Messages in this thread | | | Date | Sun, 16 Feb 2003 01:30:29 -0800 | From | Andrew Morton <> | Subject | Re: tbench as a load - DDOS attack? |
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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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