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    SubjectRe: Gigabit Performance 2.4.19-preX - Excessive locks, calls, waits
    On Mon, Mar 04, 2002 at 06:34:33PM +0000, Alan Cox wrote:
    > > > Thats up to the network adapter. In fact the Linux drivers mostly do
    > > > keep preloaded with full sized buffers and only copy if the packet size
    > > > is small (and copying 1 or 2 cache lines isnt going to hurt anyone)
    > >
    > > There's an increase in latency. For my application, I have no
    > A very tiny one (especially if you keep a small buffer pool around too).
    > To copy a packet is 2 cache line loads, which will dominate, some
    > writes that you wont be able to measure, and a writeback you won't be
    > able to instrument without a bus analyser.
    > For receive paths its up to the driver. The copy to smaller buffer is
    > something the driver can choose to do. It and it alone decide what skbuff
    > to throw at the kernel core
    > The bigger ring helping is interesting but itself begs a question. Do you
    > ever dirty rather than merely reference skbuff data. In that case a bigger

    Actually, I am plugging in my own data blocks into the skbuff->data
    pointer from the drivers themselves. When the driver allocates an
    skbuff, I use an alternate cache allocator to load the buffer
    into the skbuff, and I leave the two attached. skb_release_data does
    not release one of my data blocks. These blocks are 4K aligned so
    I am not in the middle of a cache line (I think) in the data portion
    when DMA is initiated.

    > ring may simply be hiding the fact that the recycled skbuff has dirty
    > cached data that has to be written back. Does the combination of hardware

    This is probably happening. I have an Arium here now, and am hooking
    it up. I can get memory bus traces and provide what's actually
    happening on the bus. I do not ever DMA into a partial cache line.


    > you have do the right thing when it comes to the invalidating - and do
    > you ever DMA into a partial cache line ?

    > Alan
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