Re: [Lse-tech] Re: Preliminary results of using multiblock raw I/O

Jens Axboe
2001-10-23T10:02:35Z

On Tue, Oct 23 2001, Martin Frey wrote:
> >I haven't seen the SGI rawio patch, but I'm assuming it used kiobufs to
> >pass a single unit of 1 meg down at the time. Yes currently we do incur
> >significant overhead compared to that approach.
> >
> Yes, it used kiobufs to get a gatherlist, setup a gather DMA out
> of that list and submitted it to the SCSI layer. Depending on
> the controller 1 MB could be transfered with 0 memcopies, 1 DMA,
> 1 interrupt. 200 MB/s with 10% CPU load was really impressive.

Let me repeat that the only difference between the kiobuf and the
current approach is the overhead incurred on multiple __make_request
calls. Given the current short queues, this isn't as bad as it used to
be. Of course it isn't free, though.

It's still 0 mem copies, and can be completed with 1 interrupts and DMA
operation.

--
Jens Axboe