Re: Qustions about CPU usage of "dd" process

[James Youngman]

On Dec 4, 2007 8:21 PM, Phillip Susi <address@hidden> wrote:
> Pádraig Brady wrote:
> >> The CPU percentage of "dd" process sometimes is 30% to 50%,
> >> which is higher than we expect (<= 20%), and there is no other big
> >> program running at the same time.
> >> If the disc in SATA ODD is CD-R instead of DVD-R, the percentage is
> >> much smaller(<=20).
> >
> > That just means that dd is waiting on the CD-R more than
> > on the DVD-R as the DVD-R is probably faster.
>
> iowait != busy.  It's using cpu time to actively copy data around in ram
> or to/from IO ports if not using DMA, not waiting on the hardware.

I think you may have misunderstood Pádraig's point.   Suppose that the
kernel and dd between them need X units of CPU to process 1MB of data
read from either type of disk.    As the disk's data transfer rate
rises, those X units of CPU will need to be used more often, because
those 1MB chunks are arriving from the device at shorter intervals.
For that reason, one would expect the CPU usage resulting from the use
of a faster device to be greater, even if the same total amount of CPU
work is needed to transfer the data in both cases.

However, I have no idea if the extent of this effect is sufficient to
explain the differences that you are seeing.

James.