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SubjectRe: tbench as a load - DDOS attack?
Con Kolivas <kernel@kolivas.org> wrote:
>
>
> Zwane M suggested using tbench as a load to test one of his recent patches and
> gave me the idea to try using tbench_load in contest. Here are the first set
> of results I got while running tbench 4 continuously (uniprocessor machine):
>
> tbench_load:
> Kernel [runs] Time CPU%
> test2420 1 180 38.9
> test2561 1 970 7.7
>
> This is a massive difference. Sure tbench was giving better numbers on 2.5.61
> but it caused a massive slowdown. I wondered whether this translates into
> being more susceptible to ping floods or DDOS attacks? You should have seen
> tbench 16 - 3546 seconds!
>

Yes, something is wrong with the CPU scheduler. Simple test case:

./tbench_srv &
while true
do
./tbench 4
done &
cd /usr/src/util-linux
time make -j4

The tbench activity takes the time to compile util-linux from 13 second to
133 seconds.

But that is not the whole story. The compilation appeared to make no
progress at all while tbench was running - it was only in the gaps between
ending one tbench run and starting another that the compilation did anything.

When I changed the load to one instance of

while true
do
./tbench 2
done

and one instance of

while true
do
./tbench 3
done

the compilation made no progress except for those rare instances when the two
shell scripts were restarting the tbench run at the same time. After a while
the scripts fell itno synchronism, and the compilation took 260 seconds.

Conclusion: tbench completely starves the `make'.

It mainly seems to affect uniprocessor builds. SMP was much better behaved.

Adding the sched-f3 patch basicaly fixed it all up. The 133 second build
came down to 55 seconds, and the compilation was visibly making progress
during the tbench runs.

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