Re: New net features for added performance

From: Jeff Garzik (
Date: Sat Feb 24 2001 - 19:03:38 EST

Andi Kleen wrote:
> Jeff Garzik <> 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 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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This archive was generated by hypermail 2b29 : Wed Feb 28 2001 - 21:00:09 EST