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

From: H.K. Jerry Chu
Date: Fri Jul 16 2010 - 20:37:11 EST

On Fri, Jul 16, 2010 at 10:01 AM, Patrick McManus <mcmanus@xxxxxxxxxxxx> wrote:
> On Wed, 2010-07-14 at 21:51 -0700, H.K. Jerry Chu wrote:
>>  except there are indeed bugs in the code today in that the
>> code in various places assumes initcwnd as per RFC3390. So when
>> initcwnd is raised, that actual value may be limited unnecessarily by
>> the initial wmem/sk_sndbuf.
> Thanks for the discussion!
> 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

2. the congestion backoff code is supposed to take inflight, rather than cwnd,
but initcwnd presents a special case. I don't fully understand the code yet to
propose a fix.

> 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?)

This is partly due to our load balancer policy resulting in poor cache hit,
partly due to the sheer volumes of remote clients. Some of colleagues
tried to change the host cache to a /24 subnet cache but the result wasn't
that good either (sorry I don't remember all the details.)

> article and slides much appreciated and very interetsing. I've long been
> of the opinion that the downsides of being too aggressive once in a
> while aren't all that serious anymore.. as someone else said in a
> non-reservation world you are always trying to predict the future anyhow
> and therefore overflowing a queue is always possible no matter how
> conservative.

Please voice your support to TCPM then :)


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