On Fri, Feb 23, 2001 at 07:50:50AM +0200, Jukka Marin wrote:
> On Thu, Feb 22, 2001 at 04:56:02PM -0800, sudog@sudog.com wrote:
> > 
> > definitely a common thing to hear. Unfortunately the CPU hit I am
> > forced into taking with IDE is horrendous. Comparing two cpu-intensive
> > cgi sites, one w/ scsi and one w/out, I can definitely feel the
> > difference.
> 
> What is it that causes the difference in CPU load between IDE and SCSI?
> IDE uses DMA just like SCSI, so...  Tagged queuing?

With IDE drives you've at last one interrupt per xfer. With a decent
SCSI adapter and driver you should be able to have serveral xfers per
interrupt. Also with today's IDE controllers you still have to read/write
registers on the PCI bus, before and after each xfer. This is slower than
main RAM, of course. With good SCSI controllers read/write of registers
per xfer should be much less (possibly less than 1 per xfer, as for
interrupts).

--
Manuel Bouyer <bouyer@antioche.eu.org>
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