netdev - Re: Network multiqueue question

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Date:	Thu, 15 Apr 2010 20:41:50 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jay Vosburgh <fubar@...ibm.com>
Cc:	"George B." <georgeb@...il.com>, netdev@...r.kernel.org
Subject: Re: Network multiqueue question

Le jeudi 15 avril 2010 à 11:09 -0700, Jay Vosburgh a écrit :
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
> >Vlan is multiqueue aware, but bonding is not unfortunatly at this
> >moment.
> >
> >We could let it being 'multiqueue' (a patch was submitted by Oleg A.
> >Arkhangelsky a while ago), but bonding xmit routine needs to lock a
> >central lock, shared by all queues, so it wont be very efficient...
> 
> 	The lock is a read lock, so theoretically it should be possible
> to enter the bonding transmit function on multiple CPUs at the same
> time.  The lock may thrash around, though.
> 

Yes, and with 10Gb cards, this is a limiting factor, if you want to send
14 million packets per second ;)

read_lock() is one atomic op, dirtying cacheline
read_unlock() is one atomic op, dirtying cache line again (if contended)

in active-passive mode, RCU use should be really easy, given netdevices
are already RCU compatable. This way, each cpu only reads bonding state,
without any memory changes.


> >Since this bothers me a bit, I will probably work on this in a near
> >future. (adding real multiqueue capability and RCU to bonding fast
> >paths)
> >
> >Ref: http://permalink.gmane.org/gmane.linux.network/152987
> 
> 	The question I have about it (and the above patch), is: what
> does multi-queue "awareness" really mean for a bonding device?  How does
> allocating a bunch of TX queues help, given that the determination of
> the transmitting device hasn't necessarily been made?
> 

Well, it is a problem that was also taken into account with vlan, you
might take a look at this commit :

commit 669d3e0babb40018dd6e78f4093c13a2eac73866
Author: Vasu Dev <vasu.dev@...el.com>
Date:   Tue Mar 23 14:41:45 2010 +0000

    vlan: adds vlan_dev_select_queue
    
    This is required to correctly select vlan tx queue for a driver
    supporting multi tx queue with ndo_select_queue implemented since
    currently selected vlan tx queue is unaligned to selected queue by
    real net_devce ndo_select_queue.
    
    Unaligned vlan tx queue selection causes thrash with higher vlan
    tx lock contention for least fcoe traffic and wrong socket tx
    queue_mapping for ixgbe having ndo_select_queue implemented.
    
    -v2
    
    As per Eric Dumazet<eric.dumazet@...il.com> comments, mirrored
    vlan net_device_ops to have them with and without
vlan_dev_select_queue
    and then select according to real dev ndo_select_queue present or
not
    for a vlan net_device. This is to completely skip
vlan_dev_select_queue
    calling for real net_device not supporting ndo_select_queue.
    
    Signed-off-by: Vasu Dev <vasu.dev@...el.com>
    Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
    Acked-by: Eric Dumazet <eric.dumazet@...il.com>
    Signed-off-by: David S. Miller <davem@...emloft.net>


> 	I haven't had the chance to acquire some multi-queue network
> cards and check things out with bonding, so I'm not really sure how it
> should work.  Should the bond look, from a multi-queue perspective, like
> the largest slave, or should it look like the sum of the slaves?  Some
> of this is may be mode-specific, as well.
> 




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