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SubjectLinux Interrupt Latency benchmark v2 avaible
I improved my interrupt latency benchmark generating a train of `num'
interrupt and doing some stat. I also improved the way I generate the
interrupt to account also the time that the 8259 eat in order to generate
the IRQ request to the CPU.

Here some numbers from a 166Mhz P5 noMMX.

Aug 18 20:44:33 ark kernel: Linux Interrupt Latency benchmark - Copyright
(C) 1998 Andrea Arcangeli
Aug 18 20:44:33 ark kernel: irqN 1 latency: 726
Aug 18 20:44:33 ark kernel: irqN 2 latency: 1089
Aug 18 20:44:33 ark kernel: irqN 3 latency: 892
Aug 18 20:44:33 ark kernel: irqN 4 latency: 1084
Aug 18 20:44:33 ark kernel: irqN 5 latency: 1064
Aug 18 20:44:33 ark kernel: irqN 6 latency: 759
Aug 18 20:44:33 ark kernel: irqN 7 latency: 1064
Aug 18 20:44:33 ark kernel: irqN 8 latency: 1034
Aug 18 20:44:33 ark kernel: irqN 9 latency: 1034
Aug 18 20:44:33 ark kernel: irqN 10 latency: 1064
Aug 18 20:44:33 ark kernel: latency cycles: mean 979, max 1089, min 726

The latency is measured in CPU tick.

This is the output of `insmod ./lil.o irq=3 num=10 verbose=1`.

I am monitoring the output of the insmod in realtime via network (rsh ark
cat /dev/xconsole) so after every interrupt Linux has to run the network
and the CPU put the lil code out of L1 cache.

If I run without verbose lil code remains in L1 cache and the latency
decrease a lot!

root@ark:/home/andrea# insmod ./lil.o irq=3 num=1000 verbose=0
Aug 18 20:54:45 ark kernel: Linux Interrupt Latency benchmark - Copyright
(C) 1998 Andrea Arcangeli
Aug 18 20:54:45 ark kernel: latency cycles: mean 421, max 1144, min 421

If I remove the schedule() after every interrupt also with verbose = 1 the
latency is reduced at 421 tick.

Conclusion:

With the machine in idle the latency is 421 cycles -> 2.5uSec, with the
machine loaded (so the lil code is kicked out of L1 cache) the latency is
of ~1000 cycles -> 6uSec _as_worse_. This in a "old" P5 noMMX 166Mhz
running 2.1.115.

Andrea[s] Arcangeli


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