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SubjectRe: zero-copy TCP fileserving

but isn't the real question - should the copy even take case? i have no
doubt that if a copy occurs, some cheap computation (ie. a cksum can
easily be hidden in the data transfer through the CPU).

but, why should the copy even occur? there is easily enough research
lingering around these days to show the zero copying is good (Unet for
instance as well as ExoKernel) these were direct access to user space
without copies, but the same philosophy can hold for kernel space.

Charles

>
> > If the CPU can queue posted writes for the data and is still able to feed
> > itself without stalling, and there is enough bandwidth left for any other
> > bus masters _in the unlikely event it hits main memory_ then the write is
> > pretty cheap. Since we are going to want the data again for the actual
> > I/O transaction in most cases its extremely close to zero cost.
>
> But in the case of a heavy filetransfer, the data will need to hit main
> memory, especially if the NIC can bus-master DMA from main memory. The
> thing is, in a good system, the HD controller will do BM DMA to main
> memory and there should be no need at all for the CPU to touch the data.
> DMA in, DMA out. Checksum can be handled by the NIC. Now I'm not mad
> enough to claim that direct device<->device I/O is the way to go (which
> was discussed here recently :).
>
> I fully agree that a meagre 10 MByte/s (for filling a 100 MBit/s net) is
> a tiny load on a massive P2/400 with main memory bandwidths of hundreds of
> MB/s. But the issue was scalability mainly.
>
> /Bjorn


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