netdev - Re: [PATCH net-next 2/3] mlx4_en: moderate frequency of TX completions

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Date:	Thu, 23 Feb 2012 21:26:20 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Miller <davem@...emloft.net>
CC:	<yevgenyp@...lanox.co.il>, <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next 2/3] mlx4_en: moderate frequency of TX
 completions

On Thu, 2012-02-23 at 21:13 +0000, Ben Hutchings wrote:
> On Thu, 2012-02-23 at 14:44 -0500, David Miller wrote:
> > From: Yevgeny Petrilin <yevgenyp@...lanox.co.il>
> > Date: Thu, 23 Feb 2012 15:34:27 +0200
> > 
> > > No need to ask for completion for every packet being sent.
> > > So the method is to ask for a completion every 16 packets,
> > > or when the queue is about to be full.
> > > 
> > > Signed-off-by: Yevgeny Petrilin <yevgenyp@...lanox.co.il>
> > 
> > You absolutely cannot do this, you must signal completion and free up
> > TX queue packets in a finite amount of time.
> 
> Really, sfc has been doing this forever.
> 
> Ben.
> 
> > This means that if you suddenly stop getting new packets to send
> > you must still free up all the pending TX SKBs even if no more
> > packets are given to the driver.
> > 
> > Does your hardware unconditionally give a TX completion interrupt when
> > the TX queue empties completely?  If not, then you cannot make the
> > change contained in this patch.

And it's pretty obvious the hardware has to also send a completion when
the queue is empty.  I hope.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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