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

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Date:	Fri, 16 Jul 2010 18:41:42 +0100
From:	Ed W <lists@...dgooses.com>
To:	Patrick McManus <mcmanus@...ksong.com>
CC:	"H.K. Jerry Chu" <hkjerry.chu@...il.com>,
	David Miller <davem@...emloft.net>, davidsen@....com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: Raise initial congestion window size / speedup slow start?

> and while I'm asking for info, can you expand on the conclusion
> regarding poor cache hit rates for reusing learned cwnds? (ok, I admit I
> only read the slides.. maybe the paper has more info?)
>    

My guess is that this result is specific to google and their servers?

I guess we can probably stereotype the world into two pools of devices:

1) Devices in a pool of fast networking, but connected to the rest of 
the world through a relatively slow router
2) Devices connected via a high speed network and largely the bottleneck 
device is many hops down the line and well away from us

I'm thinking here 1) client users behind broadband routers, wireless, 
3G, dialup, etc and 2) public servers that have obviously been 
deliberately placed in locations with high levels of interconnectivity.

I think history information could be more useful for clients in category 
1) because there is a much higher probability that their most 
restrictive device is one hop away and hence affects all connections and 
relatively occasionally the bottleneck is multiple hops away.  For 
devices in category 2) it's much harder because the restriction will 
usually be lots of hops away and effectively you are trying to figure 
out and cache the speed of every ADSL router out there...  For sure you 
can probably figure out how to cluster this stuff and say that pool 
there is 56K dialup, that pool there is "broadband", that pool is cell 
phone, etc, but probably it's hard to do better than that?

So my guess is this is why google have had poor results investigating 
cwnd caching?

However, I would suggest that whilst it's of little value for the server 
side, it still remains a very interesting idea for the client side and 
the cache hit ratio would seem to be dramatically higher here?

I haven't studied the code, but given there is a userspace ability to 
change init cwnd through the IP utility, it would seem likely that 
relatively little coding would now be required to implement some kind of 
limited cwnd caching and experiment with whether this is a valuable 
addition?  I would have thought if you are only fiddling with devices 
behind a broadband router then there is little chance of you "crashing 
the internet" with these kind of experiments?

Good luck

Ed W
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