netdev - Re: Flow Control and Port Mirroring Revisited

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Date:	Fri, 7 Jan 2011 10:23:58 +0900
From:	Simon Horman <horms@...ge.net.au>
To:	Jesse Gross <jesse@...ira.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	virtualization@...ts.linux-foundation.org, dev@...nvswitch.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org,
	kvm@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: Flow Control and Port Mirroring Revisited

On Thu, Jan 06, 2011 at 05:38:01PM -0500, Jesse Gross wrote:

[ snip ]
> 
> I know that everyone likes a nice netperf result but I agree with
> Michael that this probably isn't the right question to be asking.  I
> don't think that socket buffers are a real solution to the flow
> control problem: they happen to provide that functionality but it's
> more of a side effect than anything.  It's just that the amount of
> memory consumed by packets in the queue(s) doesn't really have any
> implicit meaning for flow control (think multiple physical adapters,
> all with the same speed instead of a virtual device and a physical
> device with wildly different speeds).  The analog in the physical
> world that you're looking for would be Ethernet flow control.
> Obviously, if the question is limiting CPU or memory consumption then
> that's a different story.

Point taken. I will see if I can control CPU (and thus memory) consumption
using cgroups and/or tc.

> This patch also double counts memory, since the full size of the
> packet will be accounted for by each clone, even though they share the
> actual packet data.  Probably not too significant here but it might be
> when flooding/mirroring to many interfaces.  This is at least fixable
> (the Xen-style accounting through page tracking deals with it, though
> it has its own problems).

Agreed on all counts.


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