Re: Qustions about CPU usage of "dd" process

[Pádraig Brady]

Shane Huang wrote:
> Greetings:
> 
> This is Shane, I'm sorry to disturb you guys.
> 
> I have some questions which are about CPU usage percentage of "dd"
> process.
> I'm appreciated if you can give me some help or suggestion.
> 
> When we do some test on our motherboards, we find that in some cases,
> the CPU usage percentage of "dd" is a little high when we dump data
> from SATA ODD DVD disc to SATA HDD:
> # dd if=/dev/cdwriter of=/root/temp.iso

You can get dd to read & write bigger chunks less often
by passing the bs=size option.

The optimum size will depend on the buffers between
the physical media, but '1M' might be a good first try.

> 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.

> 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?

note also you can use the `time` utility to estimate
how much time is spent by dd in its own userspace code,
and how much is spent in the system code dd is calling.

Pádraig.