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> > When transferring one or more pages via a page-alligned
> > buffer and normal read() or write(), VM tricks will be
> > used to avoid copying the data. If you touch the page
>
> You mean mmap(). You can do that already but it isnt the win you might
> think. In fact for some operations mmap is slower just due to the mmu bashing
> overhead and TLB misses.
>
> Its non intuitive that mmu operations cost more than a bulk copy but its
> frequently the case. sendfile() works nicely because it is working on data
> that isnt user mapped. O_DIRECT raw I/O has overheads too and its not clear
> where the break even points are

Yup. That's true. Like it's also nonintuitive sometimes that PIO can even
beat, in some cases, DMA (some graphics fifo devices, e.g.).


>
> > They've gotten 960 megabits/sec out of a gigabit Ethernet card
> > with this. Not stable yet.
>
> I think Jes was getting this in Linux TCP/IP without zero copy.

What was the CPU utilization?

-matt


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