netdev - Re: CPU scheduler to TXQ binding? (ixgbe vs. igb)

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Date:	Thu, 18 Sep 2014 06:41:29 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <jbrouer@...hat.com>
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: CPU scheduler to TXQ binding? (ixgbe vs. igb)

On Thu, 2014-09-18 at 06:33 -0700, Eric Dumazet wrote:

> Note that for TCP, there are packets sent by the process doing the
> send(), or packets sent by cpu doing TX completion (because of TSQ),
> but also packets sent by ACK processing done in the reverse way.
> 
> As Alexander explained, if the ACK packets are delivered into another
> CPU, then you might select another TX queue.
> 
> This is mostly prevented because of ooo_okay logic, meaning that a busy
> bulk flow should stick into a single TX queue, no matter of XPS says.
> 
> A TCP_RR workload is free to chose whatever queue, because every packet
> starting a RR block has the ooo_okay set (As prior data was delivered
> and acknowledged by the opposite peer)
> 

Last but not least, there is the fact that networking stacks use
mod_timer() to arm timers, and that by default, timer migration is on 
( cf /proc/sys/kernel/timer_migration )

We probably should use mod_timer_pinned(), but I could not really see
any difference.



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