netdev - Re: [PATCH 0/20] Batch network namespace cleanup

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Message-Id: <20091130.163454.192638570.davem@davemloft.net>
Date:	Mon, 30 Nov 2009 16:34:54 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	ebiederm@...ssion.com
Cc:	netdev@...r.kernel.org, hadi@...erus.ca, dlezcano@...ibm.com,
	adobriyan@...il.com, kaber@...sh.net
Subject: Re: [PATCH 0/20] Batch network namespace cleanup

From: ebiederm@...ssion.com (Eric W. Biederman)
Date: Sun, 29 Nov 2009 17:46:03 -0800

> Recently Jamal and Daniel perform some experiments and found that
> large numbers of network namespace exiting simultaneously is very
> inefficient.  24+ minutes in some configurations.  The cpu overhead
> was negligible but it results in long hold times of net_mutex, and
> memory being consumed a long time after the last user has gone away.
> 
> I looked into it and discovered that by batching network namespace
> cleanups I can reduce the time for 4k network namespaces exiting from
> 5-7 minutes in my configuration to 44 seconds.
> 
> This patch series is my set of changes to the network namespace core
> and associated cleanups to allow for network namespace batching.

All applied, and assuming all of the build checks pass I'll
push this out to net-next-2.6

I should look into that inet_twsk_purge performance issue you mention
when tearing down a namespace.  It walks the entire hash table and
takes a lock for every hash chain.

Eric, is it possible for us to at least slightly optimize this by
doing a peek at whether the head pointer of each chain is NULL and
bypass the spinlock and everything else in that case?  Or is this
not legal with sk_nulls?

Something like:

	if (hlist_nulls_empty(&head->twchain))
		continue;

right before the 'restart' label?
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