netdev - Re: de-indirect TCP congestion control

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Message-Id: <20180312.160516.696486782939299055.davem@davemloft.net>
Date:   Mon, 12 Mar 2018 16:05:16 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     stephen@...workplumber.org, netdev@...r.kernel.org
Subject: Re: de-indirect TCP congestion control

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 12 Mar 2018 13:03:35 -0700

> 
> 
> On 03/12/2018 12:48 PM, Stephen Hemminger wrote:
>> On Mon, 12 Mar 2018 15:04:06 -0400 (EDT)
>> David Miller <davem@...emloft.net> wrote:
>> 
>>> From: Stephen Hemminger <stephen@...workplumber.org>
>>> Date: Mon, 12 Mar 2018 11:45:52 -0700
>>>
>>>> Since indirect calls are expensive, and now even more so, perhaps we
>>>> should figure out
>>>> a way to make the default TCP congestion control hooks into direct
>>>> calls.
>>>> 99% of the users just use the single CC module compiled into the
>>>> kernel.
>>>
>>> Who is this magic user with only one CC algorithm enabled in their
>>> kernel?  I want to know who this dude is?
>>>
>>> I don't think it's going to help much since people will have I think
>>> at least two algorithms compiled into nearly everyone's tree.
>>>
>>> Distributions will enable everything.
>>>
>>> Google is going to have at least two algorithms enabled.
>>>
>>> etc. etc. etc.
>>>
>>> Getting rid of indirect calls is a fine goal, but the precondition you
>>> are mentioning to achieve this doesn't seem practical at all.
>> What I meant is that kernels with N congestion controls, almost all
>> traffic
>> uses the default So that path can be optimized. The example I gave
>> would
>> have all the others doing the same indirect call.
> 
> I do not understand. What is default_tcp_ops anyway ?
> 
> How changes to /proc/sys/net/ipv4/tcp_congestion_control will impact
> this ?

I'm also confused what is being suggested exactly and how this can
work. :-)