netdev - Re: PROBLEM: Linux kernel 2.6.31 IPv4 TCP fails to open huge amount of outgoing connections (unable to bind ... )

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Message-ID: <20100421003022.GA3107@ioremap.net>
Date:	Wed, 21 Apr 2010 04:30:22 +0400
From:	Evgeniy Polyakov <zbr@...emap.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Ben Greear <greearb@...delatech.com>,
	David Miller <davem@...emloft.net>,
	Gaspar Chilingarov <gasparch@...il.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: PROBLEM: Linux kernel 2.6.31 IPv4 TCP fails to open huge amount of outgoing connections (unable to bind ... )

On Wed, Apr 21, 2010 at 02:05:14AM +0200, Eric Dumazet (eric.dumazet@...il.com) wrote:
> I believe the bsockets 'optimization' is a bug, we should remove it.
> 
> This is a stable candidate (2.6.30+)
> 
> [PATCH net-next-2.6] tcp: remove bsockets count
> 
> Counting number of bound sockets to avoid a loop is buggy, since we cant
> know how many IP addresses are in use. When threshold is reached, we try
> 5 random slots and can fail while there are plenty available ports.

To return back to exponential bind() times you need to revert the whole
original patch including magic 5 number, not only bsockets.

But actual problem is not in this digit, but in a deeper logic.
Previously we scanned the whole table, now we have 5 attempts to
find out at least one bucket (without conflict) we will insert
new socket into. Apparently for large number of addresses it is possible
that all 5 times we will randomly select those buckets which conflicts.
As dumb solution we can increase 'attempt' number to infinite one, or
fallback to whole-table-search after several random attempts, which is a
bit more clever I think.

-- 
	Evgeniy Polyakov
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