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    SubjectRe: [RFC] [PATCH 0/3] ioat: DMA engine support
    On Wed, Nov 23, 2005 at 07:17:01PM -0500, Benjamin LaHaise wrote:
    > On Wed, Nov 23, 2005 at 11:30:08PM +0100, Andi Kleen wrote:
    > > The main problem I see is that it'll likely only pay off when you can keep
    > > the queue of copies long (to amortize the cost of
    > > talking to an external chip). At least for the standard recvmsg
    > > skb->user space, user space-> skb cases these queues are
    > > likely short in most cases. That's because most applications
    > > do relatively small recvmsg or sendmsgs.
    > Don't forget that there are benefits of not polluting the cache with the
    > traffic for the incoming skbs.

    Is that a general benefit outside benchmarks? I would expect
    most real programs to actually do something with the data
    - and that usually involves needing it in cache.

    > > But it's not clear it's a good idea: a lot of these applications prefer to
    > > have the target in cache. And IOAT will force it out of cache.
    > In the I/O AT case it might make sense to do a few prefetch()es of the
    > userland data on the return-to-userspace code path.

    Some prefetches for user space might be a good idea yes

    > Similarly, we should
    > make sure that network drivers prefetch the header at the earliest possible
    > time, too.

    It's done kind of already but tricky to get right because
    the prefetch distances upto use are not really long enough

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