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SubjectRe: New net features for added performance
Andi Kleen wrote:
>
> Jeff Garzik <jgarzik@mandrakesoft.com> writes:
>
> > Advantages: A de-allocation immediately followed by a reallocation is
> > eliminated, less L1 cache pollution during interrupt handling.
> > Potentially less DMA traffic between card and host.
> >
> > Disadvantages?
>
> You need a new mechanism to cope with low memory situations because the
> drivers can tie up quite a bit of memory (in fact you gave up unified
> memory management).

I think you misunderstand.. netif_rx frees the skb. In this example:

netif_rx(skb); /* free skb of size PKT_BUF_SZ */
skb = dev_alloc_skb(PKT_BUF_SZ)
an alloc of a PKT_BUF_SZ'd skb immediately follows a free of a
same-sized skb. 100% of the time.

It seems an obvious shortcut to me, to have __netif_rx or similar
-clear- the skb head not free it. No changes to memory management or
additional low memory situations created by this, AFAICS.


> 4) Better support for aligned RX by only copying the header, no the whole
> packet, to end up with an aligned IP header. Unless the driver knows about
> all protocol lengths this means the stack needs to support "parse header
> in this buffer, then switch to other buffer with computed offset for data"

This requires scatter-gather hardware support, right? If so, would this
support only exist for checksumming hardware -- like the current
zerocopy -- or would non-checksumming SG hardware like tulip be
supported too?

Jeff


--
Jeff Garzik | "You see, in this world there's two kinds of
Building 1024 | people, my friend: Those with loaded guns
MandrakeSoft | and those who dig. You dig." --Blondie
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