Arch Linux

Fackamato

Member

Registered: 2006-03-31

Posts: 579

What does this command do?

[root@fackamato-laptop ~]#  dd if=/dev/zero of=/dev/null bs=1M count=100000
100000+0 records in
100000+0 records out
104857600000 bytes (105 GB) copied, 10.5889 s, 9.9 GB/s
[root@fackamato-laptop ~]#  dd if=/dev/zero of=/dev/null bs=1M count=100000
100000+0 records in
100000+0 records out
104857600000 bytes (105 GB) copied, 25.8666 s, 4.1 GB/s
[root@fackamato-laptop ~]#  dd if=/dev/zero of=/dev/null bs=1M count=100000
100000+0 records in
100000+0 records out
104857600000 bytes (105 GB) copied, 16.213 s, 6.5 GB/s
[root@fackamato-laptop ~]#

I know, I mean it reads zeroes into /dev/null in 1MB chunks, but how does it work? What generates the zeroes? Fluctuation is probably because of cpufreq. Is it only cpu-bound or is it memory speed bound too? Just curious...

Mr.Elendig

#archlinux@freenode channel op

From: The intertubes

Registered: 2004-11-07

Posts: 4,094

Re: What does this command do?

/dev/zero is an endless stream of 0's

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scj

Member

From: Sweden

Registered: 2007-09-23

Posts: 158

Re: What does this command do?

My guess that it's pretty much entirely IO bound.

Fackamato

Member

Registered: 2006-03-31

Posts: 579

Re: What does this command do?

My guess that it's pretty much entirely IO bound.

Yeah sure, but where? cpu<>mem? Or entirely in software? Or cache? Or just pure instr/sec * hz..

Arkane

Member

From: Switzerland

Registered: 2008-02-18

Posts: 263

Re: What does this command do?

Cool, this gave me my first occasion to try and understand something in the kernel sources.

From what I've seen, the driver that manages /dev/zero simply puts all zeroes in the char* output buffer it's passed (actually using an asm routine for it, so it's probably as efficient as can be). Piping to /dev/null pretty much does nothing at all.
I can't really follow the call stack to those functions but it's pretty certain it totally bypasses the I/O subsystem.

As for experimental data, dding from zero to null on my machine maxes out my cpu usage and the throughput only depends on cpu availability.

Last edited by Arkane (2009-02-18 21:14:27)

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