netdev - Re: [PATCH net-next v2 2/5] tcp: metrics: Add source-address to tcp-metrics

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Message-ID: <1389222826.31367.28.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 08 Jan 2014 15:13:46 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Paasch <christoph.paasch@...ouvain.be>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
	Yuchung Cheng <ycheng@...gle.com>, Julian Anastasov <ja@....bg>
Subject: Re: [PATCH net-next v2 2/5] tcp: metrics: Add source-address to
 tcp-metrics

On Wed, 2014-01-08 at 23:43 +0100, Christoph Paasch wrote:
> Hello Eric,
> 
> On 08/01/14 - 09:55:51, Eric Dumazet wrote:
> > On Wed, 2014-01-08 at 16:05 +0100, Christoph Paasch wrote:
> > > We add the source-address to the tcp-metrics, so that different metrics
> > > will be used per source/destination-pair. We use the destination-hash to
> > > store the metric inside the hash-table. That way, deleting and dumping
> > > via "ip tcp_metrics" is easy.
> > 
> > Note that this has the following problem :
> > 
> > Some applications use a set of source IP addresses to overcome the 64K
> > port limitation.
> 
> Ok, did not know about that.
> 
> > tcp_metrics uses a hard-coded TCP_METRICS_RECLAIM_DEPTH value of 5,
> > meaning that cache wont be able to store more than 5 source IP addresses
> > (reaching one particular remote IP).
> 
> Maybe we could do something like the below (yet untested). That way we allow
> up to 32 entries with the same destination but different source and still
> only 5 with different destinations.
> 
> I guess 32 * 64K connections is enough. :)
> We could also make TCP_METRICS_RECLAIM_DEPTH(_DST) a tunable.

Well, not sure if this is a problem anyway, and if we want extra
complexity for this rare use case, considering tcp metrics for
high number of flows sharing a common path is unlikely to be useful
(with exception of Fast Open, but again it must be rare)



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