Re: I don"t understand why the following increases cpu time

In article
<1f8cf61d-933a-4171-9912-fcb9b8a1ff27@q39g2000hsf.googlegroups.com>,
grocery_stocker wrote:

> If I do the following...
>
> % cat < /dev/zero
>
> And look at top, nothing shows ups.....
>
> last pid: 83551; load averages: 1.82, 1.64, 1.38 up
> 49+10:45:28 21:26:16
> 217 processes: 2 running, 182 sleeping, 1 stopped, 32 zombie
>
> Mem: 139M Active, 632M Inact, 164M Wired, 33M Cache, 111M Buf, 28M
> Free
> Swap: 512M Total, 512M Free
>
>
> PID USERNAME THR PRI NICE SIZE RES STATE TIME WCPU
> COMMAND
> 279 buttemb 1 129 5 3212K 1808K RUN 241.8H 17.29% bash
> 82205 vrana 1 5 5 7228K 5168K ttyin 0:00 2.45% lynx
> 50756 serapis 1 5 5 8380K 6320K ttyin 0:02 1.90% lynx

When you write to the terminal, the speed of the terminal throttles how
quickly cat can run.

> However, if I change it so /dev/zero goes to /dev/nulll
>
> m-net% cat < /dev/zero > /dev/null
>
>
> And then look at top, I see that I'm taking up 52.08% CPU time...
>
> last pid: 48206; load averages: 1.44, 1.24, 1.20 up
> 49+10:41:45 21:22:33
> 215 processes: 4 running, 179 sleeping, 1 stopped, 31 zombie
>
> Mem: 136M Active, 632M Inact, 164M Wired, 33M Cache, 111M Buf, 31M
> Free
> Swap: 512M Total, 512M Free
>
>
> PID USERNAME THR PRI NICE SIZE RES STATE TIME WCPU
> COMMAND
> 47506 cdalten 1 127 5 1272K 652K RUN 0:05 52.08% cat
> 279 buttemb 1 128 5 3212K 1808K RUN 241.8H 16.11% bash
> 43171 vrana 1 5 5 7844K 5832K ttyin 0:02 5.75% lynx

The /dev/null driver is very efficient, since it doesn't have to do
anything. This allows cat to run at full speed and use more CPU time.

> The question why does copying to /dev/null make a sharp increas in CPU
> time.

--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
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