netdev - Re: RFC: Nagle latency tuning

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Message-ID: <20080923021237.GC25711@one.firstfloor.org>
Date:	Tue, 23 Sep 2008 04:12:37 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Rick Jones <rick.jones2@...com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	David Miller <davem@...emloft.net>, csnook@...hat.com,
	netdev@...r.kernel.org
Subject: Re: RFC: Nagle latency tuning

> That seems as much of a case against NAT as per-destintation attribute 
> caching.

Sure in a ideal world NAT wouldn't exist. Unfortunately we're 
not in a ideal world.

Also in general my impression is that NAT is becoming more common.
e.g. a lot of the mobile networks seem to be NATed.

> 
> If my experience at "a large company" is any indication, for 99 

My experience at a large company was different. Also see my 
second example.

> 
> And even if I were not, how is per-destination caching the possibly 
> non-optimal characteristics based on one user behind a NAT really 
> functionally different than having to tune the system-wide defaults to 
> cover that corner-case user?  

It's just wasteful on network resouces. e.g. if you start
talking to the slow link with a too large congestion window 
a lot of packets are going to be dropped. Yes TCP will
eventually adapt, but the network and the user performance
suffers and the network is ineffectively used.

> Seems that caching per-destination 
> characteristics is actually limiting the alleged brokenness to that 
> destination rather than all destinations?

Not sure what you're talking about. There's no real brokenness 
in having a slow link.  And with default startup metrics
Linux TCP has no trouble talking to a slow link.

The brokenness is using the dst_entry TCP metrics of a fast link
to talk to a slow link and that happens with NAT.

-Andi

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