Re: map_user_kiobuf problem in 2.4.0-test3

[Mark Mokryn <mark@sangate.com>]

Hi again,

Roman Zippel wrote:
> > On another interesting note: The raw devices I'm writing to are Fibre
> > Channel drives controlled by a Qlogic 2200 adapter (in 2.2.14 I'm using
> > the Qlogic driver). When writing large sequential blocks to a single
> > drive, I reached 8MB/s when the memory was mapped to the high reserved
> > region, while CPU utilization was down to about 5%. When the mapping was
> > to PCI space, I was able to write at only 4MB/s, and CPU utilization was
> > up to 60%!
> 
> The data is copied from a buffer to the pci device. DMA transfers going
> directly to pci space is impossible without (small) changes to 2.2. 2.4
> has the theoretic possibility to do it and checks already for that, but
> how it should be done practically I'd like to know too.

Okay - if you're right, then this explains the poor performance I'm
seeing for PCI-PCI raw I/O on 2.2.14... Raw I/O is senseless if bounce
buffers are used. Still, raw I/O (the kiobuf stuff) seems to be quite
broken on 2.4.0t3...

-Mark
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