Re: zero copy issue while receiving the data (counter part of sendfil e)

[Martijn van Oosterhout]

On Wed, Dec 22, 2004 at 01:51:58PM +0530, Mandeep Sandhu wrote:
> On Wed, 2004-12-22 at 01:13, Dmitry Yusupov wrote:
> > indeed :)
> > another words if you have modern NIC than you get "zero-copy"(except
> > copy_to_user()) for free :)
> what does "checksum on rx" mean??? Don't most of the NIC's support
> DMA-ing to mem on rx-ing a packet? so what does "zero-copy for free"
> mean here?

It's if the network card will check the checksums of the packets on
receiving. If it doesn't, the main CPU needs to read every byte in the
packet to calculate the checksum itself. If the CPU is doing that
anyway you can copy it elsewhere for free. 

Generally, reading from memory takes time because the CPU has to wait,
writing is free since it can be deferred in the cache (in theory
indefinitly) until there's free cycle.

In other words, if the card isn't checksumming but does DMA you're not
really saving any time over a manual copy.

Hope this helps,
-- 
Martijn van Oosterhout   <kleptog@xxxxxxxxx>   http://svana.org/kleptog/
> Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
> tool for doing 5% of the work and then sitting around waiting for someone
> else to do the other 95% so you can sue them.

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