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SubjectRe: [PATCH]sched: Isochronous class v2 for unprivileged soft rt scheduling
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Ingo Molnar <mingo@elte.hu> writes:

> thanks for the testing. The important result is that nice--20
> performance is roughly the same as SCHED_ISO. This somewhat
> reduces the urgency of the introduction of SCHED_ISO.

Doing more runs and a more thorough analysis has driven me to a
different conclusion. The important result is that *neither* nice-20
*nor* SCHED_ISO work properly in their current forms.

For further comparison, I booted an old 2.4.19 kernel with Andrew
Morton's low-latency patches and ran the same test SCHED_FIFO, with
and without background compiles. The results were roughly the same as
SCHED_FIFO on 2.6.11-rc1...

http://www.joq.us/jack/benchmarks/2.4ll-fifo
http://www.joq.us/jack/benchmarks/2.4ll-fifo+compile

In addition, I extracted some across the board information by grepping
for key results. Looking at these numbers in aggregate paints a
pretty convincing picture that neither of the new scheduler prototypes
are performing adequately compared to SCHED_FIFO on either 2.4ll or
2.6.

http://www.joq.us/jack/benchmarks/cycle_max.log
http://www.joq.us/jack/benchmarks/delay_max.log
http://www.joq.us/jack/benchmarks/xrun_count.log

Looking at delay_max broken down by directory is particularly
striking. Below, I grouped the values by scheduling class to show the
striking differences. These kinds of worst-case numbers are what
realtime applications designers are generally most interested in...

============= SCHED_FIFO ==============
...benchmarks/2.4ll-fifo...
Delay Maximum . . . . . . . . : 823 usecs
Delay Maximum . . . . . . . . : 303 usecs
...benchmarks/2.4ll-fifo+compile...
Delay Maximum . . . . . . . . : 926 usecs
Delay Maximum . . . . . . . . : 663 usecs
...benchmarks/sched-fifo...
Delay Maximum . . . . . . . . : 347 usecs
Delay Maximum . . . . . . . . : 277 usecs
Delay Maximum . . . . . . . . : 246 usecs
...benchmarks/sched-fifo+compile...
Delay Maximum . . . . . . . . : 285 usecs
Delay Maximum . . . . . . . . : 269 usecs
Delay Maximum . . . . . . . . : 277 usecs
Delay Maximum . . . . . . . . : 569 usecs
Delay Maximum . . . . . . . . : 461 usecs

============= nice(-20) ==============
...benchmarks/nice-20...
Delay Maximum . . . . . . . . : 13818 usecs
Delay Maximum . . . . . . . . : 155637 usecs
Delay Maximum . . . . . . . . : 487 usecs
Delay Maximum . . . . . . . . : 160328 usecs
Delay Maximum . . . . . . . . : 495328 usecs
...benchmarks/nice-20+compile...
Delay Maximum . . . . . . . . : 183083 usecs
Delay Maximum . . . . . . . . : 5976 usecs
Delay Maximum . . . . . . . . : 18155 usecs
Delay Maximum . . . . . . . . : 557 usecs

============= SCHED_ISO ==============
...benchmarks/sched-iso...
Delay Maximum . . . . . . . . : 21410 usecs
Delay Maximum . . . . . . . . : 36830 usecs
Delay Maximum . . . . . . . . : 4062 usecs
...benchmarks/sched-iso+compile...
Delay Maximum . . . . . . . . : 98909 usecs
Delay Maximum . . . . . . . . : 39414 usecs
Delay Maximum . . . . . . . . : 40294 usecs
Delay Maximum . . . . . . . . : 217192 usecs
Delay Maximum . . . . . . . . : 156989 usecs

Looked at this way, there really is no question. The new scheduler
prototypes are falling short significantly. Could this be due to
their lack of priority distinctions between realtime threads? Maybe.
I can't say for sure. I'll be interested to see what happens when Con
is ready for me to try his new priority-based SCHED_ISO prototype.

On a different note, the fact that 2.6 is finally performing as well
as 2.4+lowlat on this test represents significant progress. In fact,
it performed slightly better (I don't know whether that improvement is
statistically significant).

Congratulations to all who had a hand in making this happen!
--
joq
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