how to test the performance of PIO and DMA

Mark Hahn hahn at physics.mcmaster.ca
Mon Feb 4 23:56:20 EST 2002


> > > 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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