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

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Date:	Fri, 10 Jan 2014 17:37:11 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	christoph.paasch@...ouvain.be, netdev@...r.kernel.org,
	ycheng@...gle.com, ja@....bg
Subject: Re: [PATCH net-next v2 2/5] tcp: metrics: Add source-address to
 tcp-metrics

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Wed, 08 Jan 2014 15:13:46 -0800

> 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)

I think we are overthinking this, even for the aforementioned case.

If people report that this is a real problem they are hitting, and
not just with constructed test cases, we can work on a solution.
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