how to test the performance of PIO and DMA
hahn at physics.mcmaster.ca
Mon Feb 4 20:56:20 PST 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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