netdev - Re: 100Mbit ethernet performance on embedded devices

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Date:	Fri, 28 Aug 2009 18:35:13 +0100
From:	Mark Brown <broonie@...nsource.wolfsonmicro.com>
To:	Johannes Stezenbach <js@...21.net>
Cc:	Jamie Lokier <jamie@...reable.org>, linux-embedded@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: 100Mbit ethernet performance on embedded devices

On Fri, Aug 28, 2009 at 04:41:38PM +0200, Johannes Stezenbach wrote:
> On Thu, Aug 20, 2009 at 02:56:49PM +0200, Johannes Stezenbach wrote:

> > which came with the 2.6.20 kernel.  The delay between irq ->
> > netif_rx_schedule() -> NET_RX_SOFTIRQ ->  ->poll()  doesn't seem
> > to be long enough.  But of course my understanding of NAPI is
> > very limited, probably I missed something...

> It would've been nice to get a comment on this.  Yeah I know,
> old kernel, non-mainline driver...

> On this platform NAPI seems to be a win when receiving small packets,
> but not for a single max-bandwidth TCP stream.  The folks at
> stlinux.com seem to be using a dedicated hw timer to delay
> the NAPI poll() calls:
> http://www.stlinux.com/drupal/kernel/network/stmmac-optimizations

> This of course adds some latency to the packet processing,
> however in the single TCP stream case this wouldn't matter.

Does your actual system have any appreciable CPU loading?  If so that
will normally have the same effect as inserting a delay in the RX path.
Some of the numbers will often look worse with NAPI when the system is
lightly loaded (though not normally throughput).
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