Re: [patch] Real-Time Preemption, -RT-2.6.10-rc2-mm3-V0.7.32-15

[Ingo Molnar]

* Mark_H_Johnson@xxxxxxxxxxxx <Mark_H_Johnson@xxxxxxxxxxxx> wrote:

> Comparison of .32-18RT and .32-18PK results
> RT has PREEMPT_RT,
> PK has PREEMPT_DESKTOP and no threaded IRQ's.
> 2.4 has lowlat + preempt patches applied
> 
>       within 100 usec
>        CPU loop (%)   Elapsed Time (sec)    2.4
> Test   RT     PK        RT      PK   |   CPU  Elapsed
> X     90.40 100.00&     73 *    64+  |  97.20   70
> top   78.56 100.00&     39 *    31+  |  97.48   29
> neto  92.82 100.00&    350 *   184+  |  96.23   36
> neti  90.69 100.00&    350 *   170+  |  95.86   41
> diskw 82.96  99.99     360 *    61+  |  77.64   29
> diskc 91.41  99.34     350 *   310+  |  84.12   77
> diskr 88.41  99.96     360 *   310+  |  90.66   86
> total                 1881    1130   |         368
>  [higher is better]  [lower is better]
> * wide variation in audio duration
> + long stretch of audio duration "too fast"
> & 100% to digits shown, had a FEW samples > 100 usec.
> 
> WOW! Look at the 100% values measured on -18PK. The performance of 2.6
> with PREEMPT_DESKTOP is far better than 2.4 preempt+lowlat in every
> way except the non RT starvation problem. Something fixed between -12
> and -18 really made a big improvement.

yep, i guess it's the two missed-preemption points i fixed in -16.

> It is still disturbing to see the worse results for -18RT and I wish I
> knew what the cause was. Perhaps the traces I sent earlier can provide
> a clue.

are you sure you got the order of the columns right? :-|

e.g. the lt004.18PK traces you sent show a number of huge latencies,
biggest one being 1949µs. The biggest one in lt001.18RT is 250 usecs,
and much of that i believe is due to debugging overhead. (It's not
apples to apples because i dont have the RT-under-stress traces yet.) 

something's really not kosher here.

> Other notes:
> 
> [1] -PK has many more latency traces > 250 usec [some MUCH longer]
> than -RT. I ended up collecting more traces for -RT since I set the
> limit to 100 usec, but only got about 8 > 250 usec traces compared to
> 40 for -PK.

this too is suspicious to me. How can then the PREEMPT_RT kernel end up
performing within 100 usecs in only 78.56% of the measurements - it's
ridiculously low! The tracer might have a bug, but such a selective bug?

the only recent thing added was the global RT balancer, which is not
active under PREEMPT_DESKTOP. Maybe this code somehow interferes with
your workload? Could you try to disable it, by changing kernel/sched.c's
#ifdefs from:

 #if defined(CONFIG_PREEMPT_RT) && defined(CONFIG_SMP)

to:

 #if 0

(there are ~5 such places in sched.c)

	Ingo
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