Re: Qustions about CPU usage of "dd" process

[Phillip Susi]

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.

> > So my questions are:
> > (1) Is there an official normal range(or criteria) to the "dd" CPU
> > percentage?
> > (2) Can we say that it's abnormal if it is higher than 30% or even 50%?
> > (3) And what kinds of factors lead to the high CPU percentage of "dd"
> > and
> > how to decrease it?

1) No, it entirely depends on your hardware and kernel
2) I certainly don't like to see it even as high as 20% on typical 
desktop PC type hardware.  I prefer to see it less than 5%.
3) Ideas include the small block size, hardware not using DMA, hardware 
generating a lot of interrupts/only transferring a single sector at a time.