netdev - Re: [RFD] L2 Network namespace infrastructure

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Date:	Sat, 23 Jun 2007 12:40:44 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
	jamal <hadi@...erus.ca>,
	Stephen Hemminger <shemminger@...ux-foundation.org>,
	Ben Greear <greearb@...delatech.com>,
	Jeff Garzik <jeff@...zik.org>,
	YOSHIFUJI Hideaki <yoshfuji@...ux-ipv6.org>,
	Linux Containers <containers@...ts.osdl.org>
Subject: Re: [RFD] L2 Network namespace infrastructure

Eric W. Biederman wrote:
> -- The basic design
> 
> There will be a network namespace structure that holds the global
> variables for a network namespace, making those global variables
> per network namespace.
> 
> One of those per network namespace global variables will be the
> loopback device.  Which means the network namespace a packet resides
> in can be found simply by examining the network device or the socket
> the packet is traversing.
> 
> Either a pointer to this global structure will be passed into
> the functions that need to reference per network namespace variables
> or a structure that is already passed in (such as the network device)
> will be modified to contain a pointer to the network namespace
> structure.


I believe OpenVZ stores the current namespace somewhere global,
which avoids passing the namespace around. Couldn't you do this
as well?

> Depending upon the data structure it will either be modified to hold
> a per entry network namespace pointer or it there will be a separate
> copy per network namespace.  For large global data structures like
> the ipv4 routing cache hash table adding an additional pointer to the
> entries appears the more reasonable solution.


So the routing cache is shared between all namespaces?

> --- Performance
> 
> In initial measurements the only performance overhead we have been
> able to measure is getting the packet to the network namespace.
> Going through ethernet bridging or routing seems to trigger copies
> of the packet that slow things down.  When packets go directly to
> the network namespace no performance penalty has yet been measured.


It would be interesting to find out whats triggering these copies.
Do you have NAT enabled?
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