[conspire] DMA for SATA? -was: SATA HD transfers slow?

[Luke S Crawford]

Mark S Bilk <mark at cosmicpenguin.com> writes:

> But every SATA transfer is still using up one of my two CPU cores.
> See the attached jpg of the KDE4 System Monitor, also downloadable
> here:  http://cosmicpenguin.com/tmp/SysMon.jpg
> The wide peak is the transfer, and the narrow ones about every 
> second are Firefox.

Is the red line IOWAIT?

> Here are copies of the CPU line from "top".  The numbers add up
> to 100%, so the measurements for both cores are being added 
> together.  The transfer is using 30-45% wa (wait?) and 4 or 8% 
> system (for disk-disk or disk-/dev/null).  Added and multiplied 
> by 2 these equal about 100% of one of the cores.  Each transfer
> is a 700+MB file.  I used a new one each time so it wouldn't be
> in disk cache.


IOwait doesn't mean you are using pio... look in dmesg.    Iowait just
means "the cpu would have been running, but instead it was waiting for
I/O to complete"  - so if you have a slow disk, you will get lots of 
iowait.