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

From: Stephen Hemminger <[email-masked]>
Date: Thu, 15 Jul 2010 08:59:17 -0700

On Thu, 15 Jul 2010 15:33:37 +0100
Ben Hutchings <[email-masked]> wrote:

> 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.

Also many drivers copy small receives into a new buffer
which saves space and often gives better performance.