how to test the performance of PIO and DMA

alvin at alvin at
Mon Feb 4 21:19:16 PST 2002

hi mark

ditto on your comments... nobody should be using PIO mode...

for (other's) info ....
usually you can see a "*" next to the current drive settings
from   "hdparm -iv /dev/hda"
	- look for *udma5  ( for "100MByte/sec transfer" disks )

even if PIO is supported on the disk.. you dont need it or use it..

c ya
alvin ... IDE vs SCSI ...

	- note teh PIO and MultiWord DMA speed ... 
	and note the current Ultra-xxx dma modes..

On Mon, 4 Feb 2002, Mark Hahn wrote:

> > > > Title: how to test the performance of PIO and DMA
> > > 
> > >         use hdparm for simple easy tests and watch your resulting
> > >         transfer speed
> > 
> > You may also want to monitor your CPU load: there is a huge difference
> > in CPU overhead between PIO and DMA modes.
> like night and day.  PIO is a dinosaur, and any vaguely modern machine 
> doing PIO is criminally misconfigured.
> > The standard hdparm test is
> > too brief to see this,
> I have no idea why you say this: when I run 
> 	/usr/bin/time hdparm -t /dev/hda
> on the desktop I'm sitting at, I see around 24 MB/s and 9% CPU
> in dma mode, versus 8.9 MB/s and 55% CPU after hdparm -d0
> (note that the %CPU numbers are extremely approximate: the actual
> ide-dma overhead is in the 3-5% range.)
> > Moreover, I think that DMA gives you CRC checking for data going over
> > the IDE cable while PIO does not.  DMA is a clear winner, unless you
> udma, actually - plain old ide-dma doesn't do the CRC.
> all machines should be using udma on ide, and should be using
> ultra2 on SCSI (which adds CRC's as well.)

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