netdev - Re: tcp bw in 2.6

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Message-ID: <20071015084911.50229ffd@freepuppy.rosehill>
Date:	Mon, 15 Oct 2007 08:49:11 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Daniel Schaffrath <danielschaffrath@....com>
Cc:	Larry McVoy <lm@...mover.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	torvalds@...ux-foundation.org, davem@...emloft.net,
	wscott@...mover.com, Linux NetDev <netdev@...r.kernel.org>
Subject: Re: tcp bw in 2.6

On Mon, 15 Oct 2007 14:40:25 +0200
Daniel Schaffrath <danielschaffrath@....com> wrote:

> On 2007/10/02  , at 18:47, Stephen Hemminger wrote:
> 
> > On Tue, 2 Oct 2007 09:25:34 -0700
> > lm@...mover.com (Larry McVoy) wrote:
> >
> >>> If the server side is the source of the data, i.e, it's transfer  
> >>> is a
> >>> write loop, then I get the bad behaviour.
> >>> ...
> >>> So is this a bug or intentional?
> >>
> >> For whatever it is worth, I believed that we used to get better  
> >> performance
> >> from the same hardware.  My guess is that it changed somewhere  
> >> between
> >> 2.6.15-1-k7 and 2.6.18-5-k7.
> >
> > For the period from 2.6.15 to 2.6.18, the kernel by default enabled  
> > TCP
> > Appropriate Byte Counting. This caused bad performance on  
> > applications that
> > did small writes.
> Stephen, maybe you can provide me with some specifics here?
> 
> Thanks a lot!!
> Daniel
> 

Read the RFC3465 for explanation of TCP ABC.
What happens is that applications that do multiple small writes
will end up using up their window. Typically these applications are not
streaming enough data to grow the congestion window so they get held
after 4 writes until an ACK comes back.  The fix for the application
(which also helps on all OS's and TCP versions as well) is to use a call
like writev() or sendmsg() to aggregate the small header blocks together
into a single send.

-- 
Stephen Hemminger <shemminger@...ux-foundation.org>
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