netdev - Re: [PATCH 0/3] Basic deferred TX queue flushing infrastructure.

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Date:	Sat, 23 Aug 2014 20:39:05 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	Linux Netdev List <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH 0/3] Basic deferred TX queue flushing infrastructure.

On Sat, Aug 23, 2014 at 1:28 PM, David Miller <davem@...emloft.net> wrote:
>
>
> Over time, and specifically and more recently at the Networking
> Workshop during Kernel SUmmit in Chicago, we have discussed the idea
> of having some way to optimize transmits of multiple TX packets at
> a time.
>
> There are several areas of overhead that could be amortized with such
> schemes.  One has to do with locking and transactional overhead, the
> other has to do with device specific costs.
>
> This patch set here is more aimed at device specific costs.
>
> Typically a device queues up a packet in the TX queue and then has to
> do something to have the device start processing that new entry.
> Sometimes this is composed of doing an MMIO write to a "tail"
> register, and in other cases it can involve something as expensive as
> a hypervisor call.
>
> The basic setup defined here is that when the driver supports deferred
> TX queue flushing, ndo_start_xmit should no longer perform that
> operation.  Instead a new operation, ndo_xmit_flush, should do it.
>
Interesting, but I'm still wondering about MSG_MORE idea. If that is
passed with ndo_start_xmit than maybe a new ndo function might not be
needed. Also, since it's advisory, we could pass it to existing
drivers and let them decide how rather to do anything with it.

> I have converted IGB and virtio_net as example initial users.  The IGB
> conversion is tested, virtio_net is not but it does compile :-)
>
> All ndo_start_xmit call sites have been abstracted behind a new helper
> called netdev_start_xmit().
>
> This just adds the infrastructure, it does not actually add any
> instances of actually doing multiple ndo_start_xmit calls per
> ndo_xmit_flush invocation.
>
> Signed-off-by: David S. Miller <davem@...emloft.net>
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