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SubjectRe: Raise initial congestion window size / speedup slow start?
On Wed, 14 Jul 2010 19:48:36 +0100
Ed W <> wrote:

> On 14/07/2010 19:15, David Miller wrote:
> > From: Bill Davidsen<>
> > Date: Wed, 14 Jul 2010 11:21:15 -0400
> >
> >
> >> You may have to go into /proc/sys/net/core and crank up the
> >> rmem_* settings, depending on your distribution.
> >>
> > You should never, ever, have to touch the various networking sysctl
> > values to get good performance in any normal setup. If you do, it's a
> > bug, report it so we can fix it.
> >
> Just checking the basics here because I don't think this is a bug so
> much as a, less common installation that differs from the "normal" case.
> - When we create a tcp connection we always start with tcp slow start
> - This sets the congestion window to effectively 4 packets?
> - This applies in both directions?
> - Remote sender responds to my hypothetical http request with the first
> 4 packets of data
> - We need to wait one RTT for the ack to come back and now we can send
> the next 8 packets,
> - Wait for the next ack and at 16 packets we are now moving at a
> sensible fraction of the bandwidth delay product?
> So just to be clear:
> - We don't seem to have any user-space tuning knobs to influence this
> right now?
> - In this age of short attention spans, a couple of extra seconds
> between clicking something and it responding is worth optimising (IMHO)
> - I think I need to take this to netdev, but anyone else with any ideas
> happy to hear them?
> Thanks
> Ed W

TCP slow start is required by the RFC. It is there to prevent a TCP congestion
collapse. The HTTP problem is exacerbated by things beyond the user's control:
1. stupid server software that dribbles out data and doesn't used the full
payload of the packets
2. web pages with data from multiple sources (ads especially), each of which
requires a new connection
3. pages with huge graphics.

Most of this is because of sites that haven't figured out that somebody on a phone
across the globl might not have the same RTT and bandwidth that the developer on a
local network that created them. Changing the initial cwnd isn't going to fix it.

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