx,%ebx
>
> The biggest change is getting rid of the always true conditional.
>
...
>
> Overall, it really makes little difference. Actual file sizes come out the same.
> The idea is more to document what is variable
> and what is immutable in the algorithm.

SGTM, thanks Stephen !