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    SubjectRe: Is sendfile all that sexy?
    James Sutherland wrote:
    >
    > On Sat, 20 Jan 2001, Linus Torvalds wrote:
    >
    > >
    > >
    > > On Sat, 20 Jan 2001, Roman Zippel wrote:
    > > >
    > > > On Sat, 20 Jan 2001, Linus Torvalds wrote:
    > > >
    > > > > But point-to-point also means that you don't get any real advantage from
    > > > > doing things like device-to-device DMA. Because the links are
    > > > > asynchronous, you need buffers in between them anyway, and there is no
    > > > > bandwidth advantage of not going through the hub if the topology is a
    > > > > pretty normal "star" kind of thing. And you _do_ want the star topology,
    > > > > because in the end most of the bandwidth you want concentrated at the
    > > > > point that uses it.
    > > >
    > > > I agree, but who says, that the buffer always has to be the main memory?
    > >
    > > It doesn't _have_ to be.
    > >
    > > But think like a good hardware designer.
    > >
    > > In 99% of all cases, where do you want the results of a read to end up?
    > > Where do you want the contents of a write to come from?
    > >
    > > Right. Memory.
    >
    > For many applications, yes - but think about a file server for a moment.
    > 99% of the data read from the RAID (or whatever) is really aimed at the
    > appropriate NIC - going via main memory would just slow things down.
    >
    > Take a heavily laden webserver. With a nice intelligent NIC and RAID
    > controller, you might have the httpd write the header to this NIC, then
    > have the NIC and RAID controller handle the sendfile operation themselves
    > - without ever touching the OS with this data.

    And when the next user wants the same webpage/file you read it from the
    RAID again?
    Seems to me you loose the benefit of caching stuff in memory with this
    scheme.
    Sure - the RAID controller might have some cache, but it is usually
    smaller
    than main memory anyway. And then there are things like
    retransmissions...


    Helge Hafting
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