cat /dev/hdb > /dev/null DoS

cat /dev/hdb > /dev/null DoS

[Karel Kulhavý]

Hello

I have 2.8 GHz Pentium IV with 1GB RAM and 250GB ATA-133 IDE disk
on /dev/hdb.

I did cat /dev/hdb > /dev/null and after a while (5 minutes),
the system went totally unresponsive:

1) man top takes 3 minutes to display man page
2) Switching between console and X takes also a couple of minutes
3) top shows this:

Cpu(s):  1.4% us,  1.4% sy,  0.0% ni,  0.0% id,  0.0% wa, 97.3% hi,  0.0% si

/dev/hdb is auxilliary disk that is not used, is not mounted and the
root filesystem doesn't reside on it (of course). Kernel version is 2.6.3.

Is this behaviour normal?

Cl<

Re: cat /dev/hdb > /dev/null DoS

[Sven Köhler]

> I did cat /dev/hdb > /dev/null and after a while (5 minutes),
> the system went totally unresponsive:
> 
> 1) man top takes 3 minutes to display man page
> 2) Switching between console and X takes also a couple of minutes
> 3) top shows this:
> 
> Cpu(s):  1.4% us,  1.4% sy,  0.0% ni,  0.0% id,  0.0% wa, 97.3% hi,  0.0% si

i guess you have DMA enabled on /dev/hdb. I would expect, that the 
system is at least 50% idle

Re: cat /dev/hdb > /dev/null DoS

[Sven Köhler]

> i guess you have DMA enabled on /dev/hdb. I would expect, that the 
> system is at least 50% idle

i did
   dd if=/dev/hda of=/dev/null bs=1M
and noticed, that top shows 0% idle, but 95% wa - what ever that means.

Since i've got DMA turned on, i would expect the CPU to be 95% idle, 
instead 95% wa? what does "wa" stand for?

Re: cat /dev/hdb > /dev/null DoS

[Denis Vlasenko]

On Friday 09 April 2004 03:10, Sven Köhler wrote:
> > i guess you have DMA enabled on /dev/hdb. I would expect, that the
> > system is at least 50% idle
>
> i did
>    dd if=/dev/hda of=/dev/null bs=1M
> and noticed, that top shows 0% idle, but 95% wa - what ever that means.
>
> Since i've got DMA turned on, i would expect the CPU to be 95% idle,
> instead 95% wa? what does "wa" stand for?

top version? Mine is 2.0.18, looks like this:

 09:47:25  up 1 day, 17:07,  2 users,  load average: 0.42, 0.27, 0.24
116 processes: 114 sleeping, 2 running, 0 zombie, 0 stopped
CPU states:   4.2% user  52.1% system   0.0% nice  43.6% iowait   0.0% idle
Mem:   122736k av,  121088k used,    1648k free,       0k shrd,   22968k buff
        76324k active,              21000k inactive
Swap:   76792k av,   67004k used,    9788k free                   25748k cached

  PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME CPU COMMAND
15022 root      18   0  2280 1460  1200 R    38.6  1.1   0:05   0 dd
-- 
vda