netdev - Re: NAPI poll behavior in various Intel drivers

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Message-Id: <20080104.232504.238436937.davem@davemloft.net>
Date:	Fri, 04 Jan 2008 23:25:04 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	jchapman@...alix.com
Cc:	netdev@...r.kernel.org, auke-jan.h.kok@...el.com
Subject: Re: NAPI poll behavior in various Intel drivers

From: James Chapman <jchapman@...alix.com>
Date: Sat, 05 Jan 2008 00:18:31 +0000

> David Miller wrote:
> > From: James Chapman <jchapman@...alix.com>
> > Date: Fri, 04 Jan 2008 20:10:30 +0000
> > 
> >> With the latest NAPI, this code has to change. But rather than remove
> >> the tx_cleaned logic completely, shouldn't transmit processing be
> >> included in the work_done accounting when a driver does transmit cleanup
> >> processing in the poll?
> > 
> > Most other NAPI drivers don't do this, they just process all the
> > pending TX work unconditionally and do not account it into the NAPI
> > poll work.
> 
> This will cause the interface to thrash in/out of polled mode very
> quickly when it is doing almost all transmit work. That's something to
> avoid, no?

I see your point although I've never seen this in practice
with tg3 or niu.

For a heavy transmit load with TCP, the cpu killer is the ACK receive
processing.

I guess for a datagram send situation it might start to edge up.

However, you're likely deferring TX events (say every 1/4 of the TX
ring or similar) which should make the effect matter much less.

But anyways, it's someone else's driver so if they want to take
TX work into account sure. :-)  I'll keep this in mind in my NAPI
changes.

Thanks.
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