netdev - Re: Raise initial congestion window size / speedup slow start?

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Message-ID: <AANLkTilI3rXF9ikiQOIqCOXpq4s3cfqOULBn_P8jQVrp@mail.gmail.com>
Date:	Mon, 19 Jul 2010 15:51:13 -0700
From:	"H.K. Jerry Chu" <hkjerry.chu@...il.com>
To:	Rick Jones <rick.jones2@...com>
Cc:	Patrick McManus <mcmanus@...ksong.com>,
	David Miller <davem@...emloft.net>, davidsen@....com,
	lists@...dgooses.com, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: Raise initial congestion window size / speedup slow start?

 Mon, Jul 19, 2010 at 10:08 AM, Rick Jones <rick.jones2@...com> wrote:
> H.K. Jerry Chu wrote:
>>
>> On Fri, Jul 16, 2010 at 10:01 AM, Patrick McManus <mcmanus@...ksong.com>
>> wrote:
>>>
>>> can you tell us more about the impl concerns of initcwnd stored on the
>>> route?
>>
>>
>> We have found two issues when altering initcwnd through the ip route cmd:
>> 1. initcwnd is actually capped by sndbuf (i.e., tcp_wmem[1], which is
>> defaulted to a small value of 16KB). This problem has been made obscured
>> by the TSO code, which fudges the flow control limit (and could be a bug
>> by
>> itself).
>
> I'll ask my Emily Litella question of the day and inquire as to why that
> would be unique to altering initcwnd via the route?
>
> The slightly less Emily Litella-esque question is why an appliction with a
> desire to know it could send more than 16K at one time wouldn't have either
> asked via its install docs to have the minimum tweaked (certainly if one is
> already tweaking routes...), or "gone all the way" and made an explicit
> setsockopt(SO_SNDBUF) call?  We are in a realm of applications for which
> there was a proposal to allow them to pick their own initcwnd right?  Having

Per app setting of initcwnd is just one case. Another is per route setting of
initcwnd basis through the ip route cmd. For the latter the initcwnd change is
more or less supposed to be transparent to apps.

This wasn't a big issue and can probably be easily fixed by
initializing sk_sndbuf
to max(tcp_wmem[1], initcwnd) as you alluded to below. It is just our
experiements got hindered by this little bug but we weren't aware of it sooner
due to TSO fudging sndbuf.

Jerry

> them pick an SO_SNDBUF size would seem to be no more to ask.
>
> rick jones
>
> sendbuf_init = max(tcp_mem,initcwnd)?
>
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