netdev - Re: Question about way that NICs deliver packets to the kernel

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Message-Id: <1279204417.2118.12.camel@achroite.uk.solarflarecom.com>
Date:	Thu, 15 Jul 2010 15:33:37 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Junchang Wang <junchangwang@...il.com>
Cc:	romieu@...zoreil.com, netdev@...r.kernel.org
Subject: Re: Question about way that NICs deliver packets to the kernel

On Thu, 2010-07-15 at 22:24 +0800, Junchang Wang wrote:
> Hi list,
> My understand of the way that NICs deliver packets to the kernel is
> as follows. Correct me if any of this is wrong. Thanks.
> 
> 1) The device buffer is fixed. When the kernel is acknowledged arrival of a 
> new packet, it dynamically allocate a new skb and copy the packet into it. 
> For example, 8139too.
> 
> 2) The device buffer is mapped by streaming DMA. When the kernel is 
> acknowledged arrival of a new packet, it unmaps the region previously mapped. 
> Obviously, there is NO memcpy operation. Additional cost is streaming DMA 
> map/unmap operations. For example, e100 and e1000.
> 
> Here comes my question:
> 1) Is there a principle indicating which one is better? Is streaming DMA
> map/unmap operations more expensive than memcpy operation?

DMA should result in lower CPU usage and higher maximum performance.

> 2) Why does r8169 bias towards the first approach even if it support both? I 
> convert r8169 to the second one and get a 5% performance boost. Below is result
> running netperf TCP_STREAM test with 1.6K byte packet length.
>         scheme 1    scheme 2    Imp.
> r8169     683M        718M       5%
[...]

You should also compare the CPU usage.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
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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